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The SOLAR SYSTEM is the gravitationally -bound system comprising the Sun
Sun
and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun
Sun
directly, the largest eight are the planets , with the remainder being significantly smaller objects, such as dwarf planets and small Solar System
Solar System
bodies . Of the objects that orbit the Sun
Sun
indirectly, the moons , two are larger than the smallest planet, Mercury .

The Solar System
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 the majority of the remaining mass contained in Jupiter
Jupiter
. The four smaller inner planets, Mercury , Venus
Venus
, Earth
Earth
and Mars
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
Jupiter
and Saturn
Saturn
, are gas giants , being composed mainly of hydrogen and helium ; the two outermost planets, Uranus
Uranus
and Neptune , are ice giants , being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles , such as water, ammonia and methane . All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic .

The Solar System
Solar System
also contains smaller objects. The asteroid belt , which lies between the orbits of Mars
Mars
and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt
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. Such objects are categorized as dwarf planets . Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto
Pluto
and Eris . In addition to these two regions, various other small-body populations, including comets , centaurs and interplanetary dust clouds , freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites , usually termed "moons" after the Moon . Each of the outer planets is encircled by planetary rings of dust and other small objects.

The solar wind , a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere . The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium ; it extends out to the edge of the scattered disc . The Oort cloud , which is thought to be the source for long-period comets , may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System
Solar System
is located in the Orion Arm , 26,000 light-years from the center of the Milky Way
Milky Way
.

CONTENTS

* 1 Discovery and exploration

* 2 Structure and composition

* 2.1 Distances and scales

* 3 Formation and evolution * 4 Sun
Sun
* 5 Interplanetary medium
Interplanetary medium

* 6 Inner Solar System
Solar System

* 6.1 Inner planets

* 6.1.1 Mercury * 6.1.2 Venus
Venus
* 6.1.3 Earth
Earth
* 6.1.4 Mars
Mars

* 6.2 Asteroid
Asteroid
belt

* 6.2.1 Ceres * 6.2.2 Asteroid
Asteroid
groups

* 7 Outer Solar System
Solar System

* 7.1 Outer planets

* 7.1.1 Jupiter
Jupiter
* 7.1.2 Saturn
Saturn
* 7.1.3 Uranus
Uranus
* 7.1.4 Neptune
Neptune

* 7.2 Centaurs

* 8 Comets

* 9 Trans-Neptunian region

* 9.1 Kuiper belt
Kuiper belt

* 9.1.1 Pluto
Pluto
and Charon * 9.1.2 Makemake
Makemake
and Haumea
Haumea

* 9.2 Scattered disc

* 9.2.1 Eris

* 10 Farthest regions

* 10.1 Heliosphere * 10.2 Detached objects * 10.3 Oort cloud * 10.4 Boundaries

* 11 Galactic context

* 11.1 Neighbourhood * 11.2 Comparison with extrasolar systems

* 12 Visual summary * 13 See also * 14 Notes * 15 References * 16 External links

DISCOVERY AND EXPLORATION

Main article: Discovery and exploration of the Solar System _ Andreas Cellarius 's illustration of the Copernican system, from the Harmonia Macrocosmica_ (1660)

For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the Late Middle Ages – Renaissance
Renaissance
believed Earth
Earth
to be stationary at the centre of the universe 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 to develop a mathematically predictive heliocentric system. In the 17th century, Galileo Galilei
Galileo Galilei
, Johannes Kepler
Johannes Kepler
, and Isaac Newton
Isaac Newton
developed an understanding of physics that led to the gradual acceptance of the idea that Earth
Earth
moves around the Sun
Sun
and that the planets are governed by the same physical laws that governed Earth. The invention of the telescope led to the discovery of further planets and moons. Improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena, such as mountains , craters , seasonal meteorological phenomena, such as clouds , dust storms and ice caps on the other planets.

STRUCTURE AND COMPOSITION

The principal component of the Solar System
Solar System
is the Sun, a G2 main-sequence 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
Jupiter
and Saturn
Saturn
together comprising more than 90%. The remaining objects of the Solar System
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.

