Editing Solar System (section)

==General characteristics==
Astronomers sometimes divide the Solar System structure into separate regions. The [[inner Solar System]] includes Mercury, Venus, Earth, Mars, and the bodies in the [[asteroid belt]]. The [[outer Solar System]] includes Jupiter, Saturn, Uranus, Neptune, and the bodies in the [[Kuiper belt]].<ref>{{Cite web |title=The Planets |date=10 July 2023 |url=https://science.nasa.gov/solar-system/planets/ |access-date=6 April 2024 |publisher=NASA}}</ref> Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of [[Trans-Neptunian object|the objects beyond Neptune]].<ref>{{Cite web |title=Kuiper Belt: Facts |date=14 November 2017 |url=https://science.nasa.gov/solar-system/kuiper-belt/facts/ |access-date=6 April 2024 |publisher=NASA |archive-date=12 March 2024 |archive-url=https://web.archive.org/web/20240312024528/https://science.nasa.gov/solar-system/kuiper-belt/facts/ |url-status=live }}</ref>

=== Composition ===
{{Further|List of Solar System objects|List of interstellar and circumstellar molecules}}

The principal component of the Solar System is the Sun, a [[G-type main-sequence star]] that contains 99.86% of the system's known mass and dominates it gravitationally.<ref>{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |journal=[[Astronomy & Geophysics]] |volume=41 |issue=1 |pages=1.12–1.19 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |doi-access=free}}</ref> The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, [[asteroid]]s, and comets) together comprise less than 0.002% of the Solar System's total mass.{{Refn |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 arXiv |eprint=astro-ph/0512256 |first=Alessandro |last=Morbidelli |title=Origin and dynamical evolution of comets and their reservoirs |date=2005}}</ref> the Kuiper belt (estimated at 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% of 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% of the total orbiting mass.|name=footnoteD|group=lower-alpha}}

The Sun is composed of roughly 98% hydrogen and helium,<ref>{{Cite web |title=The Sun's Vital Statistics |url=http://solar-center.stanford.edu/vitalstats.html |url-status=live |archive-url=https://www.webcitation.org/6BOkQXma3?url=http://solar-center.stanford.edu/vitalstats.html |archive-date=14 October 2012 |access-date=29 July 2008 |publisher=Stanford Solar Center |postscript=,}} citing {{Cite book
|last=Eddy
|first=J.
|title=A New Sun: The Solar Results From Skylab
|url=https://history.nasa.gov/SP-402/contents.htm
|publisher=[[NASA]]
|date=1979
|page=37
|id=NASA SP-402
|access-date=12 July 2017
|archive-date=30 July 2021
|archive-url=https://web.archive.org/web/20210730024856/https://history.nasa.gov/SP-402/contents.htm
|url-status=live
}}</ref> as are Jupiter and Saturn.<ref>{{Cite web |last=Williams |first=David R. |date=7 September 2006 |title=Saturn Fact Sheet |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |url-status=dead |archive-url=https://web.archive.org/web/20110804224236/http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |archive-date=4 August 2011 |access-date=31 July 2007 |publisher=NASA }}</ref><ref name="Williams-Jupiter" /> A composition gradient exists in the Solar System, created by heat and [[light pressure]] from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.<ref>{{Cite book |last1=Weissman |first1=Paul Robert |url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 |title=Encyclopedia of the solar system |last2=Johnson |first2=Torrence V. |date=2007 |publisher=Academic Press |isbn=978-0-12-088589-3 |page=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 615]}}</ref> The boundary in the Solar System beyond which those volatile substances could coalesce is known as the [[Frost line (astrophysics)|frost line]], and it lies at roughly five times the Earth's distance from the Sun.<ref name="Mumma" />

=== Orbits ===

[[File:Solar system orrery inner planets.gif|thumb|Animations of the Solar System's [[inner planet]]s orbiting. Each frame represents 2 days of motion.]]
[[File:Solar system orrery outer planets.gif|thumb|Animations of the Solar System's [[outer planet]]s orbiting. This animation is 100 times faster than the inner planet animation.]]

