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Solar System

January 31, 2023

This article is about the Sun and its planetary system. For other uses, see Solar System (disambiguation).

The Solar System[c] is the gravitationally bound system of the Sun and the objects that orbit it. It formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority (99.86%) of the system's mass is in the Sun, with most of the remaining mass contained in the planet Jupiter. The four inner system planets—Mercury, Venus, Earth and Mars—are terrestrial planets, being composed primarily of rock and metal. The four giant planets of the outer system are substantially larger and more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, being composed mostly of volatile substances with relatively high melting points compared with hydrogen and helium, such as water, ammonia, and methane. All eight planets have nearly circular orbits that lie near the plane of Earth's orbit, called the ecliptic.

Solar System
The Sun, planets, moons and dwarf planets[a]
(true color, size to scale, distances not to scale)
Age4.568 billion years
Location
System mass1.0014 solar masses[citation needed]
Nearest star
Nearest known planetary system
Proxima Centauri system (4.2441 ly)
Planetary system
Semi-major axis of outer known planet (Neptune)
30.11 AU
(4.5 bill. km; 2.8 bill. mi)
Distance to Kuiper cliff~50 AU
Populations
Stars1 (Sun)
Known planets
Known dwarf planets
Known natural satellites
Known minor planets1,199,224[b][2]
Known comets4,402[b][2]
Identified rounded satellites19
Orbit about Galactic Center
Invariable-to-galactic plane inclination60.19° (ecliptic)
Distance to Galactic Center27,000 ± 1,000 ly
Orbital speed220 km/s; 136 mi/s
Orbital period225–250 myr
Star-related properties
Spectral typeG2V
Frost line≈5 AU[3]
Distance to heliopause≈120 AU
Hill sphere radius≈1–3 ly

There are an unknown number of smaller dwarf planets and innumerable small Solar System bodies orbiting the Sun.[d] Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, commonly called "moons" after Earth's Moon. Two natural satellites, Jupiter's moon Ganymede and Saturn's moon Titan, are larger than Mercury, the smallest terrestrial planet, though less massive, and Jupiter's moon Callisto is nearly as large. Each of the giant planets and some smaller bodies are encircled by planetary rings of ice, dust and moonlets. The asteroid belt, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of objects composed mostly of ice and rock.

In the outer reaches of the Solar System lies a class of minor planets called detached objects. There is considerable debate as to how many such objects there will prove to be.[9] Some of these objects are large enough to have rounded under their own gravity and thus to be categorized as dwarf planets. Astronomers generally accept about nine objects as dwarf planets: the asteroid Ceres, the Kuiper-belt objects Pluto, Orcus, Haumea, Quaoar, and Makemake, and the scattered-disc objects Gonggong, Eris, and Sedna.[d] Various small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between the regions of the Solar System.

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region of interplanetary medium in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky. The nearest stars are within the so-called Local Bubble, with the closest, Proxima Centauri, at 4.2441 light-years.

Formation and evolution

Main article: Formation and evolution of the Solar System
 
Artist's impression of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[e] This initial cloud was likely several light-years across and probably birthed several stars.[11] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula,[12] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[11] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi)[11] and a hot, dense protostar at the centre.[13][14] The planets formed by accretion from this disc,[15] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover minor bodies.[16][17]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun, and these would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[16] The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.[18]

 
The Helix Nebula, a planetary nebula similar to what the Sun will create when it enters its white dwarf stage

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.[19] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a main-sequence star.[20] The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun's pre-remnant life combined.[21] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space. As helium accumulates at its core the Sun is growing brighter;[22] early in its main-sequence life its brightness was 70% that of what it is today.[23]

The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (2,330 °C; 4,220 °F) at its coolest) than it is on the main sequence.[21]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well). Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf, half the original mass of the Sun but only the size of Earth.[24] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.[25]

Structure and composition

Further information: List of Solar System objects and Planet § Planetary attributes

The word solar means "pertaining to the Sun", which is derived from the Latin word sol, meaning Sun.[26] The Sun is the dominant gravitational member of the Solar System, and its planetary system is maintained in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.[27]

Orbits

 
 
Animations of the Solar System's inner planets and outer planets orbiting; the latter animation is 100 times faster than the former. Jupiter is three times as far from the Sun as Mars.

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.[28][29] Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. Many of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison.[30]

As a result of the formation of the Solar System, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.[31] There are exceptions, such as Halley's Comet.[32] Most of the larger moons orbit their planets in prograde direction, matching the planetary rotation; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner.[33] Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.[34]

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects about the Sun.[35]: 433–437  These laws stipulate that each object travels along an ellipse with the Sun at one focus, which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion.[36]: 9-6  The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these additional perturbations can be accounted for using numerical models,[36]: 9-6  but the planetary system can change chaotically over billions of years.[37]

The angular momentum of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components.[38] Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.[39][40] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.[39]

 
The orientation of the Solar System's motion

Composition

The overall structure of the charted regions of the Solar System consists of the Sun, four smaller inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets.[41] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.[42]

The principal component of the Solar System is the Sun, a low-mass star that contains 99.86% of the system's known mass and dominates it gravitationally.[43] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.[f]

The Sun is composed of roughly 98% hydrogen and helium,[47] as are Jupiter and Saturn.[48][49] A composition gradient exists in the Solar System, created by heat and light pressure from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.[50] The boundary in the Solar System beyond which those volatile substances could coalesce is known as the frost line, and it lies at roughly five times the Earth's distance from the Sun.[3]

The objects of the inner Solar System are composed mostly of rocky materials,[51] such as silicates, iron or nickel.[52] Jupiter and Saturn are composed mainly of gases with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon.[52] Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide,[51] have melting points up to a few hundred kelvins.[52] They can be found as ices, liquids, or gases in various places in the Solar System.[52] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.[51][53] Together, gases and ices are referred to as volatiles.[54]

Distances and scales

 
The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale
 
To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale.

The astronomical unit [AU] (150,000,000 km; 93,000,000 mi) would be the distance from the Earth to the Sun if the planet's orbit were perfectly circular.[55] For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).[56] Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km; 480,000,000 mi) from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km; 2.8×109 mi) from the Sun.[49][57]

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like the Titius–Bode law[58] and Johannes Kepler's model based on the Platonic solids,[59] but ongoing discoveries have invalidated these hypotheses.[60]

Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.[61] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Avicii Arena in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.[62][63]

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale.[64]

Sun

Main article: Sun
 
The Sun in true white color

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),[65] which comprises 99.86% of all the mass in the Solar System,[66] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.[67] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[68][69]

Because the Sun fuses hydrogen into helium at its core, it is a main-sequence star. More specifically, it is a G2-type main-sequence star, where the type designation refers to its effective temperature. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up about 75% of the stars in the Milky Way.[70][71]

The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars.[72] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals".[73]

Environment and habitability

Outside of the main part of the Sun's atmosphere extends the heliosphere and dominates the Solar planetary system. The vast majority of the heliosphere is occupied by a near-vacuum known as the interplanetary medium. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) called the solar wind. This stream of particles spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[74] creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU (15 billion km; 9.3 billion mi) (see § Heliosphere).[75] Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[76] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[77][78]

Earth's magnetic field stops its atmosphere from being stripped away by the solar wind.[79] Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space.[80] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.[81]

The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.[82]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the zodiacal light. It may have been formed by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is the planet Mars.[83] The second dust cloud extends from about 10 AU (1.5 billion km; 930 million mi) to about 40 AU (6.0 billion km; 3.7 billion mi), and was probably created by collisions within the Kuiper belt.[84][85]

The zone of hability of the Solar System is located in the Inner Solar System. Beside the Solar conditions for hability on Solar System objects such as Earth, habitability might be possibly in subsurface oceans of various Outer Solar System moons.

Inner Solar System

 
Overview of the Inner Solar System up to the Jovian System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[86] Composed mainly of silicates and metals,[87] the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun.[28]

Inner planets

Main article: Terrestrial planet
 
The four terrestrial planets Mercury, Venus, Earth and Mars.

