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Solar System

January 01, 1970

For other uses, see Solar System (disambiguation).

The Solar System[b] is the gravitationally bound system of the Sun and the objects that orbit it.[9] It was formed 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is an ordinary main sequence star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere.

The Sun, planets, moons and dwarf planets. The asteroid belt and Kuiper belt are not added because the individual asteroids are too small to be shown on the diagram.
Solar System
Age4.568 billion years[a]
Location
Nearest star
Population
StarsSun
Planets
Known dwarf planets
Known natural satellites758[D 3]
Known minor planets1,358,412[D 4]
Known comets4,591[D 4]
Planetary system
Star spectral typeG2V
Frost line~5 AU[5]
Semi-major axis of outermost planet30.07 AU[D 5] (Neptune)
Kuiper cliff50–70 AU[3][4]
Heliopausedetected at 120 AU[6]
Hill sphere~1–3 ly[citation needed]
Orbit about Galactic Center
Invariable-to-galactic plane inclination60.19° (ecliptic)[citation needed]
Distance to
Galactic Center
24,000–28,000 ly
[7]
Orbital speed
720,000 km/h (450,000 mi/h)[8]
Orbital period~230 million years[8]

The largest objects that orbit the Sun are the eight planets. In order from the Sun, they are four terrestrial planets (Mercury, Venus, Earth and Mars); two gas giants (Jupiter and Saturn); and two ice giants (Uranus and Neptune). All terrestrial planets have solid surfaces. Inversely, all giant planets do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn.

There is a strong consensus among astronomers[c] that the Solar System has at least eight dwarf planets: Ceres, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna. There are a vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. Some of these bodies are in the asteroid belt (between Mars's and Jupiter's orbit) and the Kuiper belt (just outside Neptune's orbit).[d] Six planets, six dwarf planets, and other bodies have orbiting natural satellites, which are commonly called 'moons'.

The Solar System is constantly flooded by the Sun's charged particles, the solar wind, forming the heliosphere. Around 75–90 astronomical units, the solar wind is halted, resulting in the heliopause. This is the boundary of the Solar System to interstellar space. The outermost region of the Solar System is the theorized Oort cloud, the source for long-period comets, extending 2,000–200,000 astronomical units (0.032–3.2 light-years). The closest star to the Solar System, Proxima Centauri, is 4.25 light-years away. Both stars belong to the Milky Way galaxy.

Formation and evolution

Main article: Formation and evolution of the Solar System

Past

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[a] 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.[12]

As the pre-solar nebula[12] collapsed, conservation of angular momentum caused it to rotate faster. The center, 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 center.[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]

 
Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

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 (within the frost line). They 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.[16]

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.[16] Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[16]

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion.[18] As helium accumulates at its core the Sun is growing brighter;[19] early in its main-sequence life its brightness was 70% that of what it is today.[20] 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.[21]

Present and future

 
The current Sun compared to its peak size in the red-giant phase

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 subsequent phases of the Sun's pre-remnant life combined.[22] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.[19]

The Solar System is in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.[23] Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.[24]

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.[22]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well).[25] 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.[22] The ejected outer layers may form a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.[26][27]

General characteristics

Astronomers sometimes divide the Solar System structure into searate regions. The inner Solar System includes the Mercury, Venus, Earth, Mars and bodies in the asteroid belt. The outer Solar System includes the Jupiter, Saturn, Uranus, Neptune and bodies in the Kuiper belt.[28] 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.[29]

Composition

Further information: List of Solar System objects and List of interstellar and circumstellar molecules

The principal component of the Solar System is the Sun, a low-mass star[e] that contains 99.86% of the system's known mass and dominates it gravitationally.[31] 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,[35] as are Jupiter and Saturn.[36][37] 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.[38] 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.[5]

Orbits

 
Animations of the Solar System's inner planets orbiting. Each frame represents 2 days of motion.
 
Animations of the Solar System's outer planets orbiting. This animation is 100 times faster than the inner planet animation.

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.[39][40] 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.[41]

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.[42] There are exceptions, such as Halley's Comet.[43] 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.[44] Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.[45]

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects around the Sun.[46]: 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.[47]: 9-6  With the exception of Mercury, 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 perturbations can be accounted for using numerical models,[47]: 9-6  but the planetary system can change chaotically over billions of years.[48]

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.[49] Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.[50][51] 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.[50]

Distances and scales

 
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.[52] For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).[53] 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.[37][54]

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 nearest 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[55] and Johannes Kepler's model based on the Platonic solids,[56] but ongoing discoveries have invalidated these hypotheses.[57]

Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.[58] 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.[59][60]

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.[61]

Habitability

Main article: Planetary habitability in the Solar System

Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields 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.[62]

The zone of habitability of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water.[63] Habitability might be possible in subsurface oceans of various outer Solar System moons.[64]

Comparison with extrasolar systems

 
Comparison between the habitable zones of the Solar System and Gliese 581 (planet d was later found to not exist). The habitable zone is highly dependent on parent star's luminosity.

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[65][66] The known Solar System lacks super-Earths, planets between one and ten times as massive as the Earth,[65] although the hypothetical Planet Nine, if it does exist, could be a super-Earth orbiting in the edge of the Solar System.[67]

Uncommonly, it has only small terrestrial 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.[65][68]

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity.[65] 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.[65][69]

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),[70] which comprises 99.86% of all the mass in the Solar System,[71] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.[72] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[73][74]

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 but shorter lived. 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.[75]

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.[76] 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".[77]

The outermost layer of the Solar atmosphere is the heliosphere, which permeates much of the Solar System. 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),[78] filling the vacuum between the bodies of the Solar System. The result is a thin, dusty atmosphere, called the interplanetary medium, which extends to at least 100 AU (15 billion km; 9.3 billion mi).[79]

Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[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 largest stable 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.[82][83]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[84] Composed mainly of silicates and metals,[85] 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 within the frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun.[39]

The inner Solar System is home to the zodiacal dust cloud. It causes the hazy zodiacal light in the dark, unpolluted sky. 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 materials from planet Mars.[86]

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 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.[87]

