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{{dablink|This article is about the Sun and its planetary system. For other systems, see [[Planetary system]] and [[Star system]]. For a list of the physical attributes of the Solar System's largest objects (radius, mass, orbital distance etc.) see [[List of Solar System objects in hydrostatic equilibrium]]}}
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[[Image:Planets2008.jpg|right|390px|thumb|[[Planet]]s and [[dwarf planet]]s of the Solar System. Sizes are to scale, but relative distances from the Sun are not.]]
The '''Solar System'''{{Ref label|A|a|none}} consists of the [[Sun]] and those [[Astronomical object|celestial objects]] bound to it by [[gravity]], all of which formed from the collapse of a [[giant molecular cloud]] approximately 4.6 billion years ago. The Sun's retinue of objects circle it in a nearly flat disc called the [[plane of the ecliptic|ecliptic plane]], most of the [[mass]] of which is contained within eight relatively solitary [[planets]] whose [[orbit]]s are almost circular. The four smaller inner planets; [[Mercury (planet)|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]], also called the [[gas giants]], are composed largely of hydrogen and helium and are far more massive than the terrestrials.  

The Solar System is also home to two main belts of [[small solar system body|small bodies]]. The [[asteroid belt]], which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. The [[Kuiper belt]] (and its subpopulation, the [[scattered disc]]), which lies beyond Neptune's orbit, is composed mostly of ices such as water, ammonia and methane. Within these belts, five individual objects, [[Ceres (dwarf planet)|Ceres]], [[Pluto]], [[Haumea (dwarf planet)|Haumea]], [[Makemake (dwarf planet)|Makemake]] and [[Eris (dwarf planet)|Eris]], are recognised to be large enough to have been rounded by their own gravity, and are thus termed [[dwarf planets]]. The hypothetical [[Oort cloud]], which acts as the source for [[long-period comet]]s, may also exist at a distance roughly a thousand times beyond these regions.

Within the Solar System, various populations of small bodies, such as [[comet]]s, [[Centaur (minor planet)|centaurs]] and [[interplanetary dust]], freely travel between these regions, while the [[solar wind]], a flow of [[plasma (physics)|plasma]] from the Sun, creates a [[stellar wind bubble|bubble]] in the [[interstellar medium]] known as the [[heliosphere]], which extends out to the edge of the scattered disc.

Six of the planets and three of the dwarf planets are orbited by [[natural satellite]]s, usually termed "moons" after Earth's [[Moon]]. Each of the outer planets is encircled by [[planetary ring]]s of dust and other particles.
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==Discovery and exploration==
{{main|Discovery and exploration of the Solar System}}
For many thousands of years, humanity, with a few notable exceptions, did not recognise the existence of the Solar System. They believed the 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 [[Indian astronomy|Indian]] mathematician-astronomer [[Aryabhata]] and the Greek philosopher [[Aristarchus of Samos]] had speculated on a heliocentric reordering of the cosmos,<ref>{{cite journal|title= The astronomical system of Copernicus|author=WC Rufus|journal=Popular Astronomy|Volume=31|pages=510|url=http://adsabs.harvard.edu/full/1923PA.....31..510R|accessdate=2009-05--09}}</ref> [[Nicolaus Copernicus]] was the first to develop a mathematically predictive heliocentric system. His 17th-century successors [[Galileo Galilei]], [[Johannes Kepler]], and [[Isaac Newton]] developed an understanding of [[physics]] which led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times, improvements in the telescope and the use of [[unmanned spacecraft]] have enabled the investigation of geological phenomena such as [[mountains]] and [[craters]] and seasonal meteorological phenomena such as [[clouds]], [[dust storms]] and [[ice caps]] on the other planets.

==Structure==
<gallery widths="170px" heights="170px" perrow="2" align="right">
File:Sun vs planets.png|The mass of the Sun compared to the remaining bodies of the Solar System. Objects smaller than Saturn are not visible at this scale.
File:Masses of the planets.png|The relative masses of the Solar planets. Jupiter at 71% of the total and Saturn at 21% dominate the system. Mercury and Mars, which together are less than 0.1%, are not visible at this scale.
</gallery>
[[Image:Oort cloud Sedna orbit.svg|thumb|400px|The [[orbit]]s of the bodies in the Solar System to scale (clockwise from top left)]]
The principal component of the Solar System is the Sun, a [[main sequence]] [[stellar classification|G2]] [[star]] that contains 99.86 percent of the system's known mass and dominates it gravitationally.<ref>{{cite journal |author=M Woolfson |title=The origin and evolution of the solar system |doi= 10.1046/j.1468-4004.2000.00012.x |year=2000 |journal=Astronomy & Geophysics |volume=41 |pages=1.12}}</ref> The Sun'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 percent.{{Ref label|C|c|none}}

Most large objects in orbit around the Sun 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.<ref>{{cite web|title=The formation of the Kuiper belt by the outward transport of bodies during Neptune’s migration|author=Harold F. Levison, Alessandro Morbidelli|url=http://www.obs-nice.fr/morby/stuff/NATURE.pdf|format=PDF|year=2003|accessdate=2007-06-25}}</ref><ref>{{cite journal|title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets|author=Harold F. Levison, Martin J Duncan|journal=Icarus
issue=1|year=1997|pages=13–32|doi=10.1006/icar.1996.5637 |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45M91DF-24&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=6fa927eab9338038f6678e6fd538d2f5|accessdate=2008-07-18|volume=127}}</ref>

All of the planets and most other objects also orbit with the Sun's rotation (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as [[Halley's Comet]].

[[Kepler's laws of planetary motion]] describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an [[ellipse]] with the Sun at one [[focus (geometry)|focus]]. Objects closer to the Sun (with smaller [[semi-major axis|semi-major axes]]) have shorter [[year]]s. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its ''[[perihelion]]'', while its most distant point from the Sun is called its ''[[aphelion]]''. Each body moves fastest at its perihelion and slowest at its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits.

To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33&nbsp;[[astronomical unit]]s (AU){{Ref label|D|d|none}} farther out than Mercury, while Saturn is 4.3&nbsp;AU out from Jupiter, and Neptune lies 10.5&nbsp;AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see [[Titius-Bode law]]),<ref>{{cite web|title=Dawn: A Journey to the Beginning of the Solar System|work=Space Physics Center: UCLA|url=http://www-ssc.igpp.ucla.edu/dawn/background.html|year=2005|accessdate=2007-11-03}}</ref> but no such theory has been accepted.

Most of the planets in the Solar System possess secondary systems of their own. Many are in turn orbited by planetary objects called [[natural satellite]]s, or moons, some of which are larger than planets. Most of the largest natural satellites are in [[synchronous rotation]], with one face permanently turned toward their parent. The four largest planets, the [[gas giant]]s, also possess [[planetary ring]]s, thin bands of tiny particles that orbit them in unison.

==Terminology==
Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets.<ref>{{cite web |title=An Overview of the Solar System |author=nineplanets.org |url=http://www.nineplanets.org/overview.html |accessdate=2007-02-15}}</ref> 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.<ref>{{cite web |title=New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt |author=Amir Alexander |work=The Planetary Society |year=2006 |url=http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html |accessdate=2006-11-08}}</ref>

Dynamically and physically, objects orbiting the Sun are officially classed into three categories: ''planets'', ''dwarf planets'' and ''small Solar System bodies''. A [[planet]] is any body in orbit around the Sun that has enough mass to form itself into a [[sphere|spherical]] shape and has [[Cleared the neighbourhood|cleared its immediate neighbourhood]] of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto does not fit this definition, as it has not cleared its orbit of surrounding Kuiper belt objects.<ref name="FinalResolution"/> A [[dwarf planet]] is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of [[planetesimal]]s and is not a satellite.<ref name="FinalResolution"/> By this definition, the Solar System has five known dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.<ref name=name>{{cite web|date=2008-11-07 <!--11:42:58-->|title=Dwarf Planets and their Systems|work= Working Group for Planetary System Nomenclature (WGPSN) |url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets| accessdate=2008-07-13 | publisher= U.S. Geological Survey }}</ref> Other objects may be classified in the future as dwarf planets, such as [[90377 Sedna|Sedna]], [[90482 Orcus|Orcus]], and [[50000 Quaoar|Quaoar]].<ref>{{cite web|title=IAU Planet Definition Committee|author=Ron Ekers|publisher=International Astronomical Union|url=http://www.iau.org/public_press/news/release/iau0601/newspaper/|accessdate=2008-10-13}}</ref> Dwarf planets that orbit in the trans-Neptunian region are called "[[plutoid]]s".<ref name="IAU0804">{{cite web
  |date=June 11, 2008, Paris
  |title=Plutoid chosen as name for Solar System objects like Pluto
  |publisher=[[International Astronomical Union]] (News Release - IAU0804)
  |url=http://www.iau.org/public_press/news/release/iau0804
  |accessdate=2008-06-11}}</ref> The remainder of the objects in orbit around the Sun are [[small Solar System body|small Solar System bodies]].<ref name="FinalResolution">{{cite news |title=The Final IAU Resolution on the definition of "planet" ready for voting |publisher=IAU |date=2006-08-24 |url=http://www.iau.org/iau0602.423.0.html |accessdate=2007-03-02}}</ref>
[[Image:Solarsys.svg|400px|thumb|left|The regions (or zones) of the Solar system: the ''inner solar system'', the ''asteroid belt'', the ''giant planets'' (Jovians) and the ''Kuiper belt''. Sizes and orbits not to scale, view is tilted.]]