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as the silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).[88]

Mercury

Main article: Mercury (planet)

Mercury (0.307–0.588 AU (45.9–88.0 million km; 28.5–54.7 million mi) from the Sun[89]) is the closest planet to the Sun. The smallest planet in the Solar System (0.055 MEarth), Mercury has no natural satellites. The dominant geological features are impact craters or basins with ejecta blankets, the remains of early volcanic activity including magma flows, and lobed ridges or rupes that were probably produced by a period of contraction early in the planet's history.[90] Mercury's very tenuous atmosphere consists of solar-wind particles trapped by Mercury's magnetic field, as well as atoms blasted off its surface by the solar wind.[91][92] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy.[93][94]

There have been searches for "Vulcanoids", asteroids in stable orbits between Mercury and the Sun, but none have been discovered.[95][96]

Venus

Main article: Venus

Venus (0.718–0.728 AU (107.4–108.9 million km; 66.7–67.7 million mi) from the Sun[89]) is close in size to Earth (0.815 MEarth) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.[97] The planet has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.[98] A relatively young planetary surface displays extensive evidence of volcanic activity, but is devoid of plate tectonics. It may undergo resurfacing episodes on a time scale of 700 million years.[99]

Earth

Main article: Earth

Earth (0.983–1.017 AU (147.1–152.1 million km; 91.4–94.5 million mi) from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist.[100] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed.[101] Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[102][103] The planetary magnetosphere shields the surface from solar and cosmic radiation, limiting atmospheric stripping and maintaining habitability.[104] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Main article: Mars

Mars (1.382–1.666 AU (206.7–249.2 million km; 128.5–154.9 million mi) from the Sun) is smaller than Earth and Venus (0.107 MEarth). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (0.088 psi; 0.18 inHg); roughly 0.6% of that of Earth but sufficient to support weather phenomena.[105] Its surface, peppered with volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[106] Its red colour comes from iron oxide (rust) in its soil.[107] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids,[108] or ejected debris from a massive impact early in Mars's history.[109]

Asteroid belt

Main articles: Asteroid belt and Asteroid
 
Linear map of the inner Solar System, showing many asteroid populations

Asteroids except for the largest, Ceres, are classified as small Solar System bodies[d] and are composed mainly of refractory rocky and metallic minerals, with some ice.[110][111] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.[112] As of 2017, the IAU designates asteroids having diameter between about 30 micrometres and 1 metre as micrometeroids, and terms smaller particles "dust".[113]

The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.[114] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[115] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[46] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[116]

Ceres

Main article: Ceres (dwarf planet)

Ceres (2.77 AU (414 million km; 257 million mi) from the Sun) is the largest asteroid, a protoplanet, and a dwarf planet.[d] It has a diameter of slightly under 1,000 km (620 mi) and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801, but as further observations revealed additional asteroids, it became common to consider it as one of the minor rather than major planets.[117] It was then reclassified again as a dwarf planet in 2006 when the IAU definition of planet was established.[118]: 218 

Pallas and Vesta

Main articles: 2 Pallas and 4 Vesta

Pallas (2.77 AU from the Sun) and Vesta (2.36 AU from the Sun) are the largest asteroids in the asteroid belt, after Ceres. They are the other two protoplanets that survive more or less intact. At about 520 km (320 mi) in diameter, they were large enough to have developed planetary geology in the past, but both have suffered large impacts and been battered out of being round.[119][120][121] Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt, as the Pallas family and Vesta family. Both were considered planets upon their discoveries in 1802 and 1807 respectively, and then like Ceres generally considered as minor planets with the discovery of more asteroids. Some authors today have begun to consider Pallas and Vesta as planets again, along with Ceres, under geophysical definitions of the term.[5]

Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Kirkwood gaps are sharp dips in the distribution of asteroid orbits that correspond to orbital resonances with Jupiter.[122] Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners (e.g. that of 90 Antiope). The asteroid belt includes main-belt comets, which may have been the source of Earth's water.[123]

Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[124] The inner Solar System contains near-Earth asteroids, many of which cross the orbits of the inner planets.[125] Some of them are potentially hazardous objects.[126]

Outer Solar System

 
Plot of objects around the Kuiper belt and other asteroid populations, the J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.[16]

Outer planets

Main article: Giant planet
 
The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right

The four outer planets, also called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun.[f] Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as gas giants.[127] Uranus and Neptune are far less massive—less than 20 Earth masses (MEarth) each—and are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants.[128] All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars.[88]

The ring–moon systems of Jupiter, Saturn, and Uranus are like miniature versions of the Solar System; that of Neptune is significantly different, having been disrupted by the capture of its largest moon Triton.[129]

Jupiter

Main article: Jupiter

Jupiter (4.951–5.457 AU (740.7–816.4 million km; 460.2–507.3 million mi) from the Sun[89]), at 318 MEarth, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. The planet possesses a 4.2–14 Gauss strength magnetosphere that spans 22–29 million km, making it, in certain respects, the largest object in the Solar System.[130] Jupiter has 80 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, are called the Galilean moons: they show similarities to the terrestrial planets, such as volcanism and internal heating.[131] Ganymede, the largest satellite in the Solar System, is larger than Mercury; Callisto is almost as large.[132]

Saturn

Main article: Saturn

Saturn (9.075–10.07 AU (1.3576–1.5065 billion km; 843.6–936.1 million mi) from the Sun[89]), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 MEarth. Saturn is the only planet of the Solar System that is less dense than water. The rings of Saturn are made up of small ice and rock particles.[133] Saturn has 83 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity;[134] they, as well as five other Saturnian moons (Iapetus, Rhea, Dione, Tethys, and Mimas), are large enough to be round. Titan, the second-largest moon in the Solar System, is bigger than Mercury and the only satellite in the Solar System to have a substantial atmosphere.[135][136]

Uranus

Main article: Uranus

Uranus (18.27–20.06 AU (2.733–3.001 billion km; 1.698–1.865 billion mi) from the Sun[89]), at 14 MEarth, has the lowest mass of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. This gives the planet extreme seasonal variation as each pole points toward and then away from the Sun.[137] It has a much colder core than the other giant planets and radiates very little heat into space.[138] As a consequence, it has the coldest planetary atmosphere in the Solar System.[139] Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.[140] Like the other giant planets, it possesses a ring system and magnetosphere.[141]

Neptune

Main article: Neptune

Neptune (29.89–30.47 AU (4.471–4.558 billion km; 2.778–2.832 billion mi) from the Sun[89]), though slightly smaller than Uranus, is more massive (17 MEarth) and hence more dense. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn.[142] Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[143] Triton is the only large satellite with a retrograde orbit, which indicates that it did not form with Neptune, but was probably captured from the Kuiper belt.[144] Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that either lead or trail the planet by about one-sixth of the way around the Sun, positions known as Lagrange points.[145]

Centaurs

Main article: Centaur (small Solar System body)

The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU (820 million km; 510 million mi)) and less than Neptune's (30 AU (4.5 billion km; 2.8 billion mi)). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi).[146] The first centaur discovered, 2060 Chiron, has also been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[147]

Comets

Main article: Comet
 
Comet Hale–Bopp seen in 1997

Comets are small Solar System bodies,[d] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.[148]

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent.[149] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[150] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.[151]

Trans-Neptunian region

 
Distribution and size of trans-Neptunian objects
 
Size comparison of some large TNOs with Earth: Pluto and its moons, Eris, Makemake, Haumea, Sedna, Gonggong, Quaoar, Orcus, Salacia, and 2002 MS4.

Inside the orbit of Neptune is the planetary region of the Solar System. Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.[152]

Kuiper belt

Main article: Kuiper belt

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[153] It extends between 30 and 50 AU (4.5 and 7.5 billion km; 2.8 and 4.6 billion mi) from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.[9] There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.[45] Many Kuiper belt objects have multiple satellites,[154] and most have orbits that take them outside the plane of the ecliptic.[155]

The Kuiper belt can be roughly divided into the "classical" belt and the resonant trans-Neptunian objects.[153] The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU (5.89 to 7.14 billion km; 3.66 to 4.43 billion mi).[156] Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated 1992 QB1; they are still in near primordial, low-eccentricity orbits.[157]

Pluto and Charon

Main articles: Pluto and Charon (moon)

The dwarf planet Pluto (with an average orbit of 39 AU (5.8 billion km; 3.6 billion mi) from the Sun) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU (4.44 billion km; 2.76 billion mi) from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU (7.41 billion km; 4.60 billion mi) at aphelion. Pluto has a 2:3 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.[158]

Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit Pluto.[159]

Others

Besides Pluto, astronomers generally agree that at least four other Kuiper belt objects are dwarf planets,[9] and additional bodies have also been proposed:[160]

  • Makemake (45.79 AU average from the Sun), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009.[161] Its orbit is far more inclined than Pluto's, at 29°.[162] It has one known moon.[163]
  • Haumea (43.13 AU average from the Sun) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune.[164] Like Makemake, it was discovered in 2005.[165] It has two known moons, Hiʻiaka and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an ellipsoid.[166]
  • Quaoar (43.69 AU average from the Sun) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea.[164] It has one known moon, Weywot.[167]
  • Orcus (39.40 AU average from the Sun) is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.[164] Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the phase of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa.[168] For this reason, it has been called the anti-Pluto.[169][170] It has one known moon, Vanth.[171]

Scattered disc

Main article: Scattered disc
 
The scattered disc object Sedna and its orbit within the Solar System.