  • Mercury (0.31–0.59 AU from the Sun)[D 6] is the smallest planet in the Solar System. Its surface is greyish, with an expansive rupes (cliff) system generated from thrust faults and bright ray systems formed by impact event remnants.[88] In the past, Mercury was volcanically active, producing smooth basaltic plains similar to the Moon.[89] It is likely that Mercury has a silicate crust and a large iron core.[90][91] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms.[92] Mercury has no natural satellites.[93]
  • Venus (0.72–0.73 AU)[D 6] has a reflective, whitish atmosphere that is mainly composed of carbon dioxide. At the surface, the atmospheric pressure is ninety times as dense as on Earth's sea level.[94] Venus is the hottest planet, with surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.[95] The planet lacks a protective magnetic field, which suggests that its atmosphere is sustained by volcanic activity.[96] Its surface displays extensive evidence of volcanic activity but devoid of plate tectonics.[97] Venus has no natural satellites.[98]
  • Earth (0.98–1.02 AU)[D 6] is the only place in the universe where life and surface liquid water known to exist.[99] Earth's atmosphere contains 78% nitrogen and 21% oxygen, which is the result of the presence of life.[100][101] The planet has a complex climate and weather system, with drastically different conditions between climate regions.[102] The solid surface of Earth is dominated by green vegetation, deserts and white ice sheets.[103][104][105] Earth's planetary magnetosphere shields the surface from radiation, limiting atmospheric stripping and maintaining life habitability.[106]
  • Mars (1.38–1.67 AU)[D 6] has a radius about half of that of Earth.[113] Most of the planet is red due to iron oxide in Martian soil,[114] and the polar regions is covered in white ice caps made of water and carbon dioxide.[115] Mars has an atmosphere composed mostly of carbon dioxide and surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.[116] Its surface is peppered with volcanoes and rift valleys, and has a rich collection of minerals.[117][118] Mars have a highly differentiated internal structure and have lost its magnetosphere 4 billion years ago.[119][120] Mars have two tiny moons: Phobos and Deimos.[121]
    • Phobos is Mars's inner moon. It is a small, irregularly shaped object with a mean radius of 11 km (7 mi). Its surface is very unreflective and dominated by impact craters.[D 7][122] In particular, Phobos's surface has a very large Stickney impact crater that is roughly 4.5 km (2.8 mi) in radius.[123]
    • Deimos is Mars's outer moon. Like Phobos, it is irregularly shaped with a mean radius of 6 km (4 mi) and its surface reflects little light.[D 8][D 9] However, Deimos have a surface that is noticeably smoother than Phobos because the regolith partially covers the impact craters.[124]

Asteroids

Main article: Asteroid
 
Overview of the inner Solar System up to Jupiter's orbit

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice.[125][126] 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.[127] As of 2017, the IAU designates asteroids having a diameter between about 30 micrometres and 1 metre as micrometeoroids, and terms smaller particles as 'dust'.[128] Asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids.[129]

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.[130] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[131] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[34] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[132]

Below are the descriptions of three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets, a precursor stage before becoming a fully-formed planet, and thus under significant scientific interest (see List of exceptional asteroids):[133][134][135]

Below are some exemplar asteroid populations that are not in the asteroid belt and within Jupiter's orbit (see also § Centaurs, trojans and resonant bodies):

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets 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, without significant rates of sublimation.[16]

The outer Solar System hosts a cosmic dust cloud. It 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.[156][157]

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, called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun.[f] All four giant planets have multiple moons and a ring system, although only Saturn's rings are easily observed from Earth.[87]

The outer atmospheres of Jupiter and Saturn are composed mainly of gases with extremely low melting points and high vapor pressure, such as hydrogen, helium, and neon,[158] hence their designation as gas giants.[159] Uranus and Neptune are far less massive at less than 20 Earth masses each and are designated as ice giants.[160] They are composed of 'ice' in astronomy sense, as in chemical compounds with melting points of up to a few hundred kelvins[158] such as water, methane, ammonia, hydrogen sulfide, and carbon dioxide.[161] Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune's orbit.[161][162]

  • Jupiter (4.95–5.46 AU)[D 6] is the biggest and most massive planet in the Solar System. It is mainly composed of hydrogen and helium. On the surface, there are orange-brown and white cloud bands moving via the law of atmospheric circulation, with giant storms swirling on the surface such as the Great Red Spot and various white 'ovals'. Jupiter possesses a very strong magnetosphere, enough to redirect ionizing radiation and cause auroras on the poles.[163] As of 2024, Jupiter has 95 confirmed satellites, which can roughly be sorted into three groups:
    • The Galilean moons, consisting of Ganymede, Callisto, Io, and Europa. They are the largest moons of Jupiter and exhibit planetary properties.[164] Notably, Ganymede is the largest satellite in the Solar System and larger than Mercury.[165]
    • The Amalthea group, consisting of Metis, Adrastea, Amalthea, and Thebe. They orbit substantially closer to Jupiter than other satellites.[166] Material from these natural satellites are the source for Jupiter's faint ring.[167]
    • Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than objects listed above.[168]
  • Saturn (9.08–10.12 AU)[D 6] has a distinctive visible ring system orbiting around its equator, which is composed of small ice and rock particles. Like Jupiter, it is mostly made of hydrogen and helium.[169] At the north and south poles, Saturn has peculiar hexagon-shaped storms larger than the diameter of Earth, rotating around themselves every 10.5 hours. Saturn has a magnetosphere capable of producing weak auroras. As of 2024, Saturn has 146 confirmed satellites, with three significant satellite groups listed below:
    • Ring moonlets and shephards, which orbit inside or close to Saturn's rings. A moonlet can only partially clear out dust in their orbit,[170] while the ring shephards are able to completely clear out dust, forming visible gaps in the rings.[171]
    • Inner large satellites, consisting of Mimas, Enceladus, Tethys, and Dione. These satellites all orbits within Saturn's E ring. They are all composed mostly of water ice and are believed to have differentiated internal structure.[172]
    • Outer large satellites, consisting of Rhea, Titan, Hyperion, and Iapetus.[172] In particular, Titan is the only satellite in the Solar System to have a substantial atmosphere.[173]
  • Uranus (18.3–20.1 AU),[D 6] uniquely among the planets, orbits the Sun on its side as its axial tilt is >90°. This gives the planet extreme seasonal variation as each pole points toward and then away from the Sun.[174] Uranus's outer layer has a muted cyan color, but underneath these clouds are many mysteries about its climate phenomena, such as Uranus's peak wind speed of 900 km/h (560 mph), polar cap variations, erratic cloud formation and unusually low internal heat. As of 2024, Uranus has 28 confirmed satellites, which are divided into three groups:
    • Inner satellites, which orbit inside Uranus's ring system.[175] They are very close to each other, which suggest that their orbits are chaotic.[176]
    • Large satellites, consisting of Titania, Oberon, Umbriel, Ariel, and Miranda.[177] Most of them have a roughly equal amounts rock and ice, except Miranda, which is made primarily of ice.[178]
    • Irregular satellites, having more distant and eccentric orbits than objects listed above.[179]
  • Neptune (29.9–30.5 AU)[D 6] is the furthest planet known in the Solar System. Its outer atmosphere has a slightly muted cyan color, with occasional storms on the surface that looks like dark spots. Like Uranus, many atmospheric phenomena of Neptune are still unexplained, such as the thermosphere's abnormally high temperature or the strong tilt (47°) of its magnetosphere. Models suggest that Neptune might have hailstone-like diamonds deep in its atmosphere. As of 2024, Neptune has 16 confirmed satellites, which are divided into two groups:
    • Regular satellites, which have circular orbits that lie near Neptune's equator.[180]
    • Irregular satellites, which as the name implies, have less regular orbits. One of them, Triton, is Neptune's largest moon. It is geologically active, with erupting geysers of liquid nitrogen.[181] Notably, Triton has a retrograde orbit, implying it was probably captured from the Kuiper belt.[182]