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the [[ecliptic]]. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.<ref name="Levison2003">{{Cite journal |last1=Levison |first1=H.F. |author-link=Harold F. Levison |last2=Morbidelli |first2=A. |date=27 November 2003 |title=The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration |journal=[[Nature (journal)|Nature]] |volume=426 |issue=6965 |pages=419–421 |bibcode=2003Natur.426..419L |doi=10.1038/nature02120 |pmid=14647375 |s2cid=4395099}}</ref><ref>{{Cite journal |last1=Levison |first1=Harold F. |last2=Duncan |first2=Martin J. |date=1997 |title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets |journal=[[Icarus (journal)|Icarus]] |volume=127 |issue=1 |pages=13–32 |bibcode=1997Icar..127...13L |doi=10.1006/icar.1996.5637}}</ref> Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are in [[synchronous rotation]], with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.<ref name="bennett_4.5">{{Cite book |last1=Bennett |first1=Jeffrey O. |title=The Cosmic Perspective |last2=Donahue |first2=Megan |last3=Schneider |first3=Nicholas |last4=Voit |first4=Mark |date=2020 |publisher=Pearson |isbn=978-0-134-87436-4 |edition=9th |location=Hoboken, NJ |chapter=4.5 Orbits, Tides, and the Acceleration of Gravity |oclc=1061866912 |author-link2=Megan Donahue}}</ref>

As a result of the [[Formation and evolution of the Solar System|formation of the Solar System]], planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.<ref>{{Cite magazine |last=Grossman |first=Lisa |date=13 August 2009 |title=Planet found orbiting its star backwards for first time |url=https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |url-status=live |magazine=New Scientist |archive-url=https://web.archive.org/web/20121017083955/http://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |archive-date=17 October 2012 |access-date=10 October 2009}}</ref> There are exceptions, such as [[Halley's Comet]].<ref>{{Cite web |last=Nakano |first=Syuichi |date=2001 |title=OAA computing section circular |url=http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |url-status=live |archive-url=https://web.archive.org/web/20190921103057/http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |archive-date=21 September 2019 |access-date=15 May 2007 |publisher=Oriental Astronomical Association}}</ref> Most of the larger moons orbit their planets in [[Retrograde and prograde motion|prograde]] direction, matching the direction of planetary rotation; Neptune's moon [[Triton (moon)|Triton]] is the largest to orbit in the opposite, retrograde manner.<ref>{{Cite journal |last1=Agnor |first1=Craig B. |last2=Hamilton |first2=Douglas P. |date=May 2006 |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |url=https://www.nature.com/articles/nature04792 |url-status=live |journal=[[Nature (journal)|Nature]] |language=en |volume=441 |issue=7090 |pages=192–194 |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |issn=1476-4687 |pmid=16688170 |s2cid=4420518 |archive-url=https://web.archive.org/web/20220415081402/https://www.nature.com/articles/nature04792 |archive-date=15 April 2022 |access-date=28 March 2022}}</ref> Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.<ref>{{Cite book |last=Gallant |first=Roy A. |url=https://www.worldcat.org/oclc/6533014 |title=National Geographic Picture Atlas of Our Universe |date=1980 |publisher=National Geographic Society |isbn=0-87044-356-9 |editor-last=Sedeen |editor-first=Margaret |location=Washington, D.C. |pages=82 |oclc=6533014 |access-date=28 March 2022 |archive-url=https://web.archive.org/web/20220420161217/https://www.worldcat.org/title/national-geographic-picture-atlas-of-our-universe/oclc/6533014 |archive-date=20 April 2022 |url-status=live}}</ref>

To a good first approximation, [[Kepler's laws of planetary motion]] describe the orbits of objects around the Sun.<ref name=":0">{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link=Steven Frautschi |author-link3=Tom M. Apostol |author-link4=David L. Goodstein}}</ref>{{Rp|pages=433–437}} These laws stipulate that each object travels along an [[ellipse]] with the Sun at one [[focus (geometry)|focus]], which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its ''[[perihelion]]'', whereas its most distant point from the Sun is called its ''[[aphelion]]''.<ref name=":8">{{Cite book |last1=Feynman |first1=Richard P. |title=The Feynman Lectures on Physics, Volume 1 |title-link=The Feynman Lectures on Physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=1989 |publisher=Addison-Wesley Pub. Co |isbn=0-201-02010-6 |location=Reading, Mass. |oclc=531535 |author-link=Richard Feynman |author-link2=Robert B. Leighton |author-link3=Matthew Sands |orig-date=1965}}</ref>{{Rp|location=9-6}} With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using [[numerical model of the Solar System|numerical models]],<ref name=":8" />{{Rp|location=9-6}} but the planetary system can change chaotically over billions of years.<ref>{{Cite journal |last1=Lecar |first1=Myron |last2=Franklin |first2=Fred A. |last3=Holman |first3=Matthew J. |last4=Murray |first4=Norman J. |date=2001 |title=Chaos in the Solar System |journal=Annual Review of Astronomy and Astrophysics |volume=39 |issue=1 |pages=581–631 |arxiv=astro-ph/0111600 |bibcode=2001ARA&A..39..581L |doi=10.1146/annurev.astro.39.1.581 |s2cid=55949289}}</ref>