The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can also be inclined up to 46.8° from the ecliptic plane.[172] Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects".[173] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[174]

Eris and Gonggong

Eris (67.78 AU average from the Sun) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto[175] and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane at an angle of 44°.[176]

Gonggong (67.38 AU average from the Sun) is in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.[177] It has one known moon, Xiangliu.[178]

Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces, the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this heliopause, the outer boundary of the heliosphere, is considered the beginning of the interstellar medium.[75] The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[179]

Edge of the heliosphere

Main article: Heliosheath
 
Artistic depiction of the Solar System's heliosphere

The Sun's stellar-wind bubble, the heliosphere, a region of space dominated by the Sun, has its boundary at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[180] Here the solar wind collides with the interstellar medium[181] and dramatically slows, condenses and becomes more turbulent,[180] forming a great oval structure known as the heliosheath. This structure has been theorized to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind.[182] Evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,[183][184] but the actual shape remains unknown.[185]

The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.[75] Voyager 1 and Voyager 2 passed the termination shock and entered the heliosheath at 94 and 84 AU from the Sun, respectively.[186][187] Voyager 1 was reported to have crossed the heliopause in August 2012, and Voyager 2 in December 2018.[188][189]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere.[180] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.[190]

Detached objects

Main articles: Detached object and Sednoid

Sedna (with an average orbit of 520 AU from the Sun) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.[191] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun.[192] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP113, discovered in 2012. Its aphelion is only about half that of Sedna's, at 458 AU.[193][194]

Oort cloud

Main article: Oort cloud

The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)) from the Sun, and possibly to as far as 100,000 AU (1.87 ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.[195][196]

Boundaries

See also: Planets beyond Neptune, Planet Nine, and List of Solar System objects by greatest aphelion

Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.[197] Most of the mass is orbiting in the region between 3,000 and 100,000 AU.[198] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. Learning about this region of space is difficult, because it depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.[199] Objects may yet be discovered in the Solar System's uncharted regions.[200] The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun.[201]

Galactic context

See also: Location of Earth and Galactic year
 
 
Diagram of the Milky Way with the position of the Solar System marked by a yellow arrow and a red dot in the Orion Arm, the dot roughly covering the large surrounding celestial area dominated by the Radcliffe wave and Split linear structures (formerly Gould Belt).[202]

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars.[203] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[204] The Sun lies about 26,660 light-years from the Galactic Center,[205] and its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.[203] This revolution is known as the Solar System's galactic year.[206] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.[207] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[g]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms.[209][210] Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.[209] However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the Shiva hypothesis or related theories, but this remains controversial.[211][212]

The Solar System lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.[209] Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz's Star, which approached to 52+23
−14 kAU of the Sun some 70+15
−10 kya, likely passing through the outer Oort cloud.[213]

Celestial neighbourhood

 
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud, here shown along with neighbouring clouds and the two closest unaided visible stars.

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.[214][215] Multiple other interstellar clouds also exist in the region within 300 light-years of the Sun, known as the Local Bubble.[215] The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.[216]

The Local Bubble is a small superbubble compared to the neighbouring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length.[202] All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye. The density of all matter in the local neighborhood is 0.097±0.013 M☉·pc−3.[217]

Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble's G-Cloud.[218] Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to Earth, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.[219]

The next closest known fusors to the Sun are the red dwarfs Barnard's Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 (8.3 ly).[220] The nearest brown dwarfs belong to the binary Luhman 16 system (6.6 ly), and the closest known rogue or free-floating planetary-mass object at less than 10 Jupiter masses is the sub-brown dwarf WISE 0855−0714 (7.4 ly).[221]

Just beyond at 8.6 ly lies Sirius, the brightest star in Earth's night sky, with roughly twice the Sun's mass, orbited by the closest white dwarf to Earth, Sirius B. Other stars within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly).[222][223] The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only about half of its luminosity.[224]

The nearest and unaided-visible group of stars beyond the immediate celestial neighbourhood is the Ursa Major Moving Group at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible star cluster the Hyades, which lie at its edge. The closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.[225]

Comparison with extrasolar systems

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[226][227] The known Solar System also lacks super-Earths, planets between one and ten times as massive as the Earth,[226] although the hypothetical Planet Nine, if it does exist, could be a super-Earth beyond the Solar System as we understand it today.[228] Uncommonly, it has only small rocky planets and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.[226][229]

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity.[226] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.[226][230]

Humanity's perspective

Main article: Discovery and exploration of the Solar System
 
Harrison H. Schmitt, an astronaut in the Apollo 17 mission, with the Moon and Earth in the background

Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle Ages–Renaissance, astronomers from Europe to India believed Earth to be stationary at the centre of the Universe[231] and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system.[232][233] Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical as well as circular, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables, which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.[234][235]

In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it.[236] Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn.[237] In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun.[238] Halley's friend Isaac Newton, in his magisterial Principia Mathematica of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity apply on Earth and in the skies.[35]: 142 

The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets.[239] In 1705, Halley realised that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun,[240] though Seneca had theorized this about comets in the 1st century.[241] Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.[242] Uranus, having occasionally been observed since antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.[243] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.[244] Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.[245]

In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space.[246] Since then, humans have landed on the Moon during the Apollo program; the Apollo 13 mission marked the furthest any human has been away from Earth at 400,171 kilometers (248,655 mi).[247] All eight planets have been visited by space probes; the outer planets were first visited by the Voyager spacecraft, one of which (Voyager 1) is the furthest object made by humankind and the first in interstellar space.[248] In addition, probes have also returned samples from comets[249] and asteroids,[250] as well as flown through the Sun's corona[251] and made fly-bys of Kuiper belt objects.[252] Six of the planets (all but Uranus and Neptune) have or had a dedicated orbiter.[253]

See also

Notes

  1. ^ The asteroid belt and Kuiper belt are not added because the individual asteroids are too small to be shown on the diagram.
  2. ^ a b As of 2 April 2022.
  3. ^ Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their naming guidelines document 25 July 2021 at the Wayback Machine. The name is commonly rendered in lower case ('solar system'), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary 27 January 2008 at the Wayback Machine.
  4. ^ a b c d e According to IAU definitions, objects orbiting the Sun are classified dynamically and physically into three categories: planets, dwarf planets, and small Solar System bodies.
  5. ^ The date is based on the oldest inclusions found to date in meteorites, 4568.2+0.2
    −0.4     million years, and is thought to be the date of the formation of the first solid material in the collapsing nebula.[10]
  6. ^ a b The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),[44] the Kuiper belt (estimated at 0.1 Earth mass)[45] and the asteroid belt (estimated to be 0.0005 Earth mass)[46] for a total, rounded upwards, of ~37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3% of the total orbiting mass.
  7. ^ If   is the angle between the north pole of the ecliptic and the north galactic pole then:
     
    where   = 27° 07′ 42.01″ and   = 12h 51m 26.282s are the declination and right ascension of the north galactic pole,[208] whereas   = 66° 33′ 38.6″ and   = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for J2000 epoch.) The result of the calculation is 60.19°.