Centaurs, trojans and resonant bodies

Main articles: Centaur and Trojan

The centaurs are icy comet-like bodies whose semi-major axes is greater than Jupiter's and less than Neptune's (between 5.5 and 30 AU). These are former Kuiper belt and scattered disc objects (SDOs) that were gravitationally perturbed closer to the Sun by the outer planets, and are expected to become comets or get ejected out of the Solar System.[33] While most centaurs are inactive and asteroid-like, some exhibit clear cometary activity, such as the first centaur discovered, 2060 Chiron, which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[183] The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi) and is one of the only few minor planets known to possess a ring system.[184][185]

Jupiter trojans are located in both of Jupiter's stable Lagrange points: L4, 60° ahead of Jupiter in its orbit, or L5, 60° behind in its orbit.[186] Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[187]

Comets

Main article: Comet
 
Comet Hale–Bopp seen in 1997

Comets are small Solar System bodies, 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.[188]

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.[189] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[190] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.[191]

Trans-Neptunian region

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.[192]

Kuiper belt

Main article: Kuiper belt
 
Plot of objects around the Kuiper belt and other asteroid populations. J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune.
 
Orbit classification of Kuiper belt objects. Some clusters that is subjected to orbital resonance are marked.

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[193] It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.[194] 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.[33] Many Kuiper belt objects have satellites,[195] and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic.[196]

The Kuiper belt can be roughly divided into the "classical" belt and the resonant trans-Neptunian objects.[193] 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.[197] 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.[198]

Currently, there are four dwarf planets in the Kuiper belt that have strong consensus among astronomers.[194][199] Many dwarf planet candidates are being considered, pending further data for verification.[200]

  • Pluto (29.7–49.3 AU) is the largest known object in the Kuiper belt. Pluto has a relatively eccentric orbit, inclined 17 degrees to the ecliptic plane. 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.[201] Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra.[202]
    • Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e. they appear to "orbit each other").
  • Makemake (38.1–52.8 AU), 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.[203] Its orbit is far more inclined than Pluto's, at 29°.[204] It has one known moon.[205]
  • Haumea (34.6–51.6 AU) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune.[206] Like Makemake, it was discovered in 2005.[207] Uniquely among the dwarf planets, Haumea possesses a ring system, two known moons named Hiʻiaka and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an ellipsoid. It is part of a collisional family of Kuiper belt objects that share similar orbits, which suggests a giant collision took place on Haumea and ejected its fragments into space billions of years ago.[208]
  • Quaoar (41.9–45.5 AU) 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.[206] It possesses a ring system and one known moon, Weywot.[209]
  • Orcus (30.3–48.1 AU), another Kuiper belt object and the largest whose dwarf planet status remains controversial: it is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.[206] 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.[210] For this reason, it has been called the anti-Pluto.[211][212] It has one known moon, Vanth.[213]

Scattered disc

Main article: Scattered disc
 
The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects

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 have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane.[214] 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".[215] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[216]

Currently, there are two dwarf planets in the scattered disc that have strong consensus among astronomers:

  • Eris (38.3–97.5 AU) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto[217] 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°.[218]
  • Gonggong (33.8–101.2 AU) is a dwarf planet in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.[D 10] It has one known moon, Xiangliu.[219]

Extreme trans-Neptunian objects

Main article: Extreme trans-Neptunian object
 
The orbits of Sedna, 2012 VP113, Leleākūhonua, and other very distant objects along with the predicted orbit of the hypothetical Planet Nine

Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short.[220] Generally, ETNOs' semi-major axis are at least 150–250 AU wide.[220][221] For example, 541132 Leleākūhonua orbits the Sun once every ~32,000 years, with a distance of 65–2000 AU from the Sun.[D 11]

This population is divided into three subgroups by astronomers. The scattered ETNOs have perihelia around 38–45 AU and an exceptionally high eccentricity of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of gravitational scattering by Neptune and still interact with the giant planets. The detached ETNOs, with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. The sednoids or inner Oort cloud objects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it.[220]

Currently, there is one ETNO that is classified as a dwarf planet:

  • Sedna (76.2–937 AU) is the first extreme trans-Neptunian object to be found. It is a large, reddish object, and it takes 11,400 years for Sedna to complete one orbit. 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.[222] The sednoids population is named after Sedna.[220]

Edge of the heliosphere

 
Diagram of the Sun's magnetosphere and helioshealth

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.[223] Here the solar wind collides with the interstellar medium[224] and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath.[223]

The heliosheath 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.[225] 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,[226][227] but the actual shape remains unknown.[228]

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.[223] The heliopause is considered the beginning of the interstellar medium.[79] 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.[229] Large objects outside the heliopause remain gravitationally bound to the sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects.[79]

Boundary area and uncertainties

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

Much of the Solar System is still unknown. Areas beyond thousands of AU away is still virtually unmapped and learning about this region of space is difficult. Study in this region 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.[230] Many objects may yet be discovered in the Solar System's uncharted regions.[231]

One of these objects might be the Oort cloud, a theorized spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets.[232][233] No direct observation of the Oort cloud is possible with present imaging technology.[234] It is theorized 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). The Oort cloud 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.[232][233]

As of the 2020s, a few astronomers hypothesized that Planet Nine (a planet beyond Neptune) might exist, based on statistical variance in the orbit of extreme trans-Neptunian objects.[235] Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.[236][237][238] However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.[239]

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.[240] Most of the mass is orbiting in the region between 3,000 and 100,000 AU.[241] The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun.[242] The Sun's Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[243]

 
The Solar System within the interstellar medium, with the different regions and their distances on a steped horizontal distance scale

Location

Celestial neighborhood

Main articles: List of nearest stars and brown dwarfs, List of nearest exoplanets, and List of nearby stellar associations and moving groups
 
Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.[244]

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.[245] 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.[246]

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.[247][248] Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble.[248] 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.[249]

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length.[250] 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.[251]

The nearest and unaided-visible group of stars beyond the immediate celestial neighborhood 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.[252]

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 ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud.[253] There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System.[254]

Galactic position and orbit

See also: Location of Earth, Galactic year, and Orbit of the Sun
 
Diagram of the Milky Way, with galactic features and the relative position of the Solar System labelled.