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

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt
Kuiper belt
of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System
Solar System
includes the four terrestrial planets and the asteroid belt. The outer Solar System
Solar System
is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System
Solar System
are considered a distinct region consisting of the objects beyond Neptune. The eight planets of the Solar System
Solar System
(by decreasing size) are Jupiter
Jupiter
, Saturn
Saturn
, Uranus
Uranus
, Neptune
Neptune
, Earth
Earth
, Venus
Venus
, Mars
Mars
and Mercury .

Most of the planets in the Solar System
Solar System
have secondary systems of their own, being orbited by planetary objects called natural satellites , or moons (two of which, Titan and Ganymede , are larger than the planet Mercury ), and, in the case of the four giant planets, by planetary rings , thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation , with one face permanently turned toward their parent. _ All planets of the Solar System
Solar System
lie very close to the ecliptic . The closer they are to the Sun, the faster they travel (inner planets on the left, all planets except Neptune
Neptune
on the right)_.

Kepler\'s laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun
Sun
at one focus . Objects closer to the Sun
Sun
(with smaller semi-major axes ) travel more quickly because they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun
Sun
varies over the course of its year. A body's closest approach to the Sun
Sun
is called its _perihelion _, whereas its most distant point from the Sun
Sun
is called its _aphelion _. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt
Kuiper belt
objects follow highly elliptical orbits. The positions of the bodies in the Solar System
Solar System
can be predicted using numerical models .

Although the Sun
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.

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. Jupiter
Jupiter
and Saturn
Saturn
, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium. A composition gradient exists in the Solar System, created by heat and light pressure from the 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
Sun
are composed largely of materials with lower melting points. The boundary in the Solar System
Solar System
beyond which those volatile substances could condense is known as the frost line , and it lies at roughly 5 AU from the Sun.

The objects of the inner Solar System
Solar System
are composed mostly of rock, the collective name for compounds with high melting points, such as silicates , iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula . Jupiter
Jupiter
and Saturn
Saturn
are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure , such as hydrogen , helium , and neon , which were always in the gaseous phase in the nebula. Ices, like water , methane , ammonia , hydrogen sulfide , and carbon dioxide , have melting points up to a few hundred kelvins. They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus
Uranus
and Neptune
Neptune
(the so-called "ice giants ") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as _volatiles _.

DISTANCES AND SCALES

The distance from Earth
Earth
to the Sun
Sun
is 1 astronomical unit (150,000,000 km ), or AU. For comparison, the radius of the Sun
Sun
is 0.0047 AU (700,000 km). Thus, the Sun
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) from the Sun
Sun
and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km) 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 nearer object to the Sun. For example, Venus
Venus
is approximately 0.33 AU farther out from the Sun
Sun
than Mercury, whereas Saturn
Saturn
is 4.3 AU out from Jupiter, and Neptune
Neptune
lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law ), but no such theory has been accepted. The images at the beginning of this section show the orbits of the various constituents of the Solar System
Solar System
on different scales.

Some Solar System
Solar System
models attempt to convey the relative scales involved in the Solar System
Solar System
on 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) Ericsson Globe in Stockholm
Stockholm
as its substitute Sun, and, following the scale, Jupiter
Jupiter
is a 7.5-metre (25-foot) sphere at Arlanda International 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
Neptune
distance is scaled to 100 metres, then the Sun would be about 3 cm in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm, and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm) at this scale. The Solar System. Distances are to scale, objects are not.

Distances of selected bodies of the Solar System
Solar System
from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity . The radius of the Sun
Sun
is 0.7 million km, and the radius of Jupiter
Jupiter
(the largest planet) is 0.07 million km, both too small to resolve on this image.

FORMATION AND EVOLUTION

Main article: Formation and evolution of the Solar System

The Solar System
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 region that would become the Solar 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 Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies . Artist's concept of the early Solar System
Solar System

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System
Solar System
close to the Sun, and these would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars
Mars
and Jupiter
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
Kuiper belt
, and Oort cloud . The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.