Planetary scientists use the terms ''gas'', ''ice'', and ''rock'' to describe the various classes of substances found throughout the Solar System.<ref name=Podolak2000>{{cite journal|last= Podolak|first=M.|coauthors=Podolak, J.I.; Marley, M.S.|title=Further investigations of random models of Uranus and Neptune |journal=Planet. Space Sci.|volume=48|pages=143&ndash;151|year=2000| url=http://adsabs.harvard.edu/abs/2000P%26SS...48..143P|doi=10.1016/S0032-0633(99)00088-4}}</ref> ''Rock'' is used to describe compounds with high [[melting point]]s that remained solid under almost all conditions in the [[protoplanetary nebula]].<ref name=Podolak2000/> Rocky substances typically include [[silicates]] and metals such as iron and nickel.<ref name=Podolak1995>{{cite journal|last=Podolak|first=M.|coauthors=Weizman, A.; Marley, M.|title=Comparative models of Uranus and Neptune|journal=Planet. Space Sci.|volume=43|issue=12|pages=1517&ndash;1522|year=1995| url=http://adsabs.harvard.edu/abs/1995P%26SS...43.1517P|doi=10.1016/0032-0633(95)00061-5}}</ref> They are prevalent in the inner Solar System, forming most of the terrestrial planets and [[asteroid]]s. ''Gases'' are materials with extremely low melting points and high [[vapor pressure]] such as [[molecular hydrogen]], [[helium]], and [[neon]], which were always in the gaseous phase in the nebula.<ref name=Podolak2000/> They dominate the middle region, comprising most of Jupiter and Saturn. ''Ices'', like [[water]], [[methane]], [[ammonia]], [[hydrogen sulfide]] and [[carbon dioxide]],<ref name=Podolak1995/> have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.<ref name=Podolak2000/> They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.<ref name=Podolak2000/> Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "[[ice giant]]s") and the numerous small objects that lie beyond Neptune's orbit.<ref name=Podolak1995/><ref name=zeilik>{{cite book | pages=240 | author=Michael Zellik| title=Astronomy: The Evolving Universe | edition=9th | year=2002 | publisher=Cambridge University Press | isbn=0521800900 | oclc=223304585 46685453}}</ref> Together, gases and ices are referred to as ''[[volatiles]]''. 
{{-}}

==Sun==
{{main|Sun}}
[[Image:Sun in X-Ray.png|thumb|right|The [[Sun]] as seen in the [[x-ray]] region of the [[electromagnetic spectrum]]]]

The Sun is the Solar System's star, and far and away its chief component. Its large mass (332,900 Earth masses)<ref>{{cite web|title=Sun: Facts & Figures|publisher=NASA|url=http://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric|accessdate=2009-05-14}}</ref> gives it an interior [[density]] high enough to sustain [[nuclear fusion]], which releases enormous amounts of [[energy]], mostly [[radiant energy|radiated]] into [[outer space|space]] as [[electromagnetic radiation]], peaking in the 400&ndash;to&ndash;700&nbsp;nm band we call [[visible light]].<ref>{{cite web|title=Why is visible light visible, but not other parts of the spectrum?|publisher=The Straight Dome|year=2003|url=http://www.straightdope.com/columns/read/2085/why-is-visible-light-visible-but-not-other-parts-of-the-spectrum|accessdate=2009-05-14}}</ref>

The Sun is classified as a moderately large [[yellow dwarf]], but this name is misleading as, compared to majority of stars in [[Milky Way|our galaxy]], the Sun is rather large and bright.<ref name=sun>{{cite news |first=Ker |last=Than |title=Astronomers Had it Wrong: Most Stars are Single |publisher=SPACE.com |date=January 30, 2006 |url=http://www.space.com/scienceastronomy/060130_mm_single_stars.html |accessdate=2007-08-01}}</ref> Stars are classified by the [[Hertzsprung-Russell diagram]], a graph which plots the brightness of stars against their surface [[temperature]]s. Generally, hotter stars are brighter. Stars following this pattern are said to be on the [[main sequence]], and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as [[red dwarf]]s, are common, making up 85 percent of the stars in the galaxy.<ref name=sun/><ref>{{cite web |year=2001 |author=Smart, R. L.; Carollo, D.; Lattanzi, M. G.; McLean, B.; Spagna, A. |title=The Second Guide Star Catalogue and Cool Stars |work=Perkins Observatory |url=http://adsabs.harvard.edu/abs/2001udns.conf..119S |accessdate=2006-12-26}}</ref>

It is believed that the Sun'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. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.<ref>{{cite journal|title=Towards a Solution to the Early Faint Sun Paradox: A Lower Cosmic Ray Flux from a Stronger Solar Wind|author=Nir J. Shaviv|journal=Journal of Geophysical Research|doi=10.1029/2003JA009997|url=http://arxiv.org/abs/astroph/0306477v2|accessdate=2009-01-26|year=2003|volume=108|pages=1437}}</ref>

The Sun is a [[metallicity#Population I stars|population I star]]; it was born in the later stages of the [[Universe#Evolution|universe's evolution]], and thus contains more elements heavier than hydrogen and helium ("[[metallicity|metals]]" in astronomical parlance) than older population II stars.<ref>{{cite journal |author=T. S. van Albada, Norman Baker |title=On the Two Oosterhoff Groups of Globular Clusters |journal=Astrophysical Journal |volume=185 |year=1973 |pages=477–498 |doi=10.1086/152434}}</ref> Elements heavier than hydrogen and helium were formed in the [[solar core|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, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a [[planetary system]], because planets form from accretion of metals.<ref> {{cite web |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |author=Charles H. Lineweaver |work=University of New South Wales |date=2001-03-09 |url=http://arxiv.org/abs/astro-ph/0012399 |accessdate=2006-07-23}}</ref>
[[Image:Heliospheric-current-sheet.gif|left|thumb|The [[heliospheric current sheet]].]]

===Interplanetary medium===
{{main|Interplanetary medium}}

Along with [[Sunlight|light]], the 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,<ref>{{cite web |title=Solar Physics: The Solar Wind |work=Marshall Space Flight Center |date=2006-07-16<!--Internet Archive estimate--> |url=http://solarscience.msfc.nasa.gov/SolarWind.shtml |accessdate=2006-10-03}}</ref> creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100&nbsp;AU (see [[#Heliopause|heliopause]]).<ref name="Voyager"/> This is known as the [[interplanetary medium]]. [[Geomagnetic storm]]s on the Sun's surface, such as [[solar flare]]s and [[coronal mass ejection]]s, disturb the heliosphere, creating [[space weather]].<ref name="SunFlip">{{cite web |url=http://science.nasa.gov/headlines/y2001/ast15feb_1.htm |title=The Sun Does a Flip |accessdate=2007-02-04 |last=Phillips |first=Tony |date=2001-02-15 |work=Science@NASA}}</ref> 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.<ref>[http://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm A Star with two North Poles], April 22, 2003, Science @ NASA</ref><ref>Riley, Pete; Linker, J. A.; Mikić, Z., "[http://adsabs.harvard.edu/abs/2002JGRA.107g.SSH8R Modeling the heliospheric current sheet: Solar cycle variations]", (2002) ''Journal of Geophysical Research'' (Space Physics), Volume 107, Issue A7, pp. SSH 8-1, CiteID 1136, DOI 10.1029/2001JA000299. ([http://ulysses.jpl.nasa.gov/science/monthly_highlights/2002-July-2001JA000299.pdf Full text])</ref>
[[Image:Aurora-SpaceShuttle-EO.jpg|thumb|right|[[Aurora australis]] seen from orbit.]]