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.[255] The Sun is part of one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[256][257]

Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.[255] This revolution is known as the Solar System's galactic year.[258] 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.[259] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[g]

The Sun follows a nearly circular orbit around the Galactic Center (where the supermassive black hole Sagittarius A* resides) at a distance of 26,660 light-years,[261] orbiting at roughly the same speed as that of the spiral arms.[262] If it orbited close to the center, 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. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life.[262]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.[262] However, according to the controversial Shiva hypothesis, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth.[263][264]

Humanity's perspective

Main article: Discovery and exploration of the Solar System
 
The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.

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 center of the universe[265] 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.[266][267]

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, 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.[268][269]

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.[270] Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn.[271] In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize 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.[272] 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.[46]: 142 

 
True-scale Solar System diagram made by Emanuel Bowen in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet.

The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets.[273] In 1705, Halley realized 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,[274] though Seneca had theorized this about comets in the 1st century.[275] 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.[276]

Uranus, having occasionally been observed since antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.[277] 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.[278] 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.[279] Mercury's orbital anomaly observations led to searches for Vulcan, a planet interior of Mercury, but these attempts were quashed with Albert Einstein's theory of general relativity in 1915.[280]

In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space since the 1960s.[281] By 1989, all eight planets have been visited by space probes.[282] Probes have returned samples from comets[283] and asteroids,[284] as well as flown through the Sun's corona[285] and visited two dwarf planets (Pluto and Ceres).[286][287] Humans have landed on the Moon during the Apollo program in the 1960s and 1970s[288] and will return to the Moon in the 2020s with the Artemis program.[289] Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006, hence the demotion of Pluto to a dwarf planet,[290] and further interest in trans-Neptunian objects.[291]