References

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solar, system, this, article, about, planetary, system, other, uses, disambiguation, gravitationally, bound, system, objects, that, orbit, formed, billion, years, from, gravitational, collapse, giant, interstellar, molecular, cloud, vast, majority, system, mas. This article is about the Sun and its planetary system For other uses see Solar System disambiguation The Solar System c is the gravitationally bound system of the Sun and the objects that orbit it It formed 4 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud The vast majority 99 86 of the system s mass is in the Sun with most of the remaining mass contained in the planet Jupiter The four inner system planets Mercury Venus Earth and Mars are terrestrial planets being composed primarily of rock and metal The four giant planets of the outer system are substantially larger and more massive than the terrestrials The two largest Jupiter and Saturn are gas giants being composed mainly of hydrogen and helium the next two Uranus and Neptune are ice giants being composed mostly of volatile substances with relatively high melting points compared with hydrogen and helium such as water ammonia and methane All eight planets have nearly circular orbits that lie near the plane of Earth s orbit called the ecliptic Solar SystemThe Sun planets moons and dwarf planets a true color size to scale distances not to scale Age4 568 billion yearsLocationLocal Interstellar Cloud Local Bubble Orion Cygnus Arm Milky WaySystem mass1 0014 solar masses citation needed Nearest starProxima Centauri 4 2441 ly Alpha Centauri 4 37 ly Nearest known planetary systemProxima Centauri system 4 2441 ly Planetary systemSemi major axis of outer known planet Neptune 30 11 AU 4 5 bill km 2 8 bill mi Distance to Kuiper cliff 50 AUPopulationsStars1 Sun Known planets8 declared by IAU MercuryVenusEarthMarsJupiterSaturnUranusNeptuneKnown dwarf planets9 by general consensus CeresOrcusPlutoHaumeaQuaoarMakemakeGonggongErisSednaKnown natural satellites657 206 planetary451 minor planetary 1 Known minor planets1 199 224 b 2 Known comets4 402 b 2 Identified rounded satellites19Orbit about Galactic CenterInvariable to galactic plane inclination60 19 ecliptic Distance to Galactic Center27 000 1 000 lyOrbital speed220 km s 136 mi sOrbital period225 250 myrStar related propertiesSpectral typeG2VFrost line 5 AU 3 Distance to heliopause 120 AUHill sphere radius 1 3 lyThere are an unknown number of smaller dwarf planets and innumerable small Solar System bodies orbiting the Sun d Six of the major planets the six largest possible dwarf planets and many of the smaller bodies are orbited by natural satellites commonly called moons after Earth s Moon Two natural satellites Jupiter s moon Ganymede and Saturn s moon Titan are larger than Mercury the smallest terrestrial planet though less massive and Jupiter s moon Callisto is nearly as large Each of the giant planets and some smaller bodies are encircled by planetary rings of ice dust and moonlets The asteroid belt which lies between the orbits of Mars and Jupiter contains objects composed of rock metal and ice Beyond Neptune s orbit lie the Kuiper belt and scattered disc which are populations of objects composed mostly of ice and rock In the outer reaches of the Solar System lies a class of minor planets called detached objects There is considerable debate as to how many such objects there will prove to be 9 Some of these objects are large enough to have rounded under their own gravity and thus to be categorized as dwarf planets Astronomers generally accept about nine objects as dwarf planets the asteroid Ceres the Kuiper belt objects Pluto Orcus Haumea Quaoar and Makemake and the scattered disc objects Gonggong Eris and Sedna d Various small body populations including comets centaurs and interplanetary dust clouds freely travel between the regions of the Solar System The solar wind a stream of charged particles flowing outwards from the Sun creates a bubble like region of interplanetary medium in the interstellar medium known as the heliosphere The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium it extends out to the edge of the scattered disc The Oort cloud which is thought to be the source for long period comets may also exist at a distance roughly a thousand times further than the heliosphere The Solar System is located 26 000 light years from the center of the Milky Way galaxy in the Orion Arm which contains most of the visible stars in the night sky The nearest stars are within the so called Local Bubble with the closest Proxima Centauri at 4 2441 light years Contents 1 Formation and evolution 2 Structure and composition 2 1 Orbits 2 2 Composition 2 3 Distances and scales 3 Sun 4 Environment and habitability 5 Inner Solar System 5 1 Inner planets 5 1 1 Mercury 5 1 2 Venus 5 1 3 Earth 5 1 4 Mars 5 2 Asteroid belt 5 2 1 Ceres 5 2 2 Pallas and Vesta 5 2 3 Asteroid groups 6 Outer Solar System 6 1 Outer planets 6 1 1 Jupiter 6 1 2 Saturn 6 1 3 Uranus 6 1 4 Neptune 6 2 Centaurs 7 Comets 8 Trans Neptunian region 8 1 Kuiper belt 8 1 1 Pluto and Charon 8 1 2 Others 8 2 Scattered disc 8 2 1 Eris and Gonggong 9 Farthest regions 9 1 Edge of the heliosphere 9 2 Detached objects 9 3 Oort cloud 9 4 Boundaries 10 Galactic context 10 1 Celestial neighbourhood 10 2 Comparison with extrasolar systems 11 Humanity s perspective 12 See also 13 Notes 14 References 15 External linksFormation and evolutionMain article Formation and evolution of the Solar System Artist s impression of the early Solar System s protoplanetary disk out of which Earth and other Solar System bodies formed The Solar System formed 4 568 billion years ago from the gravitational collapse of a region within a large molecular cloud e This initial cloud was likely several light years across and probably birthed several stars 11 As is typical of molecular clouds this one consisted mostly of hydrogen with some helium and small amounts of heavier elements fused by previous generations of stars As the region that would become the Solar System known as the pre solar nebula 12 collapsed conservation of angular momentum caused it to rotate faster The centre where most of the mass collected became increasingly hotter than the surrounding disc 11 As the contracting nebula rotated faster it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU 30 billion km 19 billion mi 11 and a hot dense protostar at the centre 13 14 The planets formed by accretion from this disc 15 in which dust and gas gravitationally attracted each other coalescing to form ever larger bodies Hundreds of protoplanets may have existed in the early Solar System but they either merged or were destroyed or ejected leaving the planets dwarf planets and leftover minor bodies 16 17 Due to their higher boiling points only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun and these would eventually form the rocky planets of Mercury Venus Earth and Mars Because metallic elements only comprised a very small fraction of the solar nebula the terrestrial planets could not grow very large The giant planets Jupiter Saturn Uranus and Neptune formed further out beyond the frost line the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets allowing them to grow massive enough to capture large atmospheres of hydrogen and helium the lightest and most abundant elements Leftover debris that never became planets congregated in regions such as the asteroid belt Kuiper belt and Oort cloud 16 The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions 18 The Helix Nebula a planetary nebula similar to what the Sun will create when it enters its white dwarf stage Within 50 million years the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion 19 The temperature reaction rate pressure and density increased until hydrostatic equilibrium was achieved the thermal pressure counterbalancing the force of gravity At this point the Sun became a main sequence star 20 The main sequence phase from beginning to end will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun s pre remnant life combined 21 Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space As helium accumulates at its core the Sun is growing brighter 22 early in its main sequence life its brightness was 70 that of what it is today 23 The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium which will occur roughly 5 billion years from now This will mark the end of the Sun s main sequence life At that time the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium and the energy output will be greater than at present The outer layers of the Sun will expand to roughly 260 times its current diameter and the Sun will become a red giant Because of its increased surface area the surface of the Sun will be cooler 2 600 K 2 330 C 4 220 F at its coolest than it is on the main sequence 21 The expanding Sun is expected to vaporize Mercury as well as Venus and render Earth uninhabitable possibly destroying it as well Eventually the core will be hot enough for helium fusion the Sun will burn helium for a fraction of the time it burned hydrogen in the core The Sun is not massive enough to commence the fusion of heavier elements and nuclear reactions in the core will dwindle Its outer layers will be ejected into space leaving behind a dense white dwarf half the original mass of the Sun but only the size of Earth 24 The ejected outer layers will form what is known as a planetary nebula returning some of the material that formed the Sun but now enriched with heavier elements like carbon to the interstellar medium 25 Structure and compositionFurther information List of Solar System objects and Planet Planetary attributes The word solar means pertaining to the Sun which is derived from the Latin word sol meaning Sun 26 The Sun is the dominant gravitational member of the Solar System and its planetary system is maintained in a relatively stable slowly evolving state by following isolated gravitationally bound orbits around the Sun 27 Orbits Animations of the Solar System s inner planets and outer planets orbiting the latter animation is 100 times faster than the former Jupiter is three times as far from the Sun as Mars The planets and other large objects in orbit around the Sun lie near the plane of Earth s orbit known as the ecliptic Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane 28 29 Most of the planets in the Solar System have secondary systems of their own being orbited by natural satellites called moons Many of the largest natural satellites are in synchronous rotation with one face permanently turned toward their parent The four giant planets have planetary rings thin bands of tiny particles that orbit them in unison 30 As a result of the formation of the Solar System planets and most other objects orbit the Sun in the same direction that the Sun is rotating That is counter clockwise as viewed from above Earth s north pole 31 There are exceptions such as Halley s Comet 32 Most of the larger moons orbit their planets in prograde direction matching the planetary rotation Neptune s moon Triton is the largest to orbit in the opposite retrograde manner 33 Most larger objects rotate around their own axes in the prograde direction relative to their orbit though the rotation of Venus is retrograde 34 To a good first approximation Kepler s laws of planetary motion describe the orbits of objects about the Sun 35 433 437 These laws stipulate that each object travels along an ellipse with the Sun at one focus which causes the body s distance from the Sun to vary over the course of its year A body s closest approach to the Sun is called its perihelion whereas its most distant point from the Sun is called its aphelion 36 9 6 The orbits of the planets are nearly circular but many comets asteroids and Kuiper belt objects follow highly elliptical orbits Kepler s laws only account for the influence of the Sun s gravity upon an orbiting body not the gravitational pulls of different bodies upon each other On a human time scale these additional perturbations can be accounted for