[[Earth's magnetic field]] stops [[Earth's atmosphere|its atmosphere]] from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space.<ref>{{cite science |last=Lundin |first=Richard |date=2001-03-09 |title=Erosion by the Solar Wind |author=Rickard Lundin |journal=Science |volume=291 |issue=5510 |pages=1909 |doi=10.1126/science.1059763 |url=http://sciencemag.org/cgi/content/full/291/5510/1909 |accessdate=2006-12-26|abstract=http://sciencemag.org/cgi/content/summary/291/5510/1909}}</ref> The interaction of the solar wind with Earth's magnetic field funnels charged particles at right angles to the Earth's upper atmosphere, where its interactions create [[Aurora (astronomy)|aurorae]] seen near the [[Earth's magnetic field#Magnetic poles|magnetic poles]].

[[Cosmic rays]] originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. 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 radiation in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{cite journal |last=Langner |first=U. W. |coauthors=M.S. Potgieter |year=2005 |title=Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays |journal=Advances in Space Research |volume=35 |issue=12 |pages=2084–2090 |doi=10.1016/j.asr.2004.12.005 |url=http://adsabs.harvard.edu/abs/2005AdSpR..35.2084L |accessdate=2007-02-11}}</ref>

The interplanetary medium is home to at least two disc-like regions of [[cosmic dust]]. The first, the [[interplanetary dust cloud|zodiacal dust cloud]], lies in the inner Solar System and causes [[zodiacal light]]. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.<ref>{{cite web |year=1998 |title=Long-term Evolution of the Zodiacal Cloud |url=http://astrobiology.arc.nasa.gov/workshops/1997/zodiac/backman/IIIc.html |accessdate=2007-02-03}}</ref> The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the [[Kuiper belt]].<ref>{{cite web |year=2003 |title=ESA scientist discovers a way to shortlist stars that might have planets |work=ESA Science and Technology |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |accessdate=2007-02-03}}</ref><ref>{{cite journal |last=Landgraf |first=M. |coauthors=Liou, J.-C.; Zook, H. A.; Grün, E. |month=May |year=2002 |title=Origins of Solar System Dust beyond Jupiter |journal=The Astronomical Journal |volume=123 |issue=5 |pages=2857–2861 |doi=10.1086/339704 |url=http://www.iop.org/EJ/article/1538-3881/123/5/2857/201502.html |accessdate=2007-02-09}}</ref>

==Inner Solar System==
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.<ref name=inner>{{cite web |title=Inner Solar System |publisher=NASA Science (Planets) |url=http://nasascience.nasa.gov/planetary-science/exploring-the-inner-solar-system |accessdate=2009-05-09}}</ref> Composed mainly of [[silicate]]s and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

===Inner planets=== <!--This heading linked from [[Extrasolar planet]]-->
{{main|Terrestrial planet}}
[[Image:Terrestrial planet size comparisons.jpg|thumb|The inner planets. From left to right: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]] (sizes to scale)]]

The four inner or terrestrial planets have dense, [[rock (geology)|rocky]] compositions, few or no [[natural satellite|moons]], and no [[planetary ring|ring systems]]. They are composed largely of minerals with high melting points, such as the [[silicate]]s which form their [[crust (geology)|crusts]] and [[mantle (geology)|mantles]], and metals such as [[iron]] and [[nickel]], which form their [[planetary core|cores]]. Three of the four inner planets (Venus, Earth and Mars) have substantial [[atmosphere]]s; all have [[impact crater]]s and [[tectonics|tectonic]] surface features such as [[rift valley]]s and [[volcano]]es. The term ''inner planet'' should not be confused with ''[[inferior planet]]'', which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).

====Mercury====
: [[Mercury (planet)|Mercury]] (0.4&nbsp;AU) is the closest planet to the Sun and the smallest planet (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.<ref>Schenk P., Melosh H.J. (1994), ''Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere'', Abstracts of the 25th Lunar and Planetary Science Conference, 1994LPI....25.1203S</ref> Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.<ref>{{cite web |year=2006 |author=Bill Arnett |title=Mercury |work=The Nine Planets |url=http://www.nineplanets.org/mercury.html |accessdate=2006-09-14}}</ref> 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, and that it was prevented from fully accreting by the young Sun's energy.<ref>Benz, W., Slattery, W. L., Cameron, A. G. W. (1988), ''Collisional stripping of Mercury's mantle'', Icarus, v. 74, p. 516–528.</ref><ref>Cameron, A. G. W. (1985), ''The partial volatilization of Mercury'', Icarus, v. 64, p. 285–294.</ref>

====Venus====
: [[Venus]] (0.7&nbsp;AU) 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 ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 [[Celsius|°C]], most likely due to the amount of [[greenhouse gas]]es in the atmosphere.<ref>{{cite paper |author=Mark Alan Bullock |title=The Stability of Climate on Venus |publisher=Southwest Research Institute |year=1997 |url=http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |format=[[Portable Document Format|PDF]] |accessdate=2006-12-26 }}</ref> 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 regularly replenished by volcanic eruptions.<ref>{{cite web |year=1999 |author=Paul Rincon |title=Climate Change as a Regulator of Tectonics on Venus |work=Johnson Space Center Houston, TX, Institute of Meteoritics, University of New Mexico, Albuquerque, NM |url=http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |format=PDF |accessdate=2006-11-19}}</ref>

====Earth====
: [[Earth]] (1&nbsp;AU) 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 [[universe]] where [[life]] is known to exist.<ref name=life>{{cite web |title=Is there life elsewhere? |publisher=NASA Science (Big Questions) |url=http://nasascience.nasa.gov/big-questions/is-there-life-elsewhere |accessdate=2009-05-21}}</ref> 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]].<ref>{{cite web |title=Earth's Atmosphere: Composition and Structure |author=Anne E. Egger, M.A./M.S. |work=VisionLearning.com |url=http://www.visionlearning.com/library/module_viewer.php?c3=&mid=107&l=|accessdate=2006-12-26}}</ref> It has one natural satellite, the [[Moon]], the only large satellite of a terrestrial planet in the Solar System.

====Mars====
: [[Mars]] (1.5&nbsp;AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of mostly [[carbon dioxide]]. 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 very recently.<ref>{{cite web |year=2004 |title=Modern Martian Marvels: Volcanoes? |author=David Noever |work=NASA Astrobiology Magazine |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1360&mode=thread&order=0&thold=0 |accessdate=2006-07-23}}</ref>   Its red colour comes from [[Iron(III) oxide|iron oxide]] (rust) in its soil.<ref>{{cite web|title=Mars: A Kid's Eye View|publisher=NASA|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mars&Display=Kids|accessdate=2009-05-14}}</ref> Mars has two tiny natural satellites ([[Deimos (moon)|Deimos]] and [[Phobos (moon)|Phobos]]) thought to be captured [[asteroid]]s.<ref>{{cite web |year=2004 |title=A Survey for Outer Satellites of Mars: Limits to Completeness |author=Scott S. Sheppard, David Jewitt, and Jan Kleyna |work=The Astronomical Journal |url=http://www.iop.org/EJ/article/1538-3881/128/5/2542/204263.html |accessdate=2006-12-26}}</ref>

===Asteroid belt===
{{main|Asteroid belt}}
[[Image:InnerSolarSystem-en.png|300px|thumb|Image of the main [[asteroid belt]] and the [[Trojan asteroids]]]]

[[Asteroid]]s are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile [[mineral]]s.<ref>{{cite web|title=Are Kuiper Belt Objects asteroids? Are large Kuiper Belt Objects planets?
|publisher=[[Cornell University]]|url=http://curious.astro.cornell.edu/question.php?number=601|accessdate=2009-03-01}}</ref>

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3&nbsp;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.<ref>{{cite journal
  | author=Petit, J.-M.; Morbidelli, A.; Chambers, J.
  | title=The Primordial Excitation and Clearing of the Asteroid Belt
  | journal=Icarus
  | year=2001
  | volume=153
  | pages=338–347
  | url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
  | format=PDF
  | accessdate=2007-03-22  | doi = 10.1006/icar.2001.6702
}}</ref>

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as [[4 Vesta|Vesta]] and [[10 Hygiea|Hygieia]] may be reclassed as dwarf planets if they are shown to have achieved [[hydrostatic equilibrium]].<ref>{{cite web|title=IAU Planet Definition Committee|publisher=International Astronomical Union|year=2006|url=http://www.iau.org/public_press/news/release/iau0601/newspaper/|accessdate=2009-03-01}}</ref>