See also

Notes

  1. ^ a b 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]
  2. ^ 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.
  3. ^ The International Astronomical Union's Minor Planet Center has yet to officially list Quaoar, Sedna and Gonggong as dwarf planets as of 2024
  4. ^ For more classifications of Solar System objects, see List of minor-planet groups and Comet § Classification.
  5. ^ "Low-mass" is a relative term; the Sun is still more massive than 95% of stars in the galaxy.[30]
  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),[32] the Kuiper belt (estimated at 0.1 Earth mass)[33] and the asteroid belt (estimated to be 0.0005 Earth mass)[34] 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,[260] 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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January 01, 1970
solar, system, other, uses, disambiguation, gravitationally, bound, system, objects, that, orbit, formed, billion, years, when, dense, region, molecular, cloud, collapsed, forming, protoplanetary, disc, ordinary, main, sequence, star, that, maintains, balanced. For other uses see Solar System disambiguation The Solar System b is the gravitationally bound system of the Sun and the objects that orbit it 9 It was formed 4 6 billion years ago when a dense region of a molecular cloud collapsed forming the Sun and a protoplanetary disc The Sun is an ordinary main sequence star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core releasing this energy from its outer photosphere The Sun planets moons and dwarf planets The asteroid belt and Kuiper belt are not added because the individual asteroids are too small to be shown on the diagram Solar SystemAge4 568 billion years a LocationLocal Interstellar CloudLocal Bubble 1 Orion Cygnus ArmMilky Way 2 Nearest starProxima Centauri 4 2465 ly D 1 Alpha Centauri 4 36 ly D 2 PopulationStarsSunPlanetsMercury Venus Earth Mars Jupiter Saturn Uranus NeptuneKnown dwarf planetsCeres Pluto Haumea Quaoar Makemake Gonggong Eris Sedna more candidates Known natural satellites758 D 3 Known minor planets1 358 412 D 4 Known comets4 591 D 4 Planetary systemStar spectral typeG2VFrost line 5 AU 5 Semi major axis of outermost planet30 07 AU D 5 Neptune Kuiper cliff50 70 AU 3 4 Heliopausedetected at 120 AU 6 Hill sphere 1 3 ly citation needed Orbit about Galactic CenterInvariable to galactic plane inclination60 19 ecliptic citation needed Distance toGalactic Center24 000 28 000 ly 7 Orbital speed720 000 km h 450 000 mi h 8 Orbital period 230 million years 8 The largest objects that orbit the Sun are the eight planets In order from the Sun they are four terrestrial planets Mercury Venus Earth and Mars two gas giants Jupiter and Saturn and two ice giants Uranus and Neptune All terrestrial planets have solid surfaces Inversely all giant planets do not have a definite surface as they are mainly composed of gases and liquids Over 99 86 of the Solar System s mass is in the Sun and nearly 90 of the remaining mass is in Jupiter and Saturn There is a strong consensus among astronomers c that the Solar System has at least eight dwarf planets Ceres Pluto Haumea Quaoar Makemake Gonggong Eris and Sedna There are a vast number of small Solar System bodies such as asteroids comets centaurs meteoroids and interplanetary dust clouds Some of these bodies are in the asteroid belt between Mars s and Jupiter s orbit and the Kuiper belt just outside Neptune s orbit d Six planets six dwarf planets and other bodies have orbiting natural satellites which are commonly called moons The Solar System is constantly flooded by the Sun s charged particles the solar wind forming the heliosphere Around 75 90 astronomical units the solar wind is halted resulting in the heliopause This is the boundary of the Solar System to interstellar space The outermost region of the Solar System is the theorized Oort cloud the source for long period comets extending 2 000 200 000 astronomical units 0 032 3 2 light years The closest star to the Solar System Proxima Centauri is 4 25 light years away Both stars belong to the Milky Way galaxy Contents 1 Formation and evolution 1 1 Past 1 2 Present and future 2 General characteristics 2 1 Composition 2 2 Orbits 2 3 Distances and scales 2 4 Habitability 2 5 Comparison with extrasolar systems 3 Sun 4 Inner Solar System 4 1 Inner planets 4 2 Asteroids 5 Outer Solar System 5 1 Outer planets 5 2 Centaurs trojans and resonant bodies 6 Comets 7 Trans Neptunian region 7 1 Kuiper belt 7 2 Scattered disc 7 3 Extreme trans Neptunian objects 7 4 Edge of the heliosphere 8 Boundary area and uncertainties 9 Location 9 1 Celestial neighborhood 9 2 Galactic position and orbit 10 Humanity s perspective 11 See also 12 Notes 13 References 13 1 Data sources 13 2 Other sources 14 External linksFormation and evolutionMain article Formation and evolution of the Solar System Past The Solar System formed 4 568 billion years ago from the gravitational collapse of a region within a large molecular cloud a 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 12 As the pre solar nebula 12 collapsed conservation of angular momentum caused it to rotate faster The center 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 center 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 nbsp Diagram of the early Solar System s protoplanetary disk out of which Earth and other Solar System bodies formed 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 within the frost line They 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 16 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 16 Leftover debris that never became planets congregated in regions such as the asteroid belt Kuiper belt and Oort cloud 16 Within 50 million years the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion 18 As helium accumulates at its core the Sun is growing brighter 19 early in its main sequence life its brightness was 70 that of what it is today 20 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 21 Present and future nbsp The current Sun compared to its peak size in the red giant phase 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 subsequent phases of the Sun s pre remnant life combined 22 Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space 19 The Solar System is in a relatively stable slowly evolving state by following isolated gravitationally bound orbits around the Sun 23 Although the Solar System has been fairly stable for billions of years it is technically chaotic and may eventually be disrupted There is a small chance that another star will pass through the Solar System in the next few billion years Although this could destabilize the system and eventually lead millions of years later to expulsion of planets collisions of planets or planets hitting the Sun it would most likely leave the Solar System much as it is today 24 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 22 The expanding Sun is expected to vaporize Mercury as well as Venus and render Earth uninhabitable possibly destroying it as well 25 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 22 The ejected outer layers may form a planetary nebula returning some of the material that formed the Sun but now enriched with heavier elements like carbon to the interstellar medium 26 27 General characteristicsAstronomers sometimes divide the Solar System structure into searate regions The inner Solar System includes the Mercury Venus Earth Mars and bodies in the asteroid belt The outer Solar System includes the Jupiter Saturn Uranus Neptune and bodies in the Kuiper belt 28 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 29 Composition Further information List of Solar System objects and List of interstellar and circumstellar molecules The principal component of the Solar System is the Sun a low mass star e that contains 99 86 of the system s known mass and dominates it gravitationally 31 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 35 as are Jupiter and Saturn 36 37 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 38 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 5 Orbits nbsp Animations of the Solar System s inner planets orbiting Each frame represents 2 days of motion nbsp Animations of the Solar System s outer planets orbiting This animation is 100 times faster than the inner planet animation 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 39 40 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 41 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 42 There are exceptions such as Halley s Comet 43 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 44 Most larger objects rotate around their own axes in the prograde direction relative to their orbit though the rotation of Venus is retrograde 45 To a good first approximation Kepler s laws of planetary motion describe the orbits of objects around the Sun 46 433 437 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 47 9 6 With the exception of Mercury 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 perturbations can be accounted for using numerical models 47 9 6 but the planetary system can change chaotically over billions of years 48 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 49 Although the Sun dominates the system by mass it accounts for only about 2 of the angular momentum 50 51 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 50 Distances and scales nbsp 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 52 For comparison the radius of the Sun is 0 0047 AU 700 000 km 400 000 mi 53 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 37 54 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 nearest 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 55 and Johannes Kepler s model based on the Platonic solids 56 but ongoing discoveries have invalidated these hypotheses 57 Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms Some are small in scale and may be mechanical called orreries whereas others extend across cities or regional areas 58 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 59 60 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 61 Habitability Main article Planetary