Sol

**Sol** (also known as the Solar System) is a star system located in the Solar Region, consis

   ting of the star Sol and 8 planets (not including several dwarf planets).

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity.

Throughout the Milky Way galaxy, the Sol System is known for being the native system of the Human race.

## Inner Sol

### Inner Planets

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

#### Mercury

*Main Article: Mercury*

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites; besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. Mercury's very tenuous 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. Hypotheses include that its outer layers were stripped off by a giant impact; or, that it was prevented from fully accreting by the young Sun's energy.

#### Venus

*Main Article: Venus*

Venus (0.7 AU from the Sun) 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. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752°F), 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, which suggests that its atmosphere is being
replenished by volcanic eruptions.

#### Earth

*Main Article: Earth*

Earth

(1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity. Its liquid hydrosphere is unique among the terrestrial planets, and it is 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, the Moon, the only large satellite of a terrestrial planet in the Solar System.

#### Mars

*Main Article: Mars*

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth). 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

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of refractory rocky and metallic minerals, with some ice. 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), depending on different, somewhat arbitrary definitions.

The asteroid belt occupies the orbit between Mars and Jupiter,
between 2.3 and 3.3 AU 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. 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 Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.

#### Ceres

*Main Article: Ceres*

Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. It has a diameter of slightly under 1,000 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 1801, and was reclassified
 to asteroid in the 1850s as further observations revealed additional
asteroids. It was classified as a dwarf planet in 2006 when the definition of a planet was created.

## Outer Sol

### Outer Planets

The four outer planets, or giant planets (sometimes called Jovian
planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together over 400 times the mass of Earth and
consist overwhelmingly of hydrogen and helium; Uranus and Neptune are
far less massive (<20 Earth masses each) and are composed primarily
of ices. For these reasons, some astronomers suggest they belong in
their own category, "ice giants". All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term *superior planet* designates planets outside Earth's orbit and thus includes both the outer planets and Mars.

#### Jupiter

*Main Article: Jupiter*

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 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 Solar System, is larger than Mercury.

#### Saturn

*Main Article: Saturn*

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. Saturn
is the only planet of the Solar System that is less dense than water. The rings of Saturn are made up of small ice and rock particles. Saturn has 62 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity. Titan, the second-largest moon in the Solar System, is larger than
Mercury and the only satellite in the Solar System with a substantial
atmosphere.

#### Uranus

*Main Article: Uranus*

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

#### Neptune

*Main Article: Neptune*

Neptune (30.1 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and hence more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has 14 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 several minor planets, termed Neptune trojans, that are in 1:1 resonance with it.

## 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.

#### Kuiper Belt

The Kuiper belt is a great ring of debris similar to the asteroid belt,
but consisting mainly of objects composed primarily of ice. It extends between 30 and 50 AU from the Sun. Though it is estimated to
contain anything from dozens to thousands of dwarf planets, it is
composed mainly of small Solar System bodies. Many of the larger Kuiper
belt objects, such as Quaoar, Varuna, and Orcus,
 may prove to be dwarf planets with further data. 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 Earth.

#### Pluto and Charon

*Main Article: Pluto*

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 the Sun at perihelion
(within the orbit of Neptune) to 49.5 AU at aphelion. Pluto has a 3:2 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. Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit within the system.

#### Makemake and Haumea

*Main Article: Makemake and Haumea*

Makemake (45.79 AU average), 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. It was named and designated a dwarf planet in 2008.

Haumea (43.13 AU average) is in an orbit similar to Makemake except that it is 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.

#### Scattered Disc

The scattered disc, which overlaps the Kuiper belt but extends much
further outwards, is thought to be the source of short-period comets.
Scattered-disc objects are thought to have been ejected into erratic
orbits by the gravitational influence of Neptune's early outward migration.
 Most scattered disc objects (SDOs) have perihelia within the Kuiper
belt but aphelia far beyond it (some more than 150 AU from the Sun).
SDOs' orbits are also highly inclined to the ecliptic plane and are
often almost perpendicular to it. 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".[109]
 Some astronomers also classify centaurs as inward-scattered Kuiper belt
 objects along with the outward-scattered residents of the scattered
disc.

#### Eris

*Main Article: Eris*

Eris
 (68 AU average) is the largest known scattered disc object, and caused a
 debate about what constitutes a planet, because 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 the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.