Category:Sol

The Sol-system consists of Sol and its planetary system of eight planets, their moons, and other non-stellar objects that formed 4.6-billion years ago from the collapse of a giant molecular cloud.

The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets Jupiter, Saturn, Uranus and Neptune, called the gas giants, are substantially more massive than the terrestrials.

The Sol-system is located within the outer Orion–Cygnus Arm of Milky Way.

Structure and composition
The principal component of the Sol-system is Sol, a G2 main-sequence star that contains 99.86 percent of the system's known mass and dominates it gravitationally.

Sol's four largest orbiting bodies, the gas giants, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90%.

Most large objects in orbit around Sol lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper Belt objects are frequently at significantly greater angles to it. All the planets and most other objects orbit Sol in the same direction that Sol is rotating (counter-clockwise, as viewed from above Sol's north pole).

Sol Regions
The overall structure of the charted regions of the Sol-system consists of Sol, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the Kuiper Belt of icy objects.

Astronomers sometimes informally divide this structure into separate regions.

The inner Sol-system includes the four terrestrial planets and the asteroid belt. The outer Sol-system is beyond the asteroids, including the four gas giants. Since the discovery of the Kuiper belt, the outermost parts of the Sol-system are considered a distinct region consisting of the objects beyond Neptune.

Formation & Evolution
The Sol-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 region that would become the Sol-system, known as the pre-solar nebula, collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre.

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 Sol-system, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies.

Due to their higher boiling points, only metals and silicates could exist in the warm inner Sol-system close to Sol, and these would form the rocky planets of Mercury, Venus, Earth, and Mars. Since metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

The gas giants (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.

Sol
Sol has the largest mass in the system (332,900 Earth masses) and produces temperatures and densities in its core high enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400–700 nm band of visible light.

Sol is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, Sol is rather large and bright. Evidence suggests that Sol's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion.

Sol is a population I star; it was born in the later stages of the universe's evolution, and thus contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars.

Inner Sol-system
The inner Sol-system is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Sol-system are relatively close to Sol; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

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



Mercury
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Mercury (0.4 AU from Sol) is the closest planet to Sol and the smallest planet in the Sol-system (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history. Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.

A handful of solar power stations exist on "peaks of eternal light" at the north and south poles of Mercury. The difficulties imposed by the planet's proximity to the sun and high orbital velocity have limited development.



Venus
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Venus (0.7 AU from Sol) is close in size to Earth (0.815 Earth masses), and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is 90 times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, its atmosphere is regularly replenished by volcanic eruptions.



Earth
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Read More: Luna

Earth (1 AU from Sol) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the Sol-system where life is known to exist. Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.

It has one natural satellite, Luna, is the only large satellite of a terrestrial planet in the Sol-system. An early source of helium-3, Luna is now mined for materials used in space habitat construction. Two dozen major stations have been constructed at Earth's L4 and L5 Lagrange points, all from lunar resources.



Mars
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Mars (1.5 AU from Sol) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% that of the Earth's). Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.

Asteroid Belt
Read More: Sol Asteroid Belt The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from Sol. It is thought to be remnants from the Sol-system's formation that failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids except the largest, Ceres, are classified as small Sol-system bodies. The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10−4 m are called meteoroids.

Ceres
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Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observations revealed additional asteroids. It was classified as a dwarf planet in 2006.

Vesta
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Vesta, minor-planet designation 4 Vesta, is one of the largest asteroids in the Sol-system, with a mean diameter of 525-kilometres. Vesta is the second-most-massive asteroid after the dwarf planet Ceres, and comprises an estimated 9% of the mass of the asteroid belt.

Hygiea
Read More: New Tencton

10 Hygiea or New Tencton is the fourth largest asteroid in the Solar System by volume and mass and is located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the largest of the class of dark C-type asteroids with a carbonaceous surface.

Asteroid Groups
Read More: Trojan Asteroids

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.

Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around Sol three times for every two Jupiter orbits.

The inner Sol-system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Outer Sol-system
The outer region of the Sol-system is home to the gas giants and their large moons. Many short-period comets, including the centaurs, also orbit in this region. Due to their greater distance from Sol, the solid objects in the outer Sol-system contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Sol-system, as the colder temperatures allow these compounds to remain solid.

Outer Planets and Gas Giants
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit Sol. Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, "ice giants". All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.



Jupiter
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Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 67 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Sol-system, is larger than Mercury.



Saturn
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Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Sol-system. The rings of Saturn are made up of small ice and rock particles. Saturn has 62 confirmed satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, the second-largest moon in the Sol-system, is larger than Mercury and the only satellite in the Sol-system with a substantial atmosphere.

Uranus
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Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits Sol on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space. Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune
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Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has 13 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune trojans, that are in 1:1 resonance with it.

Trans-Neptunian Region
The area beyond Neptune, or the "trans-Neptunian region", is still largely undeveloped or explored. It consists overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Luna) composed mainly of rock and ice. This region is sometimes known as the "outer Sol-system", though others use that term to mean the region beyond the asteroid belt.

Kuiper Belt
The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from Sol. Though it is thought to contain dozens of dwarf planets, it is composed mainly of small Sol-system bodies.

Many of the larger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may be recognized as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.

The Kuiper belt can be roughly divided into the "classical" belt and the resonances. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, 1992 QB1, and are still in near primordial, low-eccentricity orbits.

Pluto Dwarf Group
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Read More: Charon

These eight trans-Neptunian objects have the brightest absolute magnitudes, although several other TNOs have been found to be physically larger than Orcus, and several more may yet be found. The dwarf planet Pluto (39 AU average) is the largest known object in the Kuiper belt.

When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from Sol at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Charon, Pluto's largest moon, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e., they appear to "orbit each other").

Beyond Charon, four much smaller moons, P5, Nix, P4, and Hydra are known to orbit within the system. Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round Sol for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.

Makemake & Haumea
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Read More: Haumea

Makemake (45.79 AU average), while smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, it is not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was named and designated a dwarf planet in 2008. Its orbit is far more inclined than Pluto's, at 29°. Haumea (43.13 AU average) is in an orbit similar to Makemake except that it is caught in a 7:12 orbital resonance with Neptune. It is about the same size as Makemake and has two natural satellites. A rapid, 3.9-hour rotation gives it a flattened and elongated shape. It was named and designated a dwarf planet in 2008.

Eris
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Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is 25% more massive than Pluto 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 Sol) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

Oort Cloud
The Oort cloud is a spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Sol-system at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is believed to be composed of comets that were ejected from the inner Sol-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.

Location
The Sol-system is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars. Sol resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm.

Sol lies between 25,000 and 28,000 light-years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225–250 million years. This revolution is known as the Sol-system's galactic year.

Neighbourhood
The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sol-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years.

The stars next closest to Sol are the red dwarfs Barnard's Star (at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright main-sequence star roughly twice Sol's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away.

The remaining systems within 10 light years are the binary red-dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).

The Sol-system's closest solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent Sol's mass, but only 60 percent of its luminosity.