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.<ref>{{cite web |year=2002 |title=New study reveals twice as many asteroids as previously believed |work=ESA |url=http://www.esa.int/esaCP/ESAASPF18ZC_index_0.html|accessdate=2006-06-23}}</ref> Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.<ref name=Krasinsky2002>{{cite journal |authorlink=Georgij A. Krasinsky |first=G. A. |last=Krasinsky |coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..158...98K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4326fb2cf906949 |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |month=July |year=2002 |doi=10.1006/icar.2002.6837}}</ref> The main belt is very sparsely populated; [[Space probe|spacecraft]] routinely pass through without incident. Asteroids with diameters between 10 and 10<sup>−4</sup>&nbsp;[[metre|m]] are called [[meteoroid]]s.<ref>{{cite journal |author=Beech, M. |coauthors=Duncan I. Steel |year=1995 |month=September |title=On the Definition of the Term Meteoroid |journal=Quarterly Journal of the Royal Astronomical Society |volume=36 |issue=3 |pages=281–284 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44b52c369007834 |accessdate=2006-08-31}}</ref>

[[Image:Ceres optimized.jpg|thumb|150px|left|[[Ceres (dwarf planet)|Ceres]]]]

====Ceres====
: [[Ceres (dwarf planet)|Ceres]] (2.77&nbsp;AU) is the largest body in the asteroid belt and is classified as a dwarf planet. It has a diameter of slightly under 1000&nbsp;km, 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 observation revealed additional asteroids.<ref>{{cite web |title=History and Discovery of Asteroids |format=DOC |work=NASA |url=http://dawn.jpl.nasa.gov/DawnClassrooms/1_hist_dawn/history_discovery/Development/a_modeling_scale.doc |accessdate=2006-08-29}}</ref> It was again reclassified in 2006 as a dwarf planet.

====Asteroid groups====
: Asteroids in the main belt are divided into [[asteroid group]]s and [[:Category:Asteroid groups and families|families]] based on their orbital characteristics. [[Asteroid moon]]s 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 comet]]s which may have been the source of Earth's water.<ref>{{cite web |year=2006 |author=Phil Berardelli |title=Main-Belt Comets May Have Been Source Of Earths Water |work=SpaceDaily |url=http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html |accessdate=2006-06-23}}</ref>

[[Trojan asteroid]]s are located in either of Jupiter's [[Lagrangian point#L4_and_L5|L<sub>4</sub> or L<sub>5</sub> 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 family|Hilda asteroids]] are in a 2:3 [[Orbital resonance|resonance]] with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.

The inner Solar System is also dusted with [[Near-Earth asteroid|rogue asteroids]], many of which cross the orbits of the inner planets.

==Outer Solar System==
The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the [[Centaur (planetoid)|centaurs]], also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System are composed of a higher proportion of [[volatiles]] (such as water, ammonia, methane, often called ''ices'' in planetary science) than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid. 

===Outer planets===
{{main|Gas giant}}
[[Image:Gas giants in the solar system.jpg|thumb|From top to bottom: [[Neptune]], [[Uranus]], [[Saturn]], and [[Jupiter]] (not to scale)]]

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun.{{Ref label|C|c|none}} Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, “ice giants.”<ref>{{cite web |title=Formation of Giant Planets |author=Jack J. Lissauer, David J. Stevenson |work=NASA Ames Research Center; California Institute of Technology |year=2006 |url=http://www.gps.caltech.edu/uploads/File/People/djs/lissauer&stevenson(PPV).pdf|format=PDF |accessdate=2006-01-16}}</ref> All four gas giants have [[Planetary ring|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 (the outer planets and Mars).

====Jupiter====
: [[Jupiter]] (5.2&nbsp;AU), at 318 Earth masses, is 2.5 times all 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 [[Moons of Jupiter|sixty-three known satellites]]. The four largest, [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]], show similarities to the terrestrial planets, such as volcanism and internal heating.<ref>{{cite web |title=Geology of the Icy Galilean Satellites: A Framework for Compositional Studies |author=Pappalardo, R T |work=Brown University |year=1999 |url=http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |accessdate=2006-01-16}}</ref> Ganymede, the largest satellite in the Solar System, is larger than Mercury.

====Saturn====
: [[Saturn]] (9.5&nbsp;AU), distinguished by its extensive [[Rings of Saturn|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 Solar System. Saturn has [[Moons of Saturn|sixty known satellites]] (and three unconfirmed); two of which, [[Titan (moon)|Titan]] and [[Enceladus (moon)|Enceladus]], show signs of geological activity, though they are largely [[Cryovolcano|made of ice]].<ref>{{cite web |title=Cryovolcanism on the icy satellites |author=J. S. Kargel |work=U.S. Geological Survey |year=1994 |url=http://www.springerlink.com/content/n7435h4506788p22/ |accessdate=2006-01-16}}</ref> Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

====Uranus====
: [[Uranus]] (19.6&nbsp;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 gas giants, and radiates very little heat into space.<ref>{{cite web |title=10 Mysteries of the Solar System |author=Hawksett, David; Longstaff, Alan; Cooper, Keith; Clark, Stuart |work=Astronomy Now |year=2005 |url=http://adsabs.harvard.edu/abs/2005AsNow..19h..65H |accessdate=2006-01-16}}</ref> Uranus has [[Moons of Uranus|twenty-seven known satellites]], the largest ones being [[Titania (moon)|Titania]], [[Oberon (moon)|Oberon]], [[Umbriel (moon)|Umbriel]], [[Ariel (moon)|Ariel]] and [[Miranda (moon)|Miranda]].

====Neptune====
: [[Neptune]] (30&nbsp;AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more [[Density|dense]]. It radiates more internal heat, but not as much as Jupiter or Saturn.<ref>{{cite web |title=Post Voyager comparisons of the interiors of Uranus and Neptune |author=Podolak, M.; Reynolds, R. T.; Young, R. |work=NASA, Ames Research Center |year=1990 |url=http://adsabs.harvard.edu/abs/1990GeoRL..17.1737P |accessdate=2006-01-16}}</ref> Neptune has [[Moons of Neptune|thirteen known satellites]]. The largest, [[Triton (moon)|Triton]], is geologically active, with [[geyser]]s of [[liquid nitrogen]].<ref>{{cite web |title=The Plausibility of Boiling Geysers on Triton |author=Duxbury, N.S., Brown, R.H. |work=Beacon eSpace |year=1995 |url=http://trs-new.jpl.nasa.gov/dspace/handle/2014/28034?mode=full |accessdate=2006-01-16 }}</ref> Triton is the only large satellite with a [[retrograde and direct motion|retrograde]] orbit. Neptune is accompanied in its orbit by a number of [[minor planet]]s, termed [[Neptune Trojan]]s, that are in 1:1 [[Orbital resonance|resonance]] with it.

===Comets===
{{Main|Comet}}
[[Image:Comet c1995o1.jpg|right|thumb|Comet [[Hale-Bopp]]]]

Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to [[sublimation (chemistry)|sublimate]] and [[ion]]ise, creating a [[coma (cometary)|coma]]: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as [[Comet Hale-Bopp|Hale-Bopp]], are believed to originate in the Oort cloud. Many comet groups, such as the [[Kreutz Sungrazers]], formed from the breakup of a single parent.<ref>{{cite journal |author=Sekanina, Zdenek |year=2001 |title=Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration? |journal=Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic |volume=89 p.78–93}}</ref> Some comets with [[Comet#Orbital characteristics|hyperbolic]] orbits may originate outside the Solar System, but determining their precise orbits is difficult.<ref name="hyperbolic">{{cite journal |last=Królikowska |first=M. |year=2001 |title=A study of the original orbits of ''hyperbolic'' comets |journal=Astronomy & Astrophysics |volume=376 |issue=1 |pages=316–324 |doi=10.1051/0004-6361:20010945 |url=http://www.aanda.org/index.php?option=com_base_ora&url=articles/aa/full/2001/34/aa1250/aa1250.right.html&access=standard&Itemid=81 |accessdate=2007-01-02}}</ref> Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.<ref>{{cite web |url=http://www.springerlink.com/content/x0358l71h463w246/ |title=The activities of comets related to their aging and origin |author=Fred L. Whipple |accessdate=2006-12-26|date=1992-04}}</ref>

====Centaurs====
The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter (5.5&nbsp;AU) and less than Neptune (30&nbsp;AU). The largest known centaur, [[10199 Chariklo]], has a diameter of about 250&nbsp;km.<ref name=spitzer>{{cite web |title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope |author=John Stansberry, Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot |url=http://arxiv.org/abs/astro-ph/0702538v2 |year=2007 |accessdate=2008-09-21}}</ref> The first centaur discovered, [[2060 Chiron]], has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.<ref>{{cite web |year=1995 |author=Patrick Vanouplines |title=Chiron biography |work=Vrije Universitiet Brussel |url=http://www.vub.ac.be/STER/www.astro/chibio.htm |accessdate=2006-06-23}}</ref>

==Trans-Neptunian region==
The area beyond Neptune, or the "[[trans-Neptunian object|trans-Neptunian region]]", is still [[Timeline of Solar System exploration|largely unexplored]]. It appears to consist 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 Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.