habitability in the Solar System Besides solar energy the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields for those planets that have them These magnetic fields 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 62 The zone of habitability of the Solar System is conventionally located in the inner Solar System where planetary surface or atmospheric temperatures admit the possibility of liquid water 63 Habitability might be possible in subsurface oceans of various outer Solar System moons 64 Comparison with extrasolar systems nbsp Comparison between the habitable zones of the Solar System and Gliese 581 planet d was later found to not exist The habitable zone is highly dependent on parent star s luminosity Compared to many extrasolar systems the Solar System stands out in lacking planets interior to the orbit of Mercury 65 66 The known Solar System lacks super Earths planets between one and ten times as massive as the Earth 65 although the hypothetical Planet Nine if it does exist could be a super Earth orbiting in the edge of the Solar System 67 Uncommonly it has only small terrestrial 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 65 68 The orbits of Solar System planets are nearly circular Compared to other systems they have smaller orbital eccentricity 65 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 65 69 SunMain article Sun nbsp 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 70 which comprises 99 86 of all the mass in the Solar System 71 produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium 72 This releases an enormous amount of energy mostly radiated into space as electromagnetic radiation peaking in visible light 73 74 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 but shorter lived 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 75 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 76 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 77 The outermost layer of the Solar atmosphere is the heliosphere which permeates much of the Solar System 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 78 filling the vacuum between the bodies of the Solar System The result is a thin dusty atmosphere called the interplanetary medium which extends to at least 100 AU 15 billion km 9 3 billion mi 79 Activity on the Sun s surface such as solar flares and coronal mass ejections disturbs the heliosphere creating space weather and causing geomagnetic storms 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 largest stable 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 82 83 Inner Solar SystemThe inner Solar System is the region comprising the terrestrial planets and the asteroid belt 84 Composed mainly of silicates and metals 85 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 within the frost line which is a little less than 5 AU 750 million km 460 million mi from the Sun 39 The inner Solar System is home to the zodiacal dust cloud It causes the hazy zodiacal light in the dark unpolluted sky 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 materials from planet Mars 86 Inner planets Main article Terrestrial planet nbsp 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 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 87 Mercury 0 31 0 59 AU from the Sun D 6 is the smallest planet in the Solar System Its surface is greyish with an expansive rupes cliff system generated from thrust faults and bright ray systems formed by impact event remnants 88 In the past Mercury was volcanically active producing smooth basaltic plains similar to the Moon 89 It is likely that Mercury has a silicate crust and a large iron core 90 91 Mercury has a very tenuous atmosphere consisting of solar wind particles and ejected atoms 92 Mercury has no natural satellites 93 Venus 0 72 0 73 AU D 6 has a reflective whitish atmosphere that is mainly composed of carbon dioxide At the surface the atmospheric pressure is ninety times as dense as on Earth s sea level 94 Venus is the hottest planet with surface temperatures over 400 C 752 F mainly due to the amount of greenhouse gases in the atmosphere 95 The planet lacks a protective magnetic field which suggests that its atmosphere is sustained by volcanic activity 96 Its surface displays extensive evidence of volcanic activity but devoid of plate tectonics 97 Venus has no natural satellites 98 Earth 0 98 1 02 AU D 6 is the only place in the universe where life and surface liquid water known to exist 99 Earth s atmosphere contains 78 nitrogen and 21 oxygen which is the result of the presence of life 100 101 The planet has a complex climate and weather system with drastically different conditions between climate regions 102 The solid surface of Earth is dominated by green vegetation deserts and white ice sheets 103 104 105 Earth s planetary magnetosphere shields the surface from radiation limiting atmospheric stripping and maintaining life habitability 106 The Moon is Earth s only natural satellite 107 Its diameter is one quarter the size of Earth s 108 Its surface is covered in very fine regolith and dominated by impact craters 109 110 Large dark patches on the Moon maria are formed from past volcanic activity 111 The Moon s atmosphere is extremely thin indistinguishable from a vacuum 112 Mars 1 38 1 67 AU D 6 has a radius about half of that of Earth 113 Most of the planet is red due to iron oxide in Martian soil 114 and the polar regions is covered in white ice caps made of water and carbon dioxide 115 Mars has an atmosphere composed mostly of carbon dioxide and surface pressure 0 6 of that of Earth which is sufficient to support some weather phenomena 116 Its surface is peppered with volcanoes and rift valleys and has a rich collection of minerals 117 118 Mars have a highly differentiated internal structure and have lost its magnetosphere 4 billion years ago 119 120 Mars have two tiny moons Phobos and Deimos 121 Phobos is Mars s inner moon It is a small irregularly shaped object with a mean radius of 11 km 7 mi Its surface is very unreflective and dominated by impact craters D 7 122 In particular Phobos s surface has a very large Stickney impact crater that is roughly 4 5 km 2 8 mi in radius 123 Deimos is Mars s outer moon Like Phobos it is irregularly shaped with a mean radius of 6 km 4 mi and its surface reflects little light D 8 D 9 However Deimos have a surface that is noticeably smoother than Phobos because the regolith partially covers the impact craters 124 Asteroids Main article Asteroid nbsp Overview of the inner Solar System up to Jupiter s orbitAsteroids except for the largest Ceres are classified as small Solar System bodies and are composed mainly of carbonaceous refractory rocky and metallic minerals with some ice 125 126 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 127 As of 2017 update the IAU designates asteroids having a diameter between about 30 micrometres and 1 metre as micrometeoroids and terms smaller particles as dust 128 Asteroids are divided into asteroid groups and families based on their orbital characteristics Some asteroids have natural satellites that orbit them that is asteroids that orbit larger asteroids 129 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 130 The asteroid belt contains tens of thousands possibly millions of objects over one kilometre in diameter 131 Despite this the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth 34 The asteroid belt is very sparsely populated spacecraft routinely pass through without incident 132 Below are the descriptions of three largest bodies in the asteroid belt They are all considered to be relatively intact protoplanets a precursor stage before becoming a fully formed planet and thus under significant scientific interest see List of exceptional asteroids 133 134 135 Ceres 2 55 2 98 AU is the only dwarf planet in the asteroid belt 136 It is the largest object in the belt with a diameter of 940 km 580 mi 137 Its surface contains a mixture of carbon 138 frozen water and hydrated minerals 139 There are signs of past cryovolcano activity as seen in surface bright spots 140 Ceres has a very thin water vapor atmosphere but practically speaking it is indistinguishable from a vacuum 141 Vesta 2 13 3 41 AU is the second largest object in the asteroid belt 142 Its fragments survive as the Vesta asteroid family 143 and numerous HED meteorites found on Earth 144 Vesta s surface dominated by basaltic and metamorphic material has a denser composition than Ceres s 145 Its surface is marked by two giant craters Rheasilvia and Veneneia 146 Pallas 2 15 2 57 AU is the third largest object in the asteroid belt 147 It has its own Pallas asteroid family 143 Not much is known about Pallas because it has never been visited by a spacecraft 148 though its surface is predicted to be composed of silicates 149 Below are some exemplar asteroid populations that are not in the asteroid belt and within Jupiter s orbit see also Centaurs trojans and resonant bodies No vulcanoids asteroids between the orbit of Mercury and the Sun have been discovered 150 151 One asteroid is discovered to orbit within Venus s orbit as of 2024 which is 594913 ꞌAyloꞌchaxnim 152 Near Earth asteroids have orbits that approach relatively close to Earth s orbit 153 and some of them are potentially hazardous objects because they might collide with Earth in the future 154 155 Outer Solar SystemThe outer region of the Solar System is home to the giant planets and their large moons The centaurs and many short period comets 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 without significant rates of sublimation 16 The outer Solar System hosts a cosmic dust cloud It 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 156 157 Outer planets Main article Giant planet nbsp 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 called giant planets or Jovian planets collectively make up 99 of the mass known to orbit the Sun f All four giant planets have multiple moons and a ring system although only Saturn s rings are easily observed from Earth 87 The outer atmospheres of Jupiter and Saturn are composed mainly of gases with extremely low melting points and high vapor pressure such as hydrogen helium and neon 158 hence