using numerical models 36 9 6 but the planetary system can change chaotically over billions of years 37 The angular momentum of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components 38 Although the Sun dominates the system by mass it accounts for only about 2 of the angular momentum 39 40 The planets dominated by Jupiter account for most of the rest of the angular momentum due to the combination of their mass orbit and distance from the Sun with a possibly significant contribution from comets 39 The orientation of the Solar System s motion Composition The overall structure of the charted regions of the Solar System consists of the Sun four smaller inner planets surrounded by a belt of mostly rocky asteroids and four giant planets surrounded by the Kuiper belt of mostly icy objects Astronomers sometimes informally divide this structure into separate regions The inner Solar System includes the four terrestrial planets and the asteroid belt The outer Solar System is beyond the asteroids including the four giant planets 41 Since the discovery of the Kuiper belt the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune 42 The principal component of the Solar System is the Sun a low mass star that contains 99 86 of the system s known mass and dominates it gravitationally 43 The Sun s four largest orbiting bodies the giant planets account for 99 of the remaining mass with Jupiter and Saturn together comprising more than 90 The remaining objects of the Solar System including the four terrestrial planets the dwarf planets moons asteroids and comets together comprise less than 0 002 of the Solar System s total mass f The Sun is composed of roughly 98 hydrogen and helium 47 as are Jupiter and Saturn 48 49 A composition gradient exists in the Solar System created by heat and light pressure from the early Sun those objects closer to the Sun which are more affected by heat and light pressure are composed of elements with high melting points Objects farther from the Sun are composed largely of materials with lower melting points 50 The boundary in the Solar System beyond which those volatile substances could coalesce is known as the frost line and it lies at roughly five times the Earth s distance from the Sun 3 The objects of the inner Solar System are composed mostly of rocky materials 51 such as silicates iron or nickel 52 Jupiter and Saturn are composed mainly of gases with extremely low melting points and high vapour pressure such as hydrogen helium and neon 52 Ices like water methane ammonia hydrogen sulfide and carbon dioxide 51 have melting points up to a few hundred kelvins 52 They can be found as ices liquids or gases in various places in the Solar System 52 Icy substances comprise the majority of the satellites of the giant planets as well as most of Uranus and Neptune the so called ice giants and the numerous small objects that lie beyond Neptune s orbit 51 53 Together gases and ices are referred to as volatiles 54 Distances and scales The Sun s planets dwarf planets and moons size to scale labelled Distance of objects is not to scale To scale diagram of distance between planets with the white bar showing orbital variations The size of the planets is not to scale The astronomical unit AU 150 000 000 km 93 000 000 mi would be the distance from the Earth to the Sun if the planet s orbit were perfectly circular 55 For comparison the radius of the Sun is 0 0047 AU 700 000 km 400 000 mi 56 Thus the Sun occupies 0 00001 10 5 of the volume of a sphere with a radius the size of Earth s orbit whereas Earth s volume is roughly one millionth 10 6 that of the Sun Jupiter the largest planet is 5 2 astronomical units 780 000 000 km 480 000 000 mi from the Sun and has a radius of 71 000 km 0 00047 AU 44 000 mi whereas the most distant planet Neptune is 30 AU 4 5 109 km 2 8 109 mi from the Sun 49 57 With a few exceptions the farther a planet or belt is from the Sun the larger the distance between its orbit and the orbit of the next nearer object to the Sun For example Venus is approximately 0 33 AU farther out from the Sun than Mercury whereas Saturn is 4 3 AU out from Jupiter and Neptune lies 10 5 AU out from Uranus Attempts have been made to determine a relationship between these orbital distances like the Titius Bode law 58 and Johannes Kepler s model based on the Platonic solids 59 but ongoing discoveries have invalidated these hypotheses 60 Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms Some are small in scale and may be mechanical called orreries whereas others extend across cities or regional areas 61 The largest such scale model the Sweden Solar System uses the 110 metre 361 ft Avicii Arena in Stockholm as its substitute Sun and following the scale Jupiter is a 7 5 metre 25 foot sphere at Stockholm Arlanda Airport 40 km 25 mi away whereas the farthest current object Sedna is a 10 cm 4 in sphere in Lulea 912 km 567 mi away 62 63 If the Sun Neptune distance is scaled to 100 metres 330 ft then the Sun would be about 3 cm 1 2 in in diameter roughly two thirds the diameter of a golf ball the giant planets would be all smaller than about 3 mm 0 12 in and Earth s diameter along with that of the other terrestrial planets would be smaller than a flea 0 3 mm or 0 012 in at this scale 64 SunMain article Sun The Sun in true white color The Sun is the Solar System s star and by far its most massive component Its large mass 332 900 Earth masses 65 which comprises 99 86 of all the mass in the Solar System 66 produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium 67 This releases an enormous amount of energy mostly radiated into space as electromagnetic radiation peaking in visible light 68 69 Because the Sun fuses hydrogen into helium at its core it is a main sequence star More specifically it is a G2 type main sequence star where the type designation refers to its effective temperature Hotter main sequence stars are more luminous The Sun s temperature is intermediate between that of the hottest stars and that of the coolest stars Stars brighter and hotter than the Sun are rare whereas substantially dimmer and cooler stars known as red dwarfs make up about 75 of the stars in the Milky Way 70 71 The Sun is a population I star it has a higher abundance of elements heavier than hydrogen and helium metals in astronomical parlance than the older population II stars 72 Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars so the first generation of stars had to die before the universe could be enriched with these atoms The oldest stars contain few metals whereas stars born later have more This higher metallicity is thought to have been crucial to the Sun s development of a planetary system because the planets form from the accretion of metals 73 Environment and habitability The zodiacal light caused by interplanetary dust Outside of the main part of the Sun s atmosphere extends the heliosphere and dominates the Solar planetary system The vast majority of the heliosphere is occupied by a near vacuum known as the interplanetary medium Along with light the Sun radiates a continuous stream of charged particles a plasma called the solar wind This stream of particles spreads outwards at speeds from 900 000 kilometres per hour 560 000 mph to 2 880 000 kilometres per hour 1 790 000 mph 74 creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU 15 billion km 9 3 billion mi see Heliosphere 75 Activity on the Sun s surface such as solar flares and coronal mass ejections disturbs the heliosphere creating space weather and causing geomagnetic storms 76 The largest structure within the heliosphere is the heliospheric current sheet a spiral form created by the actions of the Sun s rotating magnetic field on the interplanetary medium 77 78 Earth s magnetic field stops its atmosphere from being stripped away by the solar wind 79 Venus and Mars do not have magnetic fields and as a result the solar wind is causing their atmospheres to gradually bleed away into space 80 Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun The interaction of this magnetic field and material with Earth s magnetic field funnels charged particles into Earth s upper atmosphere where its interactions create aurorae seen near the magnetic poles 81 The heliosphere and planetary magnetic fields for those planets that have them partially shield the Solar System from high energy interstellar particles called cosmic rays The density of cosmic rays in the interstellar medium and the strength of the Sun s magnetic field change on very long timescales so the level of cosmic ray penetration in the Solar System varies though by how much is unknown 82 The interplanetary medium is home to at least two disc like regions of cosmic dust The first the zodiacal dust cloud lies in the inner Solar System and causes the zodiacal light It may have been formed by collisions within the asteroid belt brought on by gravitational interactions with the planets a more recent proposed origin is the planet Mars 83 The second dust cloud extends from about 10 AU 1 5 billion km 930 million mi to about 40 AU 6 0 billion km 3 7 billion mi and was probably created by collisions within the Kuiper belt 84 85 The zone of hability of the Solar System is located in the Inner Solar System Beside the Solar conditions for hability on Solar System objects such as Earth habitability might be possibly in subsurface oceans of various Outer Solar System moons Inner Solar System Overview of the Inner Solar System up to the Jovian System The inner Solar System is the region comprising the terrestrial planets and the asteroid belt 86 Composed mainly of silicates and metals 87 the objects of the inner Solar System are relatively close to the Sun the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn This region is also within the frost line which is a little less than 5 AU 750 million km 460 million mi from the Sun 28 Inner planets Main article Terrestrial planet The four terrestrial planets Mercury Venus Earth and Mars The four terrestrial or inner planets have dense rocky compositions few or no moons and no ring systems They are composed largely of refractory minerals such as the silicates which form their crusts and mantles and metals such as iron and nickel which form their cores Three of the four inner planets Venus Earth and Mars have atmospheres substantial enough to generate weather all have impact craters and tectonic surface features such as rift valleys and volcanoes The term inner planet should not be confused with inferior planet which designates those planets that are closer to the Sun than Earth is i e Mercury and Venus 88 Mercury Main article Mercury planet Mercury 0 307 0 588 AU 45 9 88 0 million km 28 5 54 7 million mi from the Sun 89 is the closest planet to the Sun The smallest planet in the Solar System 0 055 MEarth Mercury has no natural satellites The dominant geological features are impact craters or basins with ejecta blankets the remains of early volcanic activity including magma flows and lobed ridges or rupes that were probably produced by a period of contraction early in the planet s history 90 Mercury s very tenuous atmosphere consists of solar wind particles trapped by Mercury s magnetic field as well as atoms blasted off its surface by the solar wind 91 92 Its relatively large iron core and thin mantle have not yet been adequately explained Hypotheses include that its outer layers were stripped off by a giant impact or that it was prevented from fully accreting by the young Sun s energy 93 94 There have been searches for Vulcanoids asteroids in stable orbits between Mercury and the Sun but none have been discovered 95 96 Venus Main article Venus Venus 0 718 0 728 AU 107 4 108 9 million km 66 7 67 7 million mi from the Sun 89 is close in size to Earth 0 815 MEarth and like Earth has a thick silicate mantle around an iron core a substantial atmosphere and evidence of internal geological activity It is much drier than Earth and its atmosphere is ninety times as dense Venus has no natural satellites It is the hottest planet with surface temperatures over 400 C 752 F mainly due to the amount