===Kuiper belt===
{{main|Kuiper belt}}
[[Image:Outersolarsystem objectpositions labels comp.png|thumb|300 px|Plot of all known Kuiper belt objects, set against the four outer planets]]

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&nbsp;AU from the Sun. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as [[50000 Quaoar|Quaoar]], [[20000 Varuna|Varuna]], and [[90482 Orcus|Orcus]], may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50&nbsp;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.<ref name="Delsanti-Beyond_The_Planets">{{cite web |year=2006 |author=Audrey Delsanti and David Jewitt |title=The Solar System Beyond The Planets |work=Institute for Astronomy, University of Hawaii |url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |format=PDF |accessdate=2007-01-03}}</ref> Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
[[Image:TheKuiperBelt Projections 55AU Classical Plutinos.svg|left|thumb|Diagram showing the 3:2 resonant and classical Kuiper belt divisions]]

The Kuiper belt can be roughly divided into the "[[Classical Kuiper belt object|classical]]" belt and  the [[Resonant trans-Neptunian object|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 actually begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4&nbsp;AU to 47.7&nbsp;AU.<ref>{{cite web |year=2005 |author=M. W. Buie, R. L. Millis, L. H. Wasserman, J. L. Elliot, S. D. Kern, K. B. Clancy, E. I. Chiang, A. B. Jordan, K. J. Meech, R. M. Wagner, D. E. Trilling |work=Lowell Observatory, University of Pennsylvania, Large Binocular Telescope Observatory, Massachusetts Institute of Technology, University of Hawaii, University of California at Berkeley |title=Procedures, Resources and Selected Results of the Deep Ecliptic Survey |url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0309251 |accessdate=2006-09-07}}</ref> Members of the classical Kuiper belt are classified as [[Classical Kuiper belt object|cubewanos]], after the first of their kind to be discovered, {{mpl|(15760) 1992 QB|1}}, and are still in near primordial, low-eccentricity orbits.<ref>{{cite web |url=http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |format=PDF |title=Beyond Neptune, the new frontier of the Solar System |author=E. Dotto1, M.A. Barucci2, and M. Fulchignoni |accessdate=2006-12-26 |date=2006-08-24}}</ref>

====Pluto and Charon====
: [[Pluto]] (39&nbsp;AU average), a dwarf planet, 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&nbsp;AU from the Sun at perihelion (within the orbit of Neptune) to 49.5&nbsp;AU at aphelion.
[[Image:Pluto system 2006.jpg|right|thumb|[[Pluto]] and its three known moons]]

: It is unclear whether [[Charon (moon)|Charon]], Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Both Pluto and Charon orbit a [[Center of mass#Barycenter in astronomy|barycenter]] of gravity above their surfaces, making Pluto-Charon a [[binary system (astronomy)|binary system]]. Two much smaller moons, [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]], orbit Pluto and Charon.
: Pluto lies in the resonant belt and has a 3:2 [[orbital resonance|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 [[plutino]]s.<ref name="Fajans_et_al_2001">{{cite science |last=Fajans |first=J. |coauthors=L. Frièdland |month=October |year=2001 |title=Autoresonant (nonstationary) excitation of pendulums, Plutinos, plasmas, and other nonlinear oscillators |journal=American Journal of Physics |volume=69 |issue=10 |pages=1096–1102 |doi=10.1119/1.1389278 |url=http://scitation.aip.org/journals/doc/AJPIAS-ft/vol_69/iss_10/1096_1.html |accessdate=2006-12-26 |abstract=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000069000010001096000001&idtype=cvips&gifs=yes}} </ref>

====Haumea and Makemake====
: [[Haumea (dwarf planet)|Haumea]] (43.34&nbsp;AU average), and [[Makemake (dwarf planet)|Makemake]] (45.79&nbsp;AU average) are the largest known objects in the classical Kuiper belt. Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated '''2003 EL<sub>61</sub>''' and '''2005 FY<sub>9</sub>''' respectively, they were granted names (and the status of dwarf planet) in 2008.<ref name=name/> Their orbits are far more inclined than Pluto's (28° and 29°)<ref name=Buie>{{cite web
  |author=[[Marc W. Buie]]
  |date=2008-04-05
  |title=Orbit Fit and Astrometric record for 136472
  |publisher=SwRI (Space Science Department)
  |url=http://www.boulder.swri.edu/~buie/kbo/astrom/136472.html
  |accessdate=2008-07-13}}</ref> and unlike Pluto are not affected by Neptune, being part of the classical KBO population.

===Scattered disc===
{{main|Scattered disc}}
[[Image:TheKuiperBelt Projections 100AU Classical SDO.svg|thumb|right|Projection of the aligned orbits of the scattered, classical and resonant objects. Black: scattered disc objects; blue: classical Kuiper belt objects; green: 5:2 resonant objects]]

The scattered disc overlaps the Kuiper belt but extends much further outwards. This region is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of [[Formation and evolution of the Solar System#Planetary migration|Neptune's early outward migration]]. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150&nbsp;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."<ref>{{cite web |year=2005 |author=David Jewitt |title=The 1000 km Scale KBOs |work=University of Hawaii |url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html |accessdate=2006-07-16}}</ref> Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.<ref>{{cite web |url=http://cfa-www.harvard.edu/iau/lists/Centaurs.html |title=List Of Centaurs and Scattered-Disk Objects |work=IAU: Minor Planet Center |accessdate=2007-04-02}}</ref>[[Image:Eris and dysnomia2.jpg|thumb|left|[[Eris]] and its moon [[Dysnomia (moon)|Dysnomia]]]]

====Eris====
: Eris (68&nbsp;AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400&nbsp;km (1500&nbsp;mi). It is the largest of the known dwarf planets.<ref>{{cite web |year=2005 |author=Mike Brown |title=The discovery of <s>2003 UB313</s> Eris, the <s>10th planet</s> largest known dwarf planet. |work=CalTech |url=http://www.gps.caltech.edu/~mbrown/planetlila/ |accessdate=2006-09-15}}</ref> It has one moon, [[Dysnomia (moon)|Dysnomia]]. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2&nbsp;AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6&nbsp;AU, and steeply inclined to the ecliptic plane.

==Farthest regions==
The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this ''[[heliopause]]'' is considered the beginning of the [[interstellar medium]].<ref name="Voyager" /> However, the Sun's [[Hill sphere|Roche sphere]], the effective range of its gravitational influence, is believed to extend up to a thousand times farther.

===Heliopause===
[[Image:Voyager 1 entering heliosheath region.jpg|right|thumb|300px|The [[Voyager program|Voyagers]] entering the [[heliosheath]]]]

The heliosphere is divided into two separate regions. The solar wind travels at roughly 400&nbsp;km/s until it collides with flows of plasma in the [[interstellar medium]]. The collision occurs at the [[termination shock]], which is roughly 80–100&nbsp;AU from the Sun in the upwind direction and roughly 200&nbsp;AU from the Sun downwind.<ref name=fahr /> Here the wind slows dramatically, condenses and becomes more turbulent,<ref name=fahr /> forming a great oval structure known as the [[heliosheath]] that looks and behaves very much like a comet's tail, extending outward for a further 40&nbsp;AU on the upwind side but tailing many times that distance in the opposite direction. Both ''[[Voyager 1]]'' and ''[[Voyager 2]]'' are reported to have passed the termination shock and entered the heliosheath, at 94 and 84&nbsp;AU from the Sun, respectively.<ref>{{cite journal | doi=10.1126/science.1117684 | year=2005 | month=September | author=Stone, E. C.; Cummings, A. C.; McDonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=Voyager 1 explores the termination shock region and the heliosheath beyond | volume=309 | issue=5743 | pages=2017–20 | pmid=16179468 | journal=Science (New York, N.Y.)}}</ref><ref>{{cite journal | doi=10.1038/nature07022 | year=2008 | month=July | author=Stone, E. C.; Cummings, A. C.; McDonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=An asymmetric solar wind termination shock | volume=454 | issue=7200 | pages=71–4 | pmid=18596802 | journal=Nature }}</ref> The outer boundary of the heliosphere, the [[heliopause]], is the point at which the solar wind finally terminates and is the beginning of interstellar space.<ref name="Voyager">{{cite web |url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |title=Voyager Enters Solar System's Final Frontier |work=NASA |accessdate=2007-04-02}}</ref>