their designation as gas giants 159 Uranus and Neptune are far less massive at less than 20 Earth masses each and are designated as ice giants 160 They are composed of ice in astronomy sense as in chemical compounds with melting points of up to a few hundred kelvins 158 such as water methane ammonia hydrogen sulfide and carbon dioxide 161 Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune s orbit 161 162 Jupiter 4 95 5 46 AU D 6 is the biggest and most massive planet in the Solar System It is mainly composed of hydrogen and helium On the surface there are orange brown and white cloud bands moving via the law of atmospheric circulation with giant storms swirling on the surface such as the Great Red Spot and various white ovals Jupiter possesses a very strong magnetosphere enough to redirect ionizing radiation and cause auroras on the poles 163 As of 2024 Jupiter has 95 confirmed satellites which can roughly be sorted into three groups The Galilean moons consisting of Ganymede Callisto Io and Europa They are the largest moons of Jupiter and exhibit planetary properties 164 Notably Ganymede is the largest satellite in the Solar System and larger than Mercury 165 The Amalthea group consisting of Metis Adrastea Amalthea and Thebe They orbit substantially closer to Jupiter than other satellites 166 Material from these natural satellites are the source for Jupiter s faint ring 167 Irregular satellites consisting of substantially smaller natural satellites They have more distant orbits than objects listed above 168 Saturn 9 08 10 12 AU D 6 has a distinctive visible ring system orbiting around its equator which is composed of small ice and rock particles Like Jupiter it is mostly made of hydrogen and helium 169 At the north and south poles Saturn has peculiar hexagon shaped storms larger than the diameter of Earth rotating around themselves every 10 5 hours Saturn has a magnetosphere capable of producing weak auroras As of 2024 Saturn has 146 confirmed satellites with three significant satellite groups listed below Ring moonlets and shephards which orbit inside or close to Saturn s rings A moonlet can only partially clear out dust in their orbit 170 while the ring shephards are able to completely clear out dust forming visible gaps in the rings 171 Inner large satellites consisting of Mimas Enceladus Tethys and Dione These satellites all orbits within Saturn s E ring They are all composed mostly of water ice and are believed to have differentiated internal structure 172 Outer large satellites consisting of Rhea Titan Hyperion and Iapetus 172 In particular Titan is the only satellite in the Solar System to have a substantial atmosphere 173 Uranus 18 3 20 1 AU D 6 uniquely among the planets orbits the Sun on its side as its axial tilt is gt 90 This gives the planet extreme seasonal variation as each pole points toward and then away from the Sun 174 Uranus s outer layer has a muted cyan color but underneath these clouds are many mysteries about its climate phenomena such as Uranus s peak wind speed of 900 km h 560 mph polar cap variations erratic cloud formation and unusually low internal heat As of 2024 Uranus has 28 confirmed satellites which are divided into three groups Inner satellites which orbit inside Uranus s ring system 175 They are very close to each other which suggest that their orbits are chaotic 176 Large satellites consisting of Titania Oberon Umbriel Ariel and Miranda 177 Most of them have a roughly equal amounts rock and ice except Miranda which is made primarily of ice 178 Irregular satellites having more distant and eccentric orbits than objects listed above 179 Neptune 29 9 30 5 AU D 6 is the furthest planet known in the Solar System Its outer atmosphere has a slightly muted cyan color with occasional storms on the surface that looks like dark spots Like Uranus many atmospheric phenomena of Neptune are still unexplained such as the thermosphere s abnormally high temperature or the strong tilt 47 of its magnetosphere Models suggest that Neptune might have hailstone like diamonds deep in its atmosphere As of 2024 Neptune has 16 confirmed satellites which are divided into two groups Regular satellites which have circular orbits that lie near Neptune s equator 180 Irregular satellites which as the name implies have less regular orbits One of them Triton is Neptune s largest moon It is geologically active with erupting geysers of liquid nitrogen 181 Notably Triton has a retrograde orbit implying it was probably captured from the Kuiper belt 182 Centaurs trojans and resonant bodies Main articles Centaur and Trojan The centaurs are icy comet like bodies whose semi major axes is greater than Jupiter s and less than Neptune s between 5 5 and 30 AU These are former Kuiper belt and scattered disc objects SDOs that were gravitationally perturbed closer to the Sun by the outer planets and are expected to become comets or get ejected out of the Solar System 33 While most centaurs are inactive and asteroid like some exhibit clear cometary activity such as the first centaur discovered 2060 Chiron which has been classified as a comet 95P because it develops a coma just as comets do when they approach the Sun 183 The largest known centaur 10199 Chariklo has a diameter of about 250 km 160 mi and is one of the only few minor planets known to possess a ring system 184 185 Jupiter trojans are located in both of Jupiter s stable Lagrange points L4 60 ahead of Jupiter in its orbit or L5 60 behind in its orbit 186 Hilda asteroids are in a 3 2 resonance with Jupiter that is they go around the Sun three times for every two Jupiter orbits 187 CometsMain article Comet nbsp Comet Hale Bopp seen in 1997 Comets are small Solar System bodies 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 188 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 189 Some comets with hyperbolic orbits may originate outside the Solar System but determining their precise orbits is difficult 190 Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids 191 Trans Neptunian regionBeyond 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 192 Kuiper belt Main article Kuiper belt nbsp Plot of objects around the Kuiper belt and other asteroid populations J S U and N denotes Jupiter Saturn Uranus and Neptune nbsp Orbit classification of Kuiper belt objects Some clusters that is subjected to orbital resonance are marked The Kuiper belt is a great ring of debris similar to the asteroid belt but consisting mainly of objects composed primarily of ice 193 It extends between 30 and 50 AU from the Sun It is composed mainly of small Solar System bodies although the largest few are probably large enough to be dwarf planets 194 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 33 Many Kuiper belt objects have satellites 195 and most have orbits that are substantially inclined 10 to the plane of the ecliptic 196 The Kuiper belt can be roughly divided into the classical belt and the resonant trans Neptunian objects 193 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 197 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 198 Currently there are four dwarf planets in the Kuiper belt that have strong consensus among astronomers 194 199 Many dwarf planet candidates are being considered pending further data for verification 200 Pluto 29 7 49 3 AU is the largest known object in the Kuiper belt Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane 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 201 Pluto has five moons Charon Styx Nix Kerberos and Hydra 202 Charon the largest of Pluto s moons is sometimes described as part of a binary system with Pluto as the two bodies orbit a barycenter of gravity above their surfaces i e they appear to orbit each other Makemake 38 1 52 8 AU 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 203 Its orbit is far more inclined than Pluto s at 29 204 It has one known moon 205 Haumea 34 6 51 6 AU is in an orbit similar to Makemake except that it is in a temporary 7 12 orbital resonance with Neptune 206 Like Makemake it was discovered in 2005 207 Uniquely among the dwarf planets Haumea possesses a ring system two known moons named Hiʻiaka and Namaka and rotates so quickly once every 3 9 hours that it is stretched into an ellipsoid It is part of a collisional family of Kuiper belt objects that share similar orbits which suggests a giant collision took place on Haumea and ejected its fragments into space billions of years ago 208 Quaoar 41 9 45 5 AU 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 206 It possesses a ring system and one known moon Weywot 209 Orcus 30 3 48 1 AU another Kuiper belt object and the largest whose dwarf planet status remains controversial it is in the same 2 3 orbital resonance with Neptune as Pluto and is the largest such object after Pluto itself 206 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 210 For this reason it has been called the anti Pluto 211 212 It has one known moon Vanth 213 Scattered disc Main article Scattered disc nbsp The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects 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 have perihelia within the Kuiper belt but aphelia far beyond it some more than 150 AU from the Sun SDOs orbits can be inclined up to 46 8 from the ecliptic plane 214 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 215 Some astronomers classify centaurs as inward scattered Kuiper belt objects along with the outward scattered residents of the scattered disc 216 Currently there are two dwarf planets in the scattered disc that have strong consensus among astronomers Eris 38 3 97 5 AU is the largest known scattered disc object and caused a debate about what constitutes a planet because it is 25 more massive than Pluto 217 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 44 218 Gonggong 33 8 101 2 AU is a dwarf planet in a comparable orbit to Eris except that it is in a 3 10 resonance with Neptune D 10 It has one known moon Xiangliu 219 Extreme trans Neptunian objects Main article Extreme trans Neptunian object nbsp The orbits of Sedna 2012 VP113 Leleakuhonua and other very distant objects along with the predicted orbit of the hypothetical Planet Nine