The [[angular momentum]] of the Solar System is a measure of the total amount of orbital and [[rotational momentum]] possessed by all its moving components.<ref>{{Cite book |last=Piccirillo |first=Lucio |url=https://books.google.com/books?id=W0jpDwAAQBAJ&pg=PA210 |title=Introduction to the Maths and Physics of the Solar System |date=2020 |publisher=CRC Press |isbn=978-0429682803 |page=210 |access-date=10 May 2022 |archive-url=https://web.archive.org/web/20220730084321/https://www.google.com/books/edition/Introduction_to_the_Maths_and_Physics_of/W0jpDwAAQBAJ?gbpv=1&pg=PA210 |archive-date=30 July 2022 |url-status=live}}</ref> Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.<ref name="Marochnik1995">{{Cite conference |last1=Marochnik |first1=L. |last2=Mukhin |first2=L. |date=1995 |title=Is Solar System Evolution Cometary Dominated? |series=Astronomical Society of the Pacific Conference Series |volume=74 |page=83 |bibcode=1995ASPC...74...83M |isbn=0-937707-93-7 |book-title=Progress in the Search for Extraterrestrial Life |editor=Shostak, G.S.}}</ref><ref>{{Cite journal |last1=Bi |first1=S. L. |last2=Li |first2=T. D. |last3=Li |first3=L. H. |last4=Yang |first4=W. M. |year=2011 |title=Solar Models with Revised Abundance |journal=[[The Astrophysical Journal]] |volume=731 |issue=2 |pages=L42 |arxiv=1104.1032 |bibcode=2011ApJ...731L..42B |doi=10.1088/2041-8205/731/2/L42 |s2cid=118681206}}</ref> 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.<ref name="Marochnik1995" />

=== Distances and scales ===
[[File:Solar System distance to scale.svg|center|thumb|upright=2.5|Comparison of the distances between planets, with the white bar showing orbital variations. The size of the planets is not to scale.]]
[[File:Orbital distances in the solar system linear scale.png|thumb|Relative orbital distances in the Solar System visualized as a condensed rectangle]]

The radius of the Sun is {{Cvt|0.0047|AU|km mi|sigfig=1|abbr=unit}}.<ref name="arxiv1203_4898">{{Cite journal |last1=Emilio |first1=Marcelo |last2=Kuhn |first2=Jeff R. |last3=Bush |first3=Rock I. |last4=Scholl |first4=Isabelle F. |year=2012 |title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits |journal=[[The Astrophysical Journal]] |volume=750 |issue=2 |page=135 |arxiv=1203.4898 |bibcode=2012ApJ...750..135E |doi=10.1088/0004-637X/750/2/135 |s2cid=119255559}}</ref> Thus, the Sun occupies 0.00001% (1 part in 10<sup>7</sup>) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly 1&nbsp;millionth (10<sup>−6</sup>) that of the Sun. Jupiter, the largest planet, is {{val|5.2|u=AU}} from the Sun and has a radius of {{Convert|71000|km|AU mi|abbr=on|sigfig=2}}, whereas the most distant planet, Neptune, is {{val|30|u=AU}} from the Sun.<ref name="Williams-Jupiter">{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Jupiter Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |url-status=live |archive-url=https://web.archive.org/web/20180122180353/https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |archive-date=22 January 2018 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref><ref>{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Neptune Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |url-status=live |archive-url=https://web.archive.org/web/20161119045252/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |archive-date=19 November 2016 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref>