The shape and form of the outer edge of the heliosphere is likely affected by the [[fluid dynamics]] of interactions with the interstellar medium<ref name="fahr">{{cite journal |year=2000 |author=Fahr, H. J.; Kausch, T.; Scherer, H. |title=A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction |journal=Astronomy & Astrophysics | volume=357 | pages=268 |url=http://aa.springer.de/papers/0357001/2300268.pdf | format=PDF | bibcode=2000A&A...357..268F }} See Figures 1 and 2.</ref> as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the heliopause, at around 230&nbsp;AU, lies the [[bow shock]], a plasma "wake" left by the Sun as it travels through the [[Milky Way]].<ref>{{cite web | date=June 24, 2002 |author=P. C. Frisch (University of Chicago) |title=The Sun's Heliosphere & Heliopause | work=[[Astronomy Picture of the Day]] | url=http://antwrp.gsfc.nasa.gov/apod/ap020624.html |accessdate=2006-06-23}}</ref>

No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that [[NASA]]'s [[Voyager program|Voyager spacecraft]] will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.<ref>{{cite web | year=2007 | title=Voyager: Interstellar Mission | work=NASA Jet Propulsion Laboratory | url=http://voyager.jpl.nasa.gov/mission/interstellar.html |accessdate=2008-05-08}}</ref> How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.<ref>{{cite conference |title=Innovative Interstellar Explorer |author=R. L. McNutt, Jr. et al. | booktitle= Physics of the Inner Heliosheath: Voyager Observations, Theory, and Future Prospects |publisher=AIP Conference Proceedings |volume=858 |pages=341–347 |year=2006 |url=http://adsabs.harvard.edu/abs/2006AIPC..858..341M |doi=10.1063/1.2359348}}</ref><ref>{{cite web |year=2007 |title=Interstellar space, and step on it! |work=New Scientist |url=http://space.newscientist.com/article/mg19325850.900-interstellar-space-and-step-on-it.html |date=2007-01-05 |accessdate=2007-02-05 | author=Anderson, Mark}}</ref>

===Oort cloud===
{{main|Oort cloud}}
[[File:OortCloud P-sys(PNG-fin)1.png|thumb|right|175px|3d diagram model of the Oort cloud]]

The hypothetical 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 Solar System at roughly 50,000&nbsp;AU (around 1&nbsp;[[light-year]] (LY)), and possibly to as far as 100,000&nbsp;AU (1.87&nbsp;LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the [[galactic tide]], the [[tidal force]] exerted by the [[Milky Way]].<ref>{{cite web |year=2001 |author=Stern SA, Weissman PR. |title=Rapid collisional evolution of comets during the formation of the Oort cloud. |work=Space Studies Department, Southwest Research Institute, Boulder, Colorado| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11214311&dopt=Citation |accessdate=2006-11-19}}</ref><ref>{{cite web |year=2006 |author=Bill Arnett |title=The Kuiper Belt and the Oort Cloud |work=nineplanets.org |url=http://www.nineplanets.org/kboc.html |accessdate=2006-06-23}}</ref>
[[Image:Sedna-NASA.JPG|right|thumb|Telescopic image of [[90377 Sedna|Sedna]]]]

====Sedna====
[[90377 Sedna]] (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76&nbsp;AU at perihelion to 928&nbsp;AU at aphelion and takes 12,050 years to complete. [[Michael E. Brown|Mike Brown]], who discovered the object in 2003, asserts that it cannot be part of the [[scattered disc]] or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object {{mpl|2000 CR|105}}, which has a perihelion of 45&nbsp;AU, an aphelion of 415&nbsp;AU, and an orbital period of 3,420 years.<ref>{{cite web |year=2004 |author=David Jewitt |title=Sedna – 2003 VB<sub>12</sub> |work=University of Hawaii |url=http://www.ifa.hawaii.edu/~jewitt/kb/sedna.html |accessdate=2006-06-23}}</ref> Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.<ref>{{cite web |title=Sedna |author=Mike Brown |url=http://www.gps.caltech.edu/~mbrown/sedna/ |work=CalTech |accessdate=2007-05-02}}</ref> Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

===Boundaries===
{{see also|Vulcanoid asteroid|Planets beyond Neptune|Nemesis (star)}}

Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of [[List of nearest stars|surrounding stars]] out to about two light years (125,000&nbsp;AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000&nbsp;AU.<ref>{{cite book |title=The Solar System: Third edition |author=T. Encrenaz, JP. Bibring, M. Blanc, MA. Barucci, F. Roques, PH. Zarka |publisher=Springer |year=2004 |pages=1}}</ref> Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.<ref>{{cite web |year=2004 |author=Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Levison H.F.; Hassler D.M. |title=A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images |url=http://www.ingentaconnect.com/search/expand?pub=infobike://ap/is/2000/00000148/00000001/art06520&unc=ml |accessdate=2006-07-23}}</ref> Objects may yet be discovered in the Solar System's uncharted regions.

==Galactic context==
[[Image:Milky Way Spiral Arm.svg|left|thumb|Location of the Solar System within our [[galaxy]]]]

The Solar System is located in the [[Milky Way]] [[galaxy]], a [[barred spiral galaxy]] with a diameter of about 100,000 [[light-year]]s containing about 200 billion stars.<ref>{{cite web |title=Magnetic fields in cosmology |author=A.D. Dolgov |year=2003 |url=http://arxiv.org/abs/astro-ph/0306443 |accessdate=2006-07-23}}</ref> Our Sun resides in one of the Milky Way's outer spiral arms, known as the [[Orion Arm]] or Local Spur.<ref>{{cite web |title=Three Dimensional Structure of the Milky Way Disk |author=R. Drimmel, D. N. Spergel |year=2001 |url=http://arxiv.org/abs/astro-ph/0101259 |accessdate=2006-07-23}}</ref> The Sun lies between 25,000 and 28,000 light years from the [[Galactic Centre]], and its speed within the galaxy is about 220 [[metre per second|kilometres per second]], so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's [[galactic year]].<ref>{{cite web |title=Period of the Sun's Orbit around the Galaxy (Cosmic Year |first=Stacy |last=Leong |url=http://hypertextbook.com/facts/2002/StacyLeong.shtml |year=2002 |work=The Physics Factbook |accessdate=2007-04-02}}</ref> The [[solar apex]], the direction of the Sun's path through interstellar space, is near the constellation of [[Hercules (constellation)|Hercules]] in the direction of the current location of the bright star [[Vega]].<ref>{{cite web |year=2003 |author=C. Barbieri |title=Elementi di Astronomia e Astrofisica per il Corso di Ingegneria Aerospaziale V settimana |work=IdealStars.com |url=http://dipastro.pd.astro.it/planets/barbieri/Lezioni-AstroAstrofIng04_05-Prima-Settimana.ppt |accessdate=2007-02-12}}</ref>

The Solar System's location in the galaxy is very likely a factor in the [[evolution]] of [[life]] on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous [[supernova]]e, this has given Earth long periods of interstellar stability for life to evolve.<ref name="astrobiology">{{cite web |year=2001 |author=Leslie Mullen |title=Galactic Habitable Zones |work=Astrobiology Magazine |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=139 |accessdate=2006-06-23}}</ref> The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.<ref name=astrobiology/> Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun in the form of radioactive dust grains and larger, comet-like bodies.<ref>{{cite web |year=2005 |author=|title=Supernova Explosion May Have Caused Mammoth Extinction |work=Physorg.com |url=http://www.physorg.com/news6734.html |accessdate=2007-02-02}}</ref>
[[Image:local bubble.jpg|thumb|Artist's conception of the [[Local Bubble]]]]
===Neighbourhood===

The immediate galactic neighbourhood of the Solar System is known as the [[Local Interstellar Cloud]] or Local Fluff, an area of dense cloud in an otherwise sparse region known as the [[Local Bubble]], an hourglass-shaped cavity in the [[interstellar medium]] roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.<ref>{{cite web |title=Near-Earth Supernovas |work=NASA |url=http://science.nasa.gov/headlines/y2003/06jan_bubble.htm |accessdate=2006-07-23}}</ref>

