Solar Planet

The Solar Syste is the gravitationally bound system of the Sun and the objects that orbit it. 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 a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere. Astronomers classify it as a G-type main-sequence star.

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 that the Solar System has at least nine dwarf planets: Ceres, Orcus, 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). Six planets, seven 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 from the Sun, 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 to a radius of 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 (269,000 AU) away. Both stars belong to the Milky Way galaxy.

Formation and evolution

Past

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

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.This initial cloud was likely several light-years across and probably birthed several stars. As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars.

As the pre-solar nebula collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings. As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi) and a hot, dense protostar at the center. The planets formed by accretion from this disc, 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.

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 these refractory materials only comprised a small fraction of the solar nebula, the terrestrial planets could not grow very large.

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.

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. As helium accumulates at its core, the Sun is growing brighter; early in its main-sequence life its brightness was 70% that of what it is today. 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. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.

Following the dissipation of the protoplanetary disk, the Nice model proposes that gravitational encounters between planetisimals and the gas giants caused each to migrate into different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period, the grand tack hypothesis suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the Late Heavy Bombardment of the inner planets.

Present and future

The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun. 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.

The Sun's 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. 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.

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well). Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf, half the original mass of the Sun but only the size of Earth. 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.

General characteristics

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

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 G-type main-sequence star star that contains 99.86% of the system's known mass and dominates it gravitationally.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.

The Sun is composed of roughly 98% hydrogen and helium, as are Jupiter and Saturn. 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. 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.

Orbits

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. Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All 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 discs of tiny particles that orbit them in unison.

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. There are exceptions, such as Halley's Comet. Most of the larger moons orbit their planets in prograde direction, matching the direction of planetary rotation; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner.Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects around the Sun.  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.[50]: 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,  but the planetary system can change chaotically over billions of years.

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. Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum. 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.

Distances and scales

The astronomical unit (AU; equal to 150,000,000 km; 93,000,000 mi) is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular. For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi). Thus, the Sun occupies 0.00001% (1 part in 107) 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.

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 and Johannes Kepler's model based on the Platonic solids,but ongoing discoveries have invalidated these hypotheses.

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. 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.

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.

Habitability

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.

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.Habitability might be possible in subsurface oceans of various outer Solar System moons.

Comparison with extrasolar systems

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

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.

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

Sun

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses), which comprises 99.86% of all the mass in the Solar System, produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium. This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[76][77]

Because the Sun fuses hydrogen 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 fusor stars in the Milky Way.[78]

The Sun is a population I star, having formed in the spiral arms of the Milky Way galaxy. It has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars in the galactic bulge and halo.[79] 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 formed from the accretion of "metals".[80]

The region of space dominated by the Solar magnetosphere is the heliosphere, which spans 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 spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[81] 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).[82]

Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[83] 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.[84] 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.[85][86]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[87] Composed mainly of silicates and metals,[88] 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.[42]

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.[89]

• 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.[90] In the past, Mercury was volcanically active, producing smooth basaltic plains similar to the Moon.[91] It is likely that Mercury has a silicate crust and a large iron core.[92][93] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms.[94] Mercury has no natural satellites.[95]

• 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.[96] Venus is the hottest planet, with surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.[97] The planet lacks a protective magnetic field to protect against stripping by the solar wind, which suggests that its atmosphere is sustained by volcanic activity.[98] Its surface displays extensive evidence of volcanic activity with stagnant lid tectonics.[99] Venus has no natural satellites.[100]

• Earth (0.98–1.02 AU)[D 6] is the only place in the universe where life and surface liquid water are known to exist.[101] Earth's atmosphere contains 78% nitrogen and 21% oxygen, which is the result of the presence of life.[102][103] The planet has a complex climate and weather system, with conditions differing drastically between climate regions.[104] The solid surface of Earth is dominated by green vegetation, deserts and white ice sheets.[105][106][107] Earth's surface is shaped by plate tectonics that formed the continental masses.[91] Earth's planetary magnetosphere shields the surface from radiation, limiting atmospheric stripping and maintaining life habitability.[108]

  • The Moon is Earth's only natural satellite.[109] Its diameter is one-quarter the size of Earth's.[110] Its surface is covered in very fine regolith and dominated by impact craters.[111][112] Large dark patches on the Moon, maria, are formed from past volcanic activity.[113] The Moon's atmosphere is extremely thin, consisting of a partial vacuum with particle densities of under 107 per cm−3.[114]

• Mars (1.38–1.67 AU)[D 6] has a radius about half of that of Earth.[115] Most of the planet is red due to iron oxide in Martian soil,[116] and the polar regions are covered in white ice caps made of water and carbon dioxide.[117] Mars has an atmosphere composed mostly of carbon dioxide, with surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.[118] Its surface is peppered with volcanoes and rift valleys, and has a rich collection of minerals.[119][120] Mars has a highly differentiated internal structure, and lost its magnetosphere 4 billion years ago.[121][122] Mars has two tiny moons:[123]

  • 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][124] In particular, Phobos's surface has a very large Stickney impact crater that is roughly 4.5 km (2.8 mi) in radius.[125]

  • 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, the surface of Deimos is noticeably smoother than Phobos because the regolith partially covers the impact craters.[126]

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.[127][128] They range from a few metres to hundreds of kilometres in size. Many 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 a torus-shaped region between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) from the Sun, which lies between the orbits of Mars and Jupiter. 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.[37] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[132]

The four largest asteroids: Ceres, Vesta, Pallas, Hygiea. Only Ceres and Vesta have been visited by a spacecraft and thus have a detailed picture.

Below are the descriptions of the 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 (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 cryovolcanic activity, where volatile material such as water are erupted onto the surface, 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]

• As of 2024, one asteroid has been discovered to orbit within Venus's orbit, 594913 ꞌAylóꞌchaxnim.[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 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.[18]