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33&nbsp;AU farther out from the Sun than Mercury, whereas 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 relationship between these orbital distances, like the [[Titius–Bode law]]<ref>{{Cite journal |last=Jaki |first=Stanley L. |date=1 July 1972 |title=The Early History of the Titius-Bode Law |url=https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |url-status=live |journal=American Journal of Physics |volume=40 |issue=7 |pages=1014–1023 |bibcode=1972AmJPh..40.1014J |doi=10.1119/1.1986734 |issn=0002-9505 |archive-url=https://web.archive.org/web/20220420161227/https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |archive-date=20 April 2022 |access-date=2 April 2022}}</ref> and [[Mysterium Cosmographicum|Johannes Kepler's model]] based on the [[Platonic solid]]s,<ref>{{Cite journal |last=Phillips |first=J. P. |date=1965 |title=Kepler's Echinus |journal=Isis |volume=56 |issue=2 |pages=196–200 |doi=10.1086/349957 |issn=0021-1753 |jstor=227915 |s2cid=145268784}}</ref> but ongoing discoveries have invalidated these hypotheses.<ref name="Boss">{{Cite magazine |last=Boss |first=Alan |date=October 2006 |title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries? |url=https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |url-status=live |magazine=Astronomy |volume=30 |issue=10 |page=70 |archive-url=https://web.archive.org/web/20220316135255/https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |archive-date=16 March 2022 |access-date=9 April 2022 |series=Ask Astro}}</ref>

Some [[Solar System model]]s attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called [[Orrery|orreries]])—whereas others extend across cities or regional areas.<ref>{{Cite web |last=Ottewell |first=Guy |date=1989 |title=The Thousand-Yard Model: or, Earth as a Peppercorn |url=http://www.noao.edu/education/peppercorn/pcmain.html |url-status=dead |archive-url=https://web.archive.org/web/20160710065429/http://www.noao.edu/education/peppercorn/pcmain.html |archive-date=10 July 2016 |access-date=10 May 2012 |website=NOAO Educational Outreach Office}}</ref> The largest such scale model, the [[Sweden Solar System]], uses the 110-meter (361-foot) [[Avicii Arena]] in [[Stockholm]] as its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere at [[Stockholm Arlanda Airport]], 40&nbsp;km (25&nbsp;mi) away, whereas the farthest current object, [[90377 Sedna|Sedna]], is a 10&nbsp;cm (4&nbsp;in) sphere in [[Luleå]], 912&nbsp;km (567&nbsp;mi) away.<ref>{{Cite web |title=Tours of Model Solar Systems |url=http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |url-status=dead |archive-url=https://web.archive.org/web/20110412124455/http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |archive-date=12 April 2011 |access-date=10 May 2012 |publisher=University of Illinois}}</ref><ref name="Sedna">{{Cite web |title=Luleå är Sedna. I alla fall om vår sol motsvaras av Globen i Stockholm |url=http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |url-status=dead |archive-url=https://web.archive.org/web/20100715074955/http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |archive-date=15 July 2010 |access-date=10 May 2010 |publisher=Norrbotten Kuriren (in Swedish)}}</ref> At that scale, the distance to Proxima Centauri would be roughly 8 times further than the Moon is from Earth.

If the Sun–Neptune distance is scaled to {{Convert|100|m|ft|4=-1}}, then the Sun would be about {{Convert|3|cm|abbr=on}} in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about {{Convert|3|mm|abbr=on}}, and [[Earth's diameter]] along with that of the other terrestrial planets would be smaller than a [[flea]] ({{Convert|0.3|mm|abbr=on|disp=or}}) at this scale.<ref>See, for example, {{Cite web |last=Office of Space Science |date=9 July 2004 |title=Solar System Scale |url=http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |url-status=live |archive-url=https://web.archive.org/web/20160827184323/http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |archive-date=27 August 2016 |access-date=2 April 2013 |website=NASA Educator Features}}</ref>

=== Habitability ===
{{Main|Planetary habitability in the Solar System}}
{{Multiple image|perrow = 1|total_width = 350
 | direction         = vertical

 <!--image 1-->
 | image1            = PIA21424 - The TRAPPIST-1 Habitable Zone.jpg
 | alt1              =
 | caption1          = Comparison of the habitable zones of the Solar System and [[TRAPPIST-1]], an ultracool red dwarf star known to have seven terrestrial planets in stable orbits around the star.
 <!--image 2-->
 | image2            = Diagram of different habitable zone regions by Chester Harman.jpg
 | alt2              =
 | caption2          = Comparison of the [[habitable zone]]s for different stellar temperatures, with a sample of known exoplanets plus the Earth, Mars, and Venus. From top to bottom are an [[F-type main-sequence star]], a [[G-type main-sequence star|yellow dwarf]] (G-type main-sequence star), an [[orange dwarf]] (K-type main-sequence star), a typical [[red dwarf]], and an [[ultra-cool dwarf]].
}}
Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called [[cosmic ray]]s. The density of cosmic rays in the [[interstellar medium]] and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{Cite journal |last1=Langner |first1=U. W. |last2=Potgieter |first2=M. S. |date=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 |bibcode=2005AdSpR..35.2084L |doi=10.1016/j.asr.2004.12.005}}</ref>