There are relatively few [[List of nearest stars|stars within ten light years]] (95 trillion&nbsp;km) of the Sun. 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 Sun-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 the Sun 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 the Sun's mass and orbited by a [[white dwarf]] called Sirius B. It lies 8.6 light years away. The remaining systems within ten 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).<ref>{{cite web |title=Stars within 10 light years |url=http://www.solstation.com/stars/s10ly.htm|work=SolStation |accessdate=2007-04-02}}</ref> Our closest solitary sun-like star is [[Tau Ceti]], which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.<ref>{{cite web |title=Tau Ceti |url=http://www.solstation.com/stars/tau-ceti.htm |work=SolStation |accessdate=2007-04-02}}</ref> The closest known [[extrasolar planet]] to the Sun lies around the star [[Epsilon Eridani]], a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, [[Epsilon Eridani b]], is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.<ref>{{cite web |title=HUBBLE ZEROES IN ON NEAREST KNOWN EXOPLANET |work=Hubblesite |year=2006 |url=http://hubblesite.org/newscenter/archive/releases/2006/32/text/ |accessdate-2008-01-13}}</ref>

==Formation and evolution==
{| class="wikitable" style="float:right;"
|+ Solar System's Most<br />Abundant Isotopes<ref>{{cite book |first=David |last=Arnett |year=1996 |title=Supernovae and Nucleosynthesis |edition=First |publisher=Princeton University Press |location=Princeton, New Jersey |isbn=0-691-01147-8 |oclc=33162440}}</ref>
![[Isotope]]
![[Atomic nucleus|Nuclei]]&nbsp;per<br />Million
|-
| [[Hydrogen-1]] ||style="text-align:right"| 705,700
|-
| [[Helium-4]] ||style="text-align:right"| 275,200
|-
| [[Oxygen-16]] ||style="text-align:right"| 5,920
|-
| [[Carbon-12]] ||style="text-align:right"| 3,032
|-
| [[Neon-20]] ||style="text-align:right"| 1,548
|-
| [[Iron-56]] ||style="text-align:right"| 1,169
|-
| [[Nitrogen-14]] ||style="text-align:right"| 1,105
|-
| [[Silicon-28]] ||style="text-align:right"| 653
|-
| [[Magnesium-24]] ||style="text-align:right"| 513
|-
| [[Sulfur-32]] ||style="text-align:right"| 396
|-
| [[Neon-22]] ||style="text-align:right"| 208
|-
| [[Magnesium-26]] ||style="text-align:right"| 79
|-
| [[Argon-36]] ||style="text-align:right"| 77
|-
| [[Iron-54]] ||style="text-align:right"| 72
|-
| [[Magnesium-25]] ||style="text-align:right"| 69
|-
| [[Calcium-40]] ||style="text-align:right"| 60
|-
| [[Aluminum-27]] ||style="text-align:right"| 58
|-
| [[Nickel-58]] ||style="text-align:right"| 49
|-
| [[Carbon-13]] ||style="text-align:right"| 37
|-
| [[Helium-3]] ||style="text-align:right"| 35
|-
| [[Silicon-29]] ||style="text-align:right"| 34
|-
| [[Sodium-23]] ||style="text-align:right"| 33
|-
| [[Iron-57]] ||style="text-align:right"| 28
|-
| [[Hydrogen-2]] ||style="text-align:right"| 23
|-
| [[Silicon-30]] ||style="text-align:right"| 23
|}

{{main|Formation and evolution of the Solar System}}
[[Image:M42proplyds.jpg|left|thumb|[[Hubble Space Telescope|Hubble]] image of protoplanetary disks in the [[Orion Nebula]], a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed.]]

The Solar System formed from the gravitational collapse of a giant [[molecular cloud]] 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.<ref name="Arizona">{{cite web |title=Lecture 13: The Nebular Theory of the origin of the Solar System |url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |work=University of Arizona |accessdate=2006-12-27}}</ref>

As the region that would become the Solar System, known as the [[solar nebula|pre-solar nebula]],<ref>{{cite web |title=The chemical composition of the pre-solar nebula |author=Irvine, W. M. |work=Amherst College, Massachusetts |url=http://adsabs.harvard.edu/abs/1983coex....1....3I |accessdate=2007-02-15}}</ref> collapsed, conservation of [[angular momentum]] made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.<ref name="Arizona"/> As the contracting nebula rotated, it began to flatten into a spinning [[protoplanetary disc]] with a diameter of roughly 200&nbsp;AU<ref name="Arizona"/> and a hot, dense [[protostar]] at the centre.<ref>{{cite science |last=Greaves |first=Jane S. |date=2005-01-07 |title=Disks Around Stars and the Growth of Planetary Systems |journal=Science |volume=307 | issue=5706 |pages=68–71 |doi=10.1126/science.1101979 |url=http://www.sciencemag.org/cgi/content/full/307/5706/68 | abstract=http://www.sciencemag.org/cgi/content/abstract/sci;307/5706/68 | accessdate=2006-11-16}}</ref><ref>{{cite web |date=2000-04-05 |url=http://www7.nationalacademies.org/ssb/detectionch3.html |title=Present Understanding of the Origin of Planetary Systems |publisher=National Academy of Sciences |accessdate=2007-01-19 }}</ref> At this point in its [[stellar evolution|evolution]], the Sun is believed to have been a [[T Tauri star]]. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 [[solar mass]]es, with the vast majority of the mass of the nebula in the star itself.<ref name= "Kitamara">{{cite conference | author=M. Momose, Y. Kitamura, S. Yokogawa, R. Kawabe, M. Tamura, S. Ida | title=Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a High-resolution Imaging Survey at lambda = 2 mm | booktitle=The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume I | year=2003 | publisher=Astronomical Society of the Pacific Conference Series | volume=289 | editor=Ikeuchi, S., Hearnshaw, J. and Hanawa, T. (eds.) | pages=85 | url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2003ASPC..289...85M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | format=PDF}}</ref> The planets formed by [[accretion (astrophysics)|accretion]] from this disk.<ref>{{cite journal | doi= 10.1086/429160 | title= Chondrule-forming Shock Fronts in the Solar Nebula: A Possible Unified Scenario for Planet and Chondrite Formation | year= 2005 | author= Boss, A. P. | journal= The Astrophysical Journal | volume= 621 | pages= L137}}</ref>

Within 50 million years, the pressure and density of [[hydrogen]] in the centre of the protostar became great enough for it to begin [[nuclear fusion|thermonuclear fusion]].<ref name=Yi2001>{{cite journal | author= Sukyoung Yi; Pierre Demarque; Yong-Cheol Kim; Young-Wook Lee; Chang H. Ree; Thibault Lejeune; Sydney  Barnes | title=Toward Better Age Estimates for Stellar Populations: The <math>Y^{2}</math> Isochrones for Solar Mixture | journal=Astrophysical Journal Supplement | id={{arXiv|astro-ph|0104292}} | year=2001 | volume=136 | pages=417 | doi=10.1086/321795 | url=http://adsabs.harvard.edu/abs/2001ApJS..136..417Y}}</ref> The temperature, reaction rate, pressure, and density increased until [[hydrostatic equilibrium]] was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged [[main sequence]] star.<ref>{{cite journal | author=A. Chrysostomou, P. W. Lucas | title=The Formation of Stars | journal=Contemporary Physics | year=2005 | volume=46 | pages=29 | url=http://adsabs.harvard.edu/abs/2005ConPh..46...29C | doi=10.1080/0010751042000275277}}</ref>

[[Image:redgiantsun.gif|thumb|left|Artist's conception of the future evolution of our [[Sun]]. Left: main sequence; middle: red giant; right: white dwarf]]
The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the [[Hertzsprung-Russell diagram]]. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.<ref>{{cite web|title=Science: Fiery future for planet Earth |author=Jeff Hecht |work=NewScientist |url=http://www.newscientist.com/article/mg14219191.900.html |year=1994 |accessdate=2007-10-29}}</ref>

Around 5.4&nbsp;billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a [[red giant]]. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600&nbsp;K at the coolest).<ref>{{cite journal|author=K. P. Schroder, Robert Cannon Smith|title=Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society |volume=386 |pages=155–163 |year=2008 |doi=10.1111/j.1365-2966.2008.13022.x |url=http://adsabs.harvard.edu/abs/2008MNRAS.386..155S}}</ref>

Eventually, the Sun's outer layers will fall away, leaving a [[white dwarf]], an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.<ref>{{cite web|author=Pogge, Richard W.|year=1997|url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html|title=The Once & Future Sun|format=lecture notes|work=[http://www-astronomy.mps.ohio-state.edu/Vistas/ New Vistas in Astronomy]|accessdate=2005-12-07}}</ref> The ejected outer layers will form what is known as a [[planetary nebula]], returning some of the material that formed the Sun to the interstellar medium.