of greenhouse gases in the atmosphere 97 The planet has no magnetic field that would prevent depletion of its substantial atmosphere which suggests that its atmosphere is being replenished by volcanic eruptions 98 A relatively young planetary surface displays extensive evidence of volcanic activity but is devoid of plate tectonics It may undergo resurfacing episodes on a time scale of 700 million years 99 Earth Main article Earth Earth 0 983 1 017 AU 147 1 152 1 million km 91 4 94 5 million mi from the Sun is the largest and densest of the inner planets the only one known to have current geological activity and the only place where life is known to exist 100 Its liquid hydrosphere is unique among the terrestrial planets and it is the only planet where plate tectonics has been observed 101 Earth s atmosphere is radically different from those of the other planets having been altered by the presence of life to contain 21 free oxygen 102 103 The planetary magnetosphere shields the surface from solar and cosmic radiation limiting atmospheric stripping and maintaining habitability 104 It has one natural satellite the Moon the only large satellite of a terrestrial planet in the Solar System Mars Main article Mars Mars 1 382 1 666 AU 206 7 249 2 million km 128 5 154 9 million mi from the Sun is smaller than Earth and Venus 0 107 MEarth It has an atmosphere of mostly carbon dioxide with a surface pressure of 6 1 millibars 0 088 psi 0 18 inHg roughly 0 6 of that of Earth but sufficient to support weather phenomena 105 Its surface peppered with volcanoes such as Olympus Mons and rift valleys such as Valles Marineris shows geological activity that may have persisted until as recently as 2 million years ago 106 Its red colour comes from iron oxide rust in its soil 107 Mars has two tiny natural satellites Deimos and Phobos thought to be either captured asteroids 108 or ejected debris from a massive impact early in Mars s history 109 Asteroid belt Main articles Asteroid belt and Asteroid Linear map of the inner Solar System showing many asteroid populations Asteroids except for the largest Ceres are classified as small Solar System bodies d and are composed mainly of refractory rocky and metallic minerals with some ice 110 111 They range from a few metres to hundreds of kilometres in size Asteroids smaller than one meter are usually called meteoroids and micrometeoroids grain sized with the exact division between the two categories being debated over the years 112 As of 2017 the IAU designates asteroids having diameter between about 30 micrometres and 1 metre as micrometeroids and terms smaller particles dust 113 The asteroid belt occupies the orbit between Mars and Jupiter between 2 3 and 3 3 AU 340 and 490 million km 210 and 310 million mi from the Sun It is thought to be remnants from the Solar System s formation that failed to coalesce because of the gravitational interference of Jupiter 114 The asteroid belt contains tens of thousands possibly millions of objects over one kilometre in diameter 115 Despite this the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth 46 The asteroid belt is very sparsely populated spacecraft routinely pass through without incident 116 Ceres Main article Ceres dwarf planet Ceres 2 77 AU 414 million km 257 million mi from the Sun is the largest asteroid a protoplanet and a dwarf planet d It has a diameter of slightly under 1 000 km 620 mi and a mass large enough for its own gravity to pull it into a spherical shape Ceres was considered a planet when it was discovered in 1801 but as further observations revealed additional asteroids it became common to consider it as one of the minor rather than major planets 117 It was then reclassified again as a dwarf planet in 2006 when the IAU definition of planet was established 118 218 Pallas and Vesta Main articles 2 Pallas and 4 Vesta Pallas 2 77 AU from the Sun and Vesta 2 36 AU from the Sun are the largest asteroids in the asteroid belt after Ceres They are the other two protoplanets that survive more or less intact At about 520 km 320 mi in diameter they were large enough to have developed planetary geology in the past but both have suffered large impacts and been battered out of being round 119 120 121 Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt as the Pallas family and Vesta family Both were considered planets upon their discoveries in 1802 and 1807 respectively and then like Ceres generally considered as minor planets with the discovery of more asteroids Some authors today have begun to consider Pallas and Vesta as planets again along with Ceres under geophysical definitions of the term 5 Asteroid groups Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics Kirkwood gaps are sharp dips in the distribution of asteroid orbits that correspond to orbital resonances with Jupiter 122 Asteroid moons are asteroids that orbit larger asteroids They are not as clearly distinguished as planetary moons sometimes being almost as large as their partners e g that of 90 Antiope The asteroid belt includes main belt comets which may have been the source of Earth s water 123 Jupiter trojans are located in either of Jupiter s L4 or L5 points gravitationally stable regions leading and trailing a planet in its orbit the term trojan is also used for small bodies in any other planetary or satellite Lagrange point Hilda asteroids are in a 2 3 resonance with Jupiter that is they go around the Sun three times for every two Jupiter orbits 124 The inner Solar System contains near Earth asteroids many of which cross the orbits of the inner planets 125 Some of them are potentially hazardous objects 126 Outer Solar System Plot of objects around the Kuiper belt and other asteroid populations the J S U and N denotes Jupiter Saturn Uranus and Neptune The outer region of the Solar System is home to the giant planets and their large moons The centaurs and many short period comets also orbit in this region Due to their greater distance from the Sun the solid objects in the outer Solar System contain a higher proportion of volatiles such as water ammonia and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid 16 Outer planets Main article Giant planet The outer planets Jupiter Saturn Uranus and Neptune compared to the inner planets Earth Venus Mars and Mercury at the bottom right The four outer planets also called giant planets or Jovian planets collectively make up 99 of the mass known to orbit the Sun f Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium hence their designation as gas giants 127 Uranus and Neptune are far less massive less than 20 Earth masses MEarth each and are composed primarily of ices For these reasons some astronomers suggest they belong in their own category ice giants 128 All four giant planets have rings although only Saturn s ring system is easily observed from Earth The term superior planet designates planets outside Earth s orbit and thus includes both the outer planets and Mars 88 The ring moon systems of Jupiter Saturn and Uranus are like miniature versions of the Solar System that of Neptune is significantly different having been disrupted by the capture of its largest moon Triton 129 Jupiter Main article Jupiter Jupiter 4 951 5 457 AU 740 7 816 4 million km 460 2 507 3 million mi from the Sun 89 at 318 MEarth is 2 5 times the mass of all the other planets put together It is composed largely of hydrogen and helium Jupiter s strong internal heat creates semi permanent features in its atmosphere such as cloud bands and the Great Red Spot The planet possesses a 4 2 14 Gauss strength magnetosphere that spans 22 29 million km making it in certain respects the largest object in the Solar System 130 Jupiter has 80 known satellites The four largest Ganymede Callisto Io and Europa are called the Galilean moons they show similarities to the terrestrial planets such as volcanism and internal heating 131 Ganymede the largest satellite in the Solar System is larger than Mercury Callisto is almost as large 132 Saturn Main article Saturn Saturn 9 075 10 07 AU 1 3576 1 5065 billion km 843 6 936 1 million mi from the Sun 89 distinguished by its extensive ring system has several similarities to Jupiter such as its atmospheric composition and magnetosphere Although Saturn has 60 of Jupiter s volume it is less than a third as massive at 95 MEarth Saturn is the only planet of the Solar System that is less dense than water The rings of Saturn are made up of small ice and rock particles 133 Saturn has 83 confirmed satellites composed largely of ice Two of these Titan and Enceladus show signs of geological activity 134 they as well as five other Saturnian moons Iapetus Rhea Dione Tethys and Mimas are large enough to be round Titan the second largest moon in the Solar System is bigger than Mercury and the only satellite in the Solar System to have a substantial atmosphere 135 136 Uranus Main article UranusUranus 18 27 20 06 AU 2 733 3 001 billion km 1 698 1 865 billion mi from the Sun 89 at 14 MEarth has the lowest mass of the outer planets Uniquely among the planets it orbits the Sun on its side its axial tilt is over ninety degrees to the ecliptic This gives the planet extreme seasonal variation as each pole points toward and then away from the Sun 137 It has a much colder core than the other giant planets and radiates very little heat into space 138 As a consequence it has the coldest planetary atmosphere in the Solar System 139 Uranus has 27 known satellites the largest ones being Titania Oberon Umbriel Ariel and Miranda 140 Like the other giant planets it possesses a ring system and magnetosphere 141 Neptune Main article Neptune Neptune 29 89 30 47 AU 4 471 4 558 billion km 2 778 2 832 billion mi from the Sun 89 though slightly smaller than Uranus is more massive 17 MEarth and hence more dense It radiates more internal heat than Uranus but not as much as Jupiter or Saturn 142 Neptune has 14 known satellites The largest Triton is geologically active with geysers of liquid nitrogen 143 Triton is the only large satellite with a retrograde orbit which indicates that it did not form with Neptune but was probably captured from the Kuiper belt 144 Neptune is accompanied in its orbit by several minor planets termed Neptune trojans that either lead or trail the planet by about one sixth of the way around the Sun positions known as Lagrange points 145 Centaurs Main article Centaur small Solar System body The centaurs are icy comet like bodies whose orbits have semi major axes greater than Jupiter s 5 5 AU 820 million km 510 million mi and less than Neptune s 30 AU 4 5 billion km 2 8 billion mi The largest known centaur 10199 Chariklo has a diameter of about 250 km 160 mi 146 The first centaur discovered 2060 Chiron has also been classified as a comet 95P because it develops a coma just as comets do when they approach the Sun 147 CometsMain article Comet Comet Hale Bopp seen in 1997 Comets are small Solar System bodies d typically only a few kilometres across composed largely of volatile ices They have highly eccentric orbits generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto When a comet enters the inner Solar System its proximity to the Sun causes its icy surface to sublimate and ionise creating a coma a long tail of gas and dust often visible to the naked eye 148 Short period comets have orbits lasting less than two hundred years Long period comets have orbits lasting thousands of years Short period comets are thought to originate in the Kuiper belt whereas long period comets such as Hale Bopp are thought to originate in the Oort cloud Many comet groups such as the Kreutz sungrazers formed from the breakup of a single parent 149 Some comets with hyperbolic orbits may originate outside the Solar System but determining their precise orbits is difficult 150 Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids 151 Trans Neptunian region Distribution and size of trans Neptunian objects Size comparison of some large TNOs with Earth Pluto and its moons Eris Makemake Haumea Sedna Gonggong Quaoar Orcus Salacia and 2002 MS4 Inside the orbit of Neptune is the planetary