The [[Circumstellar habitable zone|zone of habitability]] of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of [[liquid water]].<ref name="NASA-20150407">{{Cite web |last1=Dyches |first1=Preston |last2=Chou |first2=Felcia |date=7 April 2015 |title=The Solar System and Beyond is Awash in Water |url=http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water |url-status=dead |archive-url=https://web.archive.org/web/20150410113514/http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water/ |archive-date=10 April 2015 |access-date=8 April 2015 |website=[[NASA]]}}</ref> Habitability might be possible in [[subsurface ocean]]s of various outer Solar System moons.<ref>{{Cite book |last1=Robert T. Pappalardo |url=https://books.google.com/books?id=Jpcz2UoXejgC |title=Europa |last2=William B. McKinnon |last3=K. Khurana |publisher=University of Arizona Press |year=2009 |isbn=978-0-8165-2844-8 |page=658 |access-date=6 April 2023 |archive-date=6 April 2023 |archive-url=https://web.archive.org/web/20230406102731/https://books.google.com/books?id=Jpcz2UoXejgC |url-status=live }} [https://books.google.com/books?id=Jpcz2UoXejgC&pg=PA658 Extract of page 658] {{Webarchive|url=https://web.archive.org/web/20230415082720/https://books.google.com/books?id=Jpcz2UoXejgC&pg=PA658 |date=15 April 2023 }}</ref>

=== Comparison with extrasolar systems ===
Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.<ref name="Martin082015">{{Cite journal |last1=Martin |first1=Rebecca G. |last2=Livio |first2=Mario |year=2015 |title=The Solar System as an Exoplanetary System |journal=[[The Astrophysical Journal]] |volume=810 |issue=2 |page=105 |arxiv=1508.00931 |bibcode=2015ApJ...810..105M |doi=10.1088/0004-637X/810/2/105 |s2cid=119119390}}</ref><ref>{{Cite journal |last=Kohler |first=Susanna |date=25 September 2015 |title=How Normal is Our Solar System? |url=https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |url-status=live |journal=Aas Nova Highlights |publisher=American Astronomical Society |page=313 |bibcode=2015nova.pres..313K |archive-url=https://web.archive.org/web/20220407043952/https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |archive-date=7 April 2022 |access-date=31 March 2022}}</ref> The known Solar System lacks [[super-Earth]]s, planets between one and ten times as massive as the Earth,<ref name="Martin082015" /> although the hypothetical [[Planet Nine]], if it does exist, could be a super-Earth orbiting in the edge of the Solar System.<ref>{{Cite journal |last1=Sheppard |first1=Scott S. |author-link=Scott S. Sheppard |last2=Trujillo |first2=Chadwick |author-link2=Chad Trujillo |date=7 December 2016 |title=New extreme trans-Neptunian objects: Toward a super-Earth in the outer solar system |journal=The Astronomical Journal |volume=152 |issue=6 |page=221 |arxiv=1608.08772 |bibcode=2016AJ....152..221S |doi=10.3847/1538-3881/152/6/221 |issn=1538-3881 |s2cid=119187392 |doi-access=free}}</ref>

Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.<ref name="Martin082015" /><ref>{{Cite journal |last1=Volk |first1=Kathryn |last2=Gladman |first2=Brett |year=2015 |title=Consolidating and Crushing Exoplanets: Did it happen here? |journal=[[The Astrophysical Journal Letters]] |volume=806 |page=L26 |arxiv=1502.06558 |bibcode=2015ApJ...806L..26V |doi=10.1088/2041-8205/806/2/L26 |s2cid=118052299 |number=2}}</ref>

The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller [[orbital eccentricity]].<ref name="Martin082015" /> Although there are attempts to explain it partly with a bias in the [[Doppler spectroscopy|radial-velocity detection method]] and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.<ref name="Martin082015" /><ref>{{Cite journal |last1=Goldreich |first1=Peter |last2=Lithwick |first2=Yoram |last3=Sari |first3=Re'em |year=2004 |title=Final Stages of Planet Formation |journal=[[The Astrophysical Journal]] |volume=614 |issue=1 |pages=497–507 |arxiv=astro-ph/0404240 |bibcode=2004ApJ...614..497G |doi=10.1086/423612 |s2cid=16419857}}</ref>