==See also==
{{portal|Solar System|Solar system.jpg}}
* [[Astronomical symbols]]
* [[Celestia]] – 3D computer space-simulation program
* [[Family Portrait (Voyager)]]
* [[Geological features of the solar system]]
* [[Geocentrism]]
* List of Solar System objects:
**[[Timeline of discovery of Solar System planets and their moons|by discovery date]]
**[[List of Solar System objects|by orbit]]
**[[List of Solar System objects by size|by size]]
* [[New Horizons]]
* [[Numerical model of solar system]]
* [[Pioneer 10]]
* [[Pioneer 11]]
* [[Planetary mnemonic]]
* [[Solar System in fiction]]
* [[Solar system model]]
* [[Space colonization]]
* [[Voyager program]]

==Notes==
<div class="references-small">
<ol type="a">
<li>{{Note label|A|a|none}}[[Capitalization]] of the name varies. The [[International Astronomical Union|IAU]], the authoritative body regarding astronomical nomenclature, specifies [http://www.iau.org/public_press/themes/naming/ capitalizing the names of all individual astronomical objects] ('''Solar System'''). However, the name is commonly rendered in lower case ('''solar system''') including in the ''[[Oxford English Dictionary]]'', [http://www.m-w.com/dictionary/solar%20system ''Merriam-Webster's 11th Collegiate Dictionary''], and [http://www.britannica.com/eb/article-9110143 ''Encyclopædia Britannica''].</li>

<li>{{Note label|B|b|none}}See [[List of moons]] for the full list of natural satellites of the eight planets and five dwarf planets.</li>

<li>{{Note label|C|c|none}}The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),<ref>{{cite web|title=Origin and dynamical evolution of comets and their reservoirs|author=Alessandro Morbidelli|work=CNRS, Observatoire de la Côte d’Azur|year=2006|url=http://arxiv.org/abs/astro-ph/0512256v1|accessdate=2007-08-03}}</ref> the Kuiper Belt (estimated at roughly 0.1 Earth mass)<ref name="Delsanti-Beyond_The_Planets"/> and the asteroid belt (estimated to be 0.0005 Earth mass)<ref name="Krasinsky2002" /> for a total, rounded upwards, of ~37 Earth masses, or 8.1 percent the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3 percent of the total.</li>

<li>{{note label|D|d|none}}Astronomers usually measure distances within the Solar System in [[astronomical unit]]s (AU). One AU is approximately the average distance between the Earth and the Sun, or 149,598,000&nbsp;km. Pluto is roughly 38&nbsp;AU from the Sun, while Jupiter lies at roughly 5.2&nbsp;AU. One [[light-year]] is 63,240&nbsp;AU.</li></ol></div>

==References==
{{reflist|2}}

==External links==
{{Commonscat|Solar System}}
{{sisterlinks|Solar System}}
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys&Display=Overview Solar System Profile] by [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration]
* [http://space.jpl.nasa.gov NASA's Solar System Simulator]
* [http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
* [http://www.nineplanets.org/ The Nine Planets – Comprehensive Solar System site by Bill Arnett]
* [http://www.space.com/solarsystem/ SPACE.com: All About the Solar System]
* [http://www.classzone.com/books/earth_science/terc/content/visualizations/es2701/es2701page01.cfm?chapter_no=27 Illustration of the distance between planets]
* [http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the sun, and other stars]
* [http://www.psrd.hawaii.edu/ Planetary Science Research Discoveries] Educational journal with articles about Solar System bodies

{{Solar system table}}
{{Earth's location}}
{{Systems}}
{{featured article}}

[[Category:Solar System| ]]
[[Category:Planetary science]]
[[Category:Space science]]
[[Category:Planetary systems]]

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[[br:Koskoriad an Heol]]
[[bg:Слънчева система]]
[[ca:Sistema solar]]
[[cv:Хĕвел системи]]
[[cs:Sluneční soustava]]
[[cy:Cysawd yr Haul]]
[[da:Solsystem]]
[[de:Sonnensystem]]
[[dv:އިރަވީ ނިޒާމު]]
[[et:Päikesesüsteem]]
[[el:Ηλιακό σύστημα]]
[[es:Sistema Solar]]
[[eo:Sunsistemo]]
[[eu:Eguzki-sistema]]
[[fa:منظومه شمسی]]
[[fo:Sólskipanin]]
[[fr:Système solaire]]
[[fy:Sinnestelsel]]
[[fur:Sisteme solâr]]
[[ga:Grianchóras]]
[[gv:Corys ny Greiney]]
[[gd:An Crios-Grèine]]
[[gl:Sistema Solar]]
[[gan:太陽系]]
[[gu:સૂર્યમંડળ]]
[[ko:태양계]]
[[haw:Kahua o nā hōkū]]
[[hy:Արեգակնային համակարգ]]
[[hi:सौर मंडल]]
[[hr:Sunčev sustav]]
[[io:Sunala sistemo]]
[[ilo:Sistema a Solar]]
[[id:Tata Surya]]
[[ia:Systema solar]]
[[ie:Solari sistema]]
[[os:Хурон системæ]]
[[is:Sólkerfið]]
[[it:Sistema solare]]
[[he:מערכת השמש]]
[[jv:Tata Surya]]
[[kn:ಜ್ಯೋತಿರ್ಮಂಡಲ]]
[[pam:Solar System]]
[[ka:მზის სისტემა]]
[[sw:Mfumo wa jua na sayari zake]]
[[ht:Sistèm solè]]
[[ku:Pergala rojê]]
[[lo:ລະບົບສຸຣິຍະ]]
[[la:Systema solare]]
[[lv:Saules sistēma]]
[[lb:Sonnesystem]]
[[lt:Saulės sistema]]
[[lij:Scistema Solare]]
[[li:Zonnestèlsel]]
[[ln:Minzɔ́tɔ mwa Mói]]
[[jbo:solri ciste]]
[[hu:Naprendszer]]
[[mk:Сончев систем]]
[[ml:സൗരയൂഥം]]
[[mt:Sistema Solari]]
[[mr:सूर्यमाला]]
[[arz:نظام شمسى]]
[[ms:Sistem Suria]]
[[mn:Нарны аймаг]]
[[my:နေအဖွဲ့အစည်း]]
[[nah:Tōnatiuh Īchān]]
[[nl:Zonnestelsel]]
[[nds-nl:Zunnesysteem]]
[[ne:सौर्यमण्डल]]
[[ja:太陽系]]
[[no:Solsystem]]
[[nn:Solsystemet]]
[[nov:Sunal sisteme]]
[[oc:Sistèma Solar]]
[[uz:Quyosh tizimi]]
[[pa:ਸੂਰਜ ਮੰਡਲ]]
[[km:ប្រព័ន្ធព្រះអាទិត្យ]]
[[pms:Sistema solar]]
[[nds:Sünnsystem]]
[[pl:Układ Słoneczny]]
[[pt:Sistema Solar]]
[[ksh:Sunnesystem]]
[[ro:Sistemul solar]]
[[rm:Sistem Solar]]
[[qu:Inti Llika]]
[[ru:Солнечная система]]
[[sah:Күн системата]]
[[se:Beaivvášgoddi]]
[[sco:Solar seestem]]
[[stq:Sunnensystem]]
[[sq:Sistemi diellor]]
[[scn:Sistema sulari]]
[[si:සෞරග්‍රහ මණ්ඩලය]]
[[simple:Solar System]]
[[sk:Slnečná sústava]]
[[sl:Osončje]]
[[sr:Сунчев систем]]
[[sh:Sunčev sistem]]
[[su:Sistim tatasurya]]
[[fi:Aurinkokunta]]
[[sv:Solsystemet]]
[[tl:Sistemang Pang-araw]]
[[ta:சூரியக் குடும்பம்]]
[[tt:Кояш системасы]]
[[te:సౌరమండలము]]
[[th:ระบบสุริยะ]]
[[tg:Системаи офтобӣ]]
[[tr:Güneş Sistemi]]
[[uk:Сонячна система]]
[[ur:نظام شمسی]]
[[ug:قۇياش سىستېمىسى]]
[[vec:Sistema sołare]]
[[vi:Hệ Mặt Trời]]
[[fiu-vro:Pääväsüstem]]
[[wa:Sistinme solrece]]
[[vls:Zunnestelsel]]
[[war:Sistema solar]]
[[wo:Nosteg jant]]
[[wuu:太阳系]]
[[yi:זון סיסטעם]]
[[yo:Ètò òòrùn]]
[[zh-yue:太陽系]]
[[diq:Sistemê Roci]]
[[bat-smg:Saulės sėstema]]
[[zh:太阳系]]