region of the Solar System Beyond the orbit of Neptune lies the area of the trans Neptunian region with the doughnut shaped Kuiper belt home of Pluto and several other dwarf planets and an overlapping disc of scattered objects which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt The entire region is still largely unexplored It appears to consist overwhelmingly of many thousands of small worlds the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon composed mainly of rock and ice This region is sometimes described as the third zone of the Solar System enclosing the inner and the outer Solar System 152 Kuiper belt Main article Kuiper beltThe Kuiper belt is a great ring of debris similar to the asteroid belt but consisting mainly of objects composed primarily of ice 153 It extends between 30 and 50 AU 4 5 and 7 5 billion km 2 8 and 4 6 billion mi from the Sun It is composed mainly of small Solar System bodies although the largest few are probably large enough to be dwarf planets 9 There are estimated to be over 100 000 Kuiper belt objects with a diameter greater than 50 km 30 mi but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth 45 Many Kuiper belt objects have multiple satellites 154 and most have orbits that take them outside the plane of the ecliptic 155 The Kuiper belt can be roughly divided into the classical belt and the resonant trans Neptunian objects 153 The latter have orbits whose periods are in a simple ratio to that of Neptune for example going around the Sun twice for every three times that Neptune does or once for every two The classical belt consists of objects having no resonance with Neptune and extends from roughly 39 4 to 47 7 AU 5 89 to 7 14 billion km 3 66 to 4 43 billion mi 156 Members of the classical Kuiper belt are sometimes called cubewanos after the first of their kind to be discovered originally designated 1992 QB1 they are still in near primordial low eccentricity orbits 157 Pluto and Charon Main articles Pluto and Charon moon The dwarf planet Pluto with an average orbit of 39 AU 5 8 billion km 3 6 billion mi from the Sun is the largest known object in the Kuiper belt When discovered in 1930 it was considered to be the ninth planet this changed in 2006 with the adoption of a formal definition of planet Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29 7 AU 4 44 billion km 2 76 billion mi from the Sun at perihelion within the orbit of Neptune to 49 5 AU 7 41 billion km 4 60 billion mi at aphelion Pluto has a 2 3 resonance with Neptune meaning that Pluto orbits twice round the Sun for every three Neptunian orbits Kuiper belt objects whose orbits share this resonance are called plutinos 158 Charon the largest of Pluto s moons is sometimes described as part of a binary system with Pluto as the two bodies orbit a barycentre of gravity above their surfaces i e they appear to orbit each other Beyond Charon four much smaller moons Styx Nix Kerberos and Hydra orbit Pluto 159 Others Besides Pluto astronomers generally agree that at least four other Kuiper belt objects are dwarf planets 9 and additional bodies have also been proposed 160 Makemake 45 79 AU average from the Sun although smaller than Pluto is the largest known object in the classical Kuiper belt that is a Kuiper belt object not in a confirmed resonance with Neptune Makemake is the brightest object in the Kuiper belt after Pluto Discovered in 2005 it was officially named in 2009 161 Its orbit is far more inclined than Pluto s at 29 162 It has one known moon 163 Haumea 43 13 AU average from the Sun is in an orbit similar to Makemake except that it is in a temporary 7 12 orbital resonance with Neptune 164 Like Makemake it was discovered in 2005 165 It has two known moons Hiʻiaka and Namaka and rotates so quickly once every 3 9 hours that it is stretched into an ellipsoid 166 Quaoar 43 69 AU average from the Sun is the second largest known object in the classical Kuiper belt after Makemake Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea 164 It has one known moon Weywot 167 Orcus 39 40 AU average from the Sun is in the same 2 3 orbital resonance with Neptune as Pluto and is the largest such object after Pluto itself 164 Its eccentricity and inclination are similar to Pluto s but its perihelion lies about 120 from that of Pluto Thus the phase of Orcus s orbit is opposite to Pluto s Orcus is at aphelion most recently in 2019 around when Pluto is at perihelion most recently in 1989 and vice versa 168 For this reason it has been called the anti Pluto 169 170 It has one known moon Vanth 171 Scattered disc Main article Scattered disc The scattered disc object Sedna and its orbit within the Solar System The scattered disc which overlaps the Kuiper belt but extends out to near 500 AU is thought to be the source of short period comets Scattered disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune s early outward migration Most scattered disc objects SDOs have perihelia within the Kuiper belt but aphelia far beyond it some more than 150 AU from the Sun SDOs orbits can also be inclined up to 46 8 from the ecliptic plane 172 Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered disc objects as scattered Kuiper belt objects 173 Some astronomers also classify centaurs as inward scattered Kuiper belt objects along with the outward scattered residents of the scattered disc 174 Eris and Gonggong Eris 67 78 AU average from the Sun is the largest known scattered disc object and caused a debate about what constitutes a planet because it is 25 more massive than Pluto 175 and about the same diameter It is the most massive of the known dwarf planets It has one known moon Dysnomia Like Pluto its orbit is highly eccentric with a perihelion of 38 2 AU roughly Pluto s distance from the Sun and an aphelion of 97 6 AU and steeply inclined to the ecliptic plane at an angle of 44 176 Gonggong 67 38 AU average from the Sun is in a comparable orbit to Eris except that it is in a 3 10 resonance with Neptune 177 It has one known moon Xiangliu 178 Farthest regionsThe point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces the solar wind and the Sun s gravity The limit of the solar wind s influence is roughly four times Pluto s distance from the Sun this heliopause the outer boundary of the heliosphere is considered the beginning of the interstellar medium 75 The Sun s Hill sphere the effective range of its gravitational dominance is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud 179 Edge of the heliosphere Main article Heliosheath Artistic depiction of the Solar System s heliosphere The Sun s stellar wind bubble the heliosphere a region of space dominated by the Sun has its boundary at the termination shock which is roughly 80 100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind 180 Here the solar wind collides with the interstellar medium 181 and dramatically slows condenses and becomes more turbulent 180 forming a great oval structure known as the heliosheath This structure has been theorized to look and behave very much like a comet s tail extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind 182 Evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field 183 184 but the actual shape remains unknown 185 The outer boundary of the heliosphere the heliopause is the point at which the solar wind finally terminates and is the beginning of interstellar space 75 Voyager 1 and Voyager 2 passed the termination shock and entered the heliosheath at 94 and 84 AU from the Sun respectively 186 187 Voyager 1 was reported to have crossed the heliopause in August 2012 and Voyager 2 in December 2018 188 189 The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south e g it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere 180 Beyond the heliopause at around 230 AU lies the bow shock a plasma wake left by the Sun as it travels through the Milky Way 190 Detached objects Main articles Detached object and SednoidSedna with an average orbit of 520 AU from the Sun is a large reddish object with a gigantic highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11 400 years to complete Mike Brown who discovered the object in 2003 asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune s migration He and other astronomers consider it to be the first in an entirely new population sometimes termed distant detached objects DDOs which also may include the object 2000 CR105 which has a perihelion of 45 AU an aphelion of 415 AU and an orbital period of 3 420 years 191 Brown terms this population the inner Oort cloud because it may have formed through a similar process although it is far closer to the Sun 192 Sedna is very likely a dwarf planet though its shape has yet to be determined The second unequivocally detached object with a perihelion farther than Sedna s at roughly 81 AU is 2012 VP113 discovered in 2012 Its aphelion is only about half that of Sedna s at 458 AU 193 194 Oort cloud Main article Oort cloud The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long period comets and to surround the Solar System at roughly 50 000 AU around 1 light year ly from the Sun and possibly to as far as 100 000 AU 1 87 ly It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets Oort cloud objects move very slowly and can be perturbed by infrequent events such as collisions the gravitational effects of a passing star or the galactic tide the tidal force exerted by the Milky Way 195 196 Boundaries See also Planets beyond Neptune Planet Nine and List of Solar System objects by greatest aphelion Much of the Solar System is still unknown The Sun s gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years 125 000 AU Lower estimates for the radius of the Oort cloud by contrast do not place it farther than 50 000 AU 197 Most of the mass is orbiting in the region between 3 000 and 100 000 AU 198 Despite discoveries such as Sedna the region between the Kuiper belt and the Oort cloud an area tens of thousands of AU in radius is still virtually unmapped Learning about this region of space is difficult because it depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun and even then detecting these objects has often been possible only when they happened to become bright enough to register as comets 199 Objects may yet be discovered in the Solar System s uncharted regions 200 The furthest known objects such as Comet West have aphelia around 70 000 AU from the Sun 201 Galactic contextSee also Location of Earth and Galactic year Diagram of the Milky Way with the position of the Solar System marked by a yellow arrow and a red dot in the Orion Arm the dot roughly covering the large surrounding celestial area dominated by the Radcliffe wave and Split linear structures formerly Gould Belt 202 The Solar System is located in the Milky Way a barred spiral galaxy with a diameter of about 100 000 light years containing more than 100 billion stars 203 The Sun resides in one of the Milky Way s outer spiral arms known as the Orion Cygnus Arm or Local Spur 204 The Sun lies about 26 660 light years from the Galactic Center 205 and its speed around the center of the Milky Way is about 220 km s so that it completes one revolution every 240 million years 203 This revolution is known as the Solar System s galactic year 206 The solar apex the direction of the Sun s path through interstellar space is near the constellation Hercules in the direction of the current location of the bright star Vega 207 The plane of the ecliptic lies at an angle of about 60 to the galactic plane g The Solar System s location in the Milky Way is a factor in the evolutionary history of life on Earth Its orbit is close to circular and orbits near the Sun are at roughly the same speed as that of the spiral arms 209 210 Therefore the Sun passes through arms only rarely Because spiral arms are home to a far larger concentration of supernovae gravitational instabilities and radiation that could disrupt the Solar System this has given Earth long periods of stability for life to evolve 209 However the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth according to the Shiva hypothesis or related