Some objects in the Solar System have a very large orbit and therefore are much less affected by the known giant planets than other minor planet populations These bodies are called extreme trans Neptunian objects or ETNOs for short 220 Generally ETNOs semi major axis are at least 150 250 AU wide 220 221 For example 541132 Leleakuhonua orbits the Sun once every 32 000 years with a distance of 65 2000 AU from the Sun D 11 This population is divided into three subgroups by astronomers The scattered ETNOs have perihelia around 38 45 AU and an exceptionally high eccentricity of more than 0 85 As with the regular scattered disc objects they were likely formed as result of gravitational scattering by Neptune and still interact with the giant planets The detached ETNOs with perihelia approximately between 40 45 and 50 60 AU are less affected by Neptune than the scattered ETNOs but are still relatively close to Neptune The sednoids or inner Oort cloud objects with perihelia beyond 50 60 AU are too far from Neptune to be strongly influenced by it 220 Currently there is one ETNO that is classified as a dwarf planet Sedna 76 2 937 AU is the first extreme trans Neptunian object to be found It is a large reddish object and it takes 11 400 years for Sedna to complete one orbit 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 222 The sednoids population is named after Sedna 220 Edge of the heliosphere nbsp Diagram of the Sun s magnetosphere and helioshealth 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 223 Here the solar wind collides with the interstellar medium 224 and dramatically slows condenses and becomes more turbulent forming a great oval structure known as the heliosheath 223 The heliosheath 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 225 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 226 227 but the actual shape remains unknown 228 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 223 The heliopause is considered the beginning of the interstellar medium 79 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 229 Large objects outside the heliopause remain gravitationally bound to the sun but the flow of matter in the interstellar medium homogenizes the distribution of micro scale objects 79 Boundary area and uncertaintiesSee also Planets beyond Neptune and List of Solar System objects by greatest aphelion Much of the Solar System is still unknown Areas beyond thousands of AU away is still virtually unmapped and learning about this region of space is difficult Study in this region 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 230 Many objects may yet be discovered in the Solar System s uncharted regions 231 One of these objects might be the Oort cloud a theorized spherical cloud of up to a trillion icy objects that is thought to be the source for all long period comets 232 233 No direct observation of the Oort cloud is possible with present imaging technology 234 It is theorized 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 The Oort cloud 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 232 233 As of the 2020s a few astronomers hypothesized that Planet Nine a planet beyond Neptune might exist based on statistical variance in the orbit of extreme trans Neptunian objects 235 Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted suggesting that a large planet might be influencing their orbit over millions of years 236 237 238 However some astronomers said that this observation might be credited to observational biases or just sheer coincidence 239 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 240 Most of the mass is orbiting in the region between 3 000 and 100 000 AU 241 The furthest known objects such as Comet West have aphelia around 70 000 AU from the Sun 242 The Sun s Hill sphere with respect to the galactic nucleus the effective range of its gravitational influence is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud 243 nbsp The Solar System within the interstellar medium with the different regions and their distances on a steped horizontal distance scaleLocationCelestial neighborhood Main articles List of nearest stars and brown dwarfs List of nearest exoplanets and List of nearby stellar associations and moving groups nbsp Diagram of the Local Interstellar Cloud the G Cloud and surrounding stars As of 2022 the precise location of the Solar System in the clouds is an open question in astronomy 244 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 245 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 246 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 247 248 Multiple other interstellar clouds exist in the region within 300 light years of the Sun known as the Local Bubble 248 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 249 The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures formerly Gould Belt each of which are some thousands of light years in length 250 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 251 The nearest and unaided visible group of stars beyond the immediate celestial neighborhood 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 252 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 50 000 AU of the Sun some 70 thousands years ago likely passing through the outer Oort cloud 253 There is a 1 chance every billion years that a star will pass within 100 AU of the Sun potentially disrupting the Solar System 254 Galactic position and orbit See also Location of Earth Galactic year and Orbit of the Sun nbsp Diagram of the Milky Way with galactic features and the relative position of the Solar System labelled 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 255 The Sun is part of one of the Milky Way s outer spiral arms known as the Orion Cygnus Arm or Local Spur 256 257 Its speed around the center of the Milky Way is about 220 km s so that it completes one revolution every 240 million years 255 This revolution is known as the Solar System s galactic year 258 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 259 The plane of the ecliptic lies at an angle of about 60 to the galactic plane g The Sun follows a nearly circular orbit around the Galactic Center where the supermassive black hole Sagittarius A resides at a distance of 26 660 light years 261 orbiting at roughly the same speed as that of the spiral arms 262 If it orbited close to the center 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 In this scenario the intense radiation of the Galactic Center could interfere with the development of complex life 262 The Solar System s location in the Milky Way is a factor in the evolutionary history of life on Earth Spiral arms are home to a far larger concentration of supernovae gravitational instabilities and radiation that could disrupt the Solar System but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms this has given Earth long periods of stability for life to evolve 262 However according to the controversial Shiva hypothesis the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth 263 264 Humanity s perspectiveMain article Discovery and exploration of the Solar System nbsp The motion of lights moving across the sky is the basis of the classical definition of planets wandering stars 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 center of the universe 265 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 266 267 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 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 268 269 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 270 Christiaan Huygens followed on from these observations by discovering Saturn s moon Titan and the shape of the rings of Saturn 271 In 1677 Edmond Halley observed a transit of Mercury across the Sun leading him to realize 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 272 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 46 142 nbsp True scale Solar System diagram made by Emanuel Bowen in 1747 At that time Uranus Neptune nor the asteroid belts have been discovered yet The term Solar System entered the English language by 1704 when John Locke used it to refer to the Sun planets and comets 273 In 1705 Halley realized 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 274 though Seneca had theorized this about comets in the 1st century 275 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 276 Uranus having occasionally been observed since antiquity was recognized to be a planet orbiting beyond Saturn by 1783 277 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 278 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 279 Mercury s orbital anomaly observations led to searches for Vulcan a planet interior of Mercury but these attempts were quashed with Albert Einstein s theory of general relativity in 1915 280 In the 20th century humans began their space exploration around the Solar System starting with placing telescopes in space since the 1960s 281 By 1989 all eight planets have been visited by space probes 282 Probes have returned samples from comets 283 and asteroids 284 as well as flown through the Sun s corona 285 and visited two dwarf planets Pluto and Ceres 286 287 Humans have landed on the Moon during the Apollo program in the 1960s and 1970s 288 and will return to the Moon in the 2020s with the Artemis program 289 Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006 hence the demotion of Pluto to a dwarf planet 290 and further interest in trans Neptunian objects 291 See also nbsp solar system portal nbsp outer space portal nbsp astronomy portal Lists of geological features of the Solar System List of gravitationally rounded objects of the Solar System List of Solar System extremes List of Solar System objects by size Outline of the Solar System Planetary mnemonic Phrase used to remember the planets of the Solar SystemNotes a b 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 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 The International Astronomical Union s Minor Planet Center has yet to officially list Quaoar Sedna and Gonggong as dwarf planets as of 2024 For more classifications of Solar System objects see List of 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