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

The Solar System
Solar System
will remain roughly as we know it today until the hydrogen in the core of the Sun
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 this time, the core of the Sun will collapse, and the energy output will be much greater than at present. The outer layers of the Sun
Sun
will expand to roughly 260 times its current diameter, and the Sun
Sun
will become a red giant . Because of its vastly increased surface area, the surface of the Sun
Sun
will be considerably cooler (2,600 K at its coolest) than it is on the main sequence. The expanding Sun
Sun
is expected to vaporize Mercury and render Earth
Earth
uninhabitable. Eventually, the core will be hot enough for helium fusion; the Sun
Sun
will burn helium for a fraction of the time it burned hydrogen in the core. The Sun
Sun
is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will move away into space, leaving a white dwarf , an extraordinarily dense object, half the original mass of the Sun
Sun
but only the size of Earth. The ejected outer layers will form what is known as a planetary nebula , returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.

SUN

Main article: Sun
Sun
Size comparison of the Sun
Sun
and the planets

The Sun
Sun
is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth
Earth
masses) produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium , making it a main-sequence star. This releases an enormous amount of energy , mostly radiated into space as electromagnetic radiation peaking in visible light .

The Sun
Sun
is a G2-type main-sequence star . Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun
Sun
are rare, whereas substantially dimmer and cooler stars, known as red dwarfs , make up 85% of the stars in the Milky Way.

The Sun
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. 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 high 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".

INTERPLANETARY MEDIUM

Main articles: Interplanetary medium
Interplanetary medium
and Solar wind The heliospheric current sheet

The vast majority of the Solar System
Solar System
consists of a near-vacuum known as the interplanetary medium . Along with light , the Sun
Sun
radiates a continuous stream of charged particles (a plasma ) known as the solar wind . This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU _(see § Heliosphere )_. Activity on the Sun's surface, such as solar flares and coronal mass ejections , disturb the heliosphere, creating space weather and causing geomagnetic storms . The largest structure within the heliosphere is the heliospheric current sheet , a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.

Earth\'s magnetic field stops its atmosphere from being stripped away by the solar wind. Venus
Venus
and Mars
Mars
do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space. 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 .

The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System
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
Solar System
varies, though by how much is unknown.

The interplanetary medium is home to at least two disc-like regions of cosmic dust . The first, the zodiacal dust cloud , lies in the inner Solar System
Solar System
and causes the zodiacal light . It was likely formed by collisions within the asteroid belt brought on by gravitational interactions with the planets. The second dust cloud extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt
Kuiper belt
.

INNER SOLAR SYSTEM

The INNER SOLAR SYSTEM is the region comprising the terrestrial planets and the asteroid belt . Composed mainly of silicates and metals, the objects of the inner Solar System
Solar System
are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter
Jupiter
and Saturn. This region is also within the frost line , which is a little less than 5 AU (about 700 million km) from the Sun.

INNER PLANETS

Main article: Terrestrial planet The inner planets. From left to right: Earth
Earth
, Mars
Mars
, Venus
Venus
, and Mercury (sizes to scale).

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
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
Sun
than Earth
Earth
is (i.e. Mercury and Venus).

Mercury

Main article: Mercury (planet) Mercury (0.4 AU from the Sun) is the closest planet to the Sun
Sun
and the smallest planet in the Solar System (0.055 Earth
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
Venus
Venus
(0.7 AU from the Sun) is close in size to Earth
Earth
(0.815 Earth
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
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
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, and the only place where life is known to exist. 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
Moon
, the only large satellite of a terrestrial planet in the Solar System.

Mars

Main article: Mars
Mars
Mars
Mars
(1.5 AU from the Sun) is smaller than Earth and Venus
Venus
(0.107 Earth
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

Main article: Asteroid
Asteroid
belt The donut-shaped asteroid belt is located between the orbits of Mars
Mars
and Jupiter
Jupiter
.

Sun
Sun
Jupiter
Jupiter
trojans Planetary orbit Asteroid
Asteroid
belt Hilda asteroids NEOs (selection)

Asteroids except for the largest, Ceres, are classified as small Solar System
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
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 (dwarf planet) 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.

Asteroid
Asteroid
Groups

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

OUTER SOLAR SYSTEM

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

OUTER PLANETS

Main articles: Outer planets and Giant planet From top to bottom: Neptune
Neptune
, Uranus
Uranus
, Saturn
Saturn
, and Jupiter
Jupiter
(Montage with approximate colour and relative size)

The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter
Jupiter
and Saturn
Saturn
are together over 400 times the mass of Earth
Earth
and consist overwhelmingly of hydrogen and helium; Uranus
Uranus
and Neptune