theories but this remains controversial 211 212 The Solar System lies well outside the star crowded environs of the galactic centre Near the centre gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System producing collisions with potentially catastrophic implications for life on Earth The intense radiation of the galactic centre could also interfere with the development of complex life 209 Stellar flybys that pass within 0 8 light years of the Sun occur roughly once every 100 000 years The closest well measured approach was Scholz s Star which approached to 52 23 14 kAU of the Sun some 70 15 10 kya likely passing through the outer Oort cloud 213 Celestial neighbourhood Beyond the heliosphere is the interstellar medium consisting of various clouds of gases The Solar System currently moves through the Local Interstellar Cloud here shown along with neighbouring clouds and the two closest unaided visible stars The Solar System is surrounded by the Local Interstellar Cloud although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud s edge 214 215 Multiple other interstellar clouds also exist in the region within 300 light years of the Sun known as the Local Bubble 215 The latter feature is an hourglass shaped cavity or superbubble in the interstellar medium roughly 300 light years across The bubble is suffused with high temperature plasma suggesting that it may be the product of several recent supernovae 216 The Local Bubble is a small superbubble compared to the neighbouring wider Radcliffe Wave and Split linear structures formerly Gould Belt each of which are some thousands of light years in length 202 All these structures are part of the Orion Arm which contains most of the stars in the Milky Way that are visible to the unaided eye The density of all matter in the local neighborhood is 0 097 0 013 M pc 3 217 Within ten light years of the Sun there are relatively few stars the closest being the triple star system Alpha Centauri which is about 4 4 light years away and may be in the Local Bubble s G Cloud 218 Alpha Centauri A and B are a closely tied pair of Sun like stars whereas the closest star to Earth the small red dwarf Proxima Centauri orbits the pair at a distance of 0 2 light year In 2016 a potentially habitable exoplanet was found to be orbiting Proxima Centauri called Proxima Centauri b the closest confirmed exoplanet to the Sun 219 The next closest known fusors to the Sun are the red dwarfs Barnard s Star at 5 9 ly Wolf 359 7 8 ly and Lalande 21185 8 3 ly 220 The nearest brown dwarfs belong to the binary Luhman 16 system 6 6 ly and the closest known rogue or free floating planetary mass object at less than 10 Jupiter masses is the sub brown dwarf WISE 0855 0714 7 4 ly 221 Just beyond at 8 6 ly lies Sirius the brightest star in Earth s night sky with roughly twice the Sun s mass orbited by the closest white dwarf to Earth Sirius B Other stars within ten light years are the binary red dwarf system Luyten 726 8 8 7 ly and the solitary red dwarf Ross 154 9 7 ly 222 223 The closest solitary Sun like star to the Solar System is Tau Ceti at 11 9 light years It has roughly 80 of the Sun s mass but only about half of its luminosity 224 The nearest and unaided visible group of stars beyond the immediate celestial neighbourhood is the Ursa Major Moving Group at roughly 80 light years which is within the Local Bubble like the nearest as well as unaided visible star cluster the Hyades which lie at its edge The closest star forming regions are the Corona Australis Molecular Cloud the Rho Ophiuchi cloud complex and the Taurus molecular cloud the latter lies just beyond the Local Bubble and is part of the Radcliffe wave 225 Comparison with extrasolar systems Compared to many extrasolar systems the Solar System stands out in lacking planets interior to the orbit of Mercury 226 227 The known Solar System also lacks super Earths planets between one and ten times as massive as the Earth 226 although the hypothetical Planet Nine if it does exist could be a super Earth beyond the Solar System as we understand it today 228 Uncommonly it has only small rocky planets and large gas giants elsewhere planets of intermediate size are typical both rocky and gas so there is no gap as seen between the size of Earth and of Neptune with a radius 3 8 times as large As many of these super Earths are closer to their respective stars than Mercury is to the sun a hypothesis has arisen that all planetary systems start with many close in planets and that typically a sequence of their collisions causes consolidation of mass into few larger planets but in case of the Solar System the collisions caused their destruction and ejection 226 229 The orbits of Solar System planets are nearly circular Compared to other systems they have smaller orbital eccentricity 226 Although there are attempts to explain it partly with a bias in the radial velocity detection method and partly with long interactions of a quite high number of planets the exact causes remain undetermined 226 230 Humanity s perspectiveMain article Discovery and exploration of the Solar System Harrison H Schmitt an astronaut in the Apollo 17 mission with the Moon and Earth in the background Humanity s knowledge of the Solar System has grown incrementally over the centuries Up to the Late Middle Ages Renaissance astronomers from Europe to India believed Earth to be stationary at the centre of the Universe 231 and categorically different from the divine or ethereal objects that moved through the sky Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system 232 233 Heliocentrism did not triumph immediately over geocentrism but the work of Copernicus had its champions notably Johannes Kepler Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical as well as circular and the precise observational data of Tycho Brahe Kepler produced the Rudolphine Tables which enabled accurate computations of the positions of the then known planets Pierre Gassendi used them to predict a transit of Mercury in 1631 and Jeremiah Horrocks did the same for a transit of Venus in 1639 This provided a strong vindication of heliocentrism and Kepler s elliptical orbits 234 235 In the 17th century Galileo publicized the use of the telescope in astronomy he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it 236 Christiaan Huygens followed on from these observations by discovering Saturn s moon Titan and the shape of the rings of Saturn 237 In 1677 Edmond Halley observed a transit of Mercury across the Sun leading him to realise that observations of the solar parallax of a planet more ideally using the transit of Venus could be used to trigonometrically determine the distances between Earth Venus and the Sun 238 Halley s friend Isaac Newton in his magisterial Principia Mathematica of 1687 demonstrated that celestial bodies are not quintessentially different from Earthly ones the same laws of motion and of gravity apply on Earth and in the skies 35 142 The term Solar System entered the English language by 1704 when John Locke used it to refer to the Sun planets and comets 239 In 1705 Halley realised that repeated sightings of a comet were of the same object returning regularly once every 75 76 years This was the first evidence that anything other than the planets repeatedly orbited the Sun 240 though Seneca had theorized this about comets in the 1st century 241 Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth Sun distance as 93 726 900 miles 150 838 800 km only 0 8 greater than the modern value 242 Uranus having occasionally been observed since antiquity was recognized to be a planet orbiting beyond Saturn by 1783 243 In 1838 Friedrich Bessel successfully measured a stellar parallax an apparent shift in the position of a star created by Earth s motion around the Sun providing the first direct experimental proof of heliocentrism 244 Neptune was identified as a planet some years later in 1846 thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus 245 In the 20th century humans began their space exploration around the Solar System starting with placing telescopes in space 246 Since then humans have landed on the Moon during the Apollo program the Apollo 13 mission marked the furthest any human has been away from Earth at 400 171 kilometers 248 655 mi 247 All eight planets have been visited by space probes the outer planets were first visited by the Voyager spacecraft one of which Voyager 1 is the furthest object made by humankind and the first in interstellar space 248 In addition probes have also returned samples from comets 249 and asteroids 250 as well as flown through the Sun s corona 251 and made fly bys of Kuiper belt objects 252 Six of the planets all but Uranus and Neptune have or had a dedicated orbiter 253 See also solar system portal outer space portal astronomy portalList of gravitationally rounded objects of the Solar System List of Solar System objects by size Lists of geological features of the Solar System List of Solar System extremes Outline of the Solar SystemNotes The asteroid belt and Kuiper belt are not added because the individual asteroids are too small to be shown on the diagram a b As of 2 April 2022 Capitalization of the name varies The International Astronomical Union the authoritative body regarding astronomical nomenclature specifies capitalizing the names of all individual astronomical objects but uses mixed Solar System and solar system structures in their naming guidelines document Archived 25 July 2021 at the Wayback Machine The name is commonly rendered in lower case solar system as for example in the Oxford English Dictionary and Merriam Webster s 11th Collegiate Dictionary Archived 27 January 2008 at the Wayback Machine a b c d e According to IAU definitions objects orbiting the Sun are classified dynamically and physically into three categories planets dwarf planets and small Solar System bodies A planet is any body orbiting the Sun whose mass is sufficient for gravity to have pulled it into a near spherical shape and that has cleared its immediate neighbourhood of all smaller objects By this definition the Solar System has eight planets Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune Because it has not cleared its neighbourhood of other Kuiper belt objects Pluto does not fit this definition 4 A dwarf planet is a body orbiting the Sun that is massive enough to be made near spherical by its own gravity but that has not cleared planetesimals from its neighbourhood and is also not a satellite 4 Dwarf planets are considered planets by some planetologists but not by the IAU 5 The IAU has recognized four other bodies in the Solar System as dwarf planets Ceres Haumea Makemake and Eris 6 7 Other objects commonly accepted as dwarf planets include Gonggong Sedna Orcus and Quaoar In a reference to Pluto other dwarf planets orbiting in the trans Neptunian region are sometimes called plutoids 8 though this term is seldom used The remaining objects orbiting the Sun are known as small Solar System bodies 4 The date is based on the oldest inclusions found to date in meteorites 4568 2 0 2 0 4 million years and is thought to be the date of the formation of the first solid material in the collapsing nebula 10 a b The mass of the Solar System excluding the Sun Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud estimated at roughly 3 Earth masses 44 the Kuiper belt estimated at 0 1 Earth mass 45 and the asteroid belt estimated to be 0 0005 Earth mass 46 for a total rounded upwards of 37 Earth masses or 8 1 of the mass in orbit around the Sun With the combined masses of Uranus and Neptune 31 Earth masses subtracted the remaining 6 Earth masses of material comprise 1 3 of the total orbiting mass If ps displaystyle psi is the angle between the north pole of the ecliptic and the north galactic pole then cos ps cos b g cos b e cos a g a e sin b g sin b e displaystyle cos psi cos beta g cos beta e cos alpha g alpha e sin beta g sin beta e where b g displaystyle beta g 27 07 42 01 and a g displaystyle alpha g 12h 51m 26 282s are the declination and 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