View source for Formation and evolution of the Solar System

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[[File:The_Mysterious_Case_of_the_Disappearing_Dust.jpg|thumb|upright=1.5|Artist's conception of a [[protoplanetary disk]]]]

There is evidence that the formation of the [[Solar System]] began about 4.6 [[bya|billion years ago]] with the [[gravitational collapse]] of a small part of a giant [[molecular cloud]].<ref name="Bouvier" /> Most of the collapsing mass collected in the center, forming the [[Sun]], while the rest flattened into a [[protoplanetary disk]] out of which the [[planet]]s, [[Natural satellite|moons]], [[asteroid]]s, and other [[small Solar System bodies]] formed.

This model, known as the [[nebular hypothesis]], was first developed in the 18th century by [[Emanuel Swedenborg]], [[Immanuel Kant]], and [[Pierre-Simon Laplace]]. Its subsequent development has interwoven a variety of scientific disciplines including [[astronomy]], [[chemistry]], [[geology]], [[physics]], and [[planetary science]]. Since the dawn of the [[Space Age]] in the 1950s and the discovery of [[exoplanet]]s in the 1990s, the model has been both challenged and refined to account for new observations.

The [[Solar System]] has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later to have been captured by their planets. Still others, such as Earth's [[Moon]], may be the result of [[Giant-impact hypothesis|giant collisions]]. Collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System. Beyond Neptune, many sub-planet sized objects formed. Several thousand [[trans-Neptunian object]]s have been observed. Unlike the planets, these trans-Neptunian objects mostly move on eccentric orbits, inclined to the plane of the planets. The positions of the planets might have shifted due to gravitational interactions.<ref name="Gomes" /> The process of [[planetary migration]] explains parts of the Solar System's current structure.<ref name="Levison2007" />

In roughly 5&nbsp;billion years, the Sun will cool and expand outward to many times its current diameter (becoming a [[red giant]]), before casting off its outer layers as a [[planetary nebula]] and leaving behind a stellar remnant known as a [[white dwarf]]. In the distant future, the gravity of passing stars will gradually reduce the Sun's retinue of planets. Some planets will be destroyed, and others ejected into [[interstellar space]]. Ultimately, over the course of [[Timeline of the far future|tens of billions]] of years, it is likely that the Sun will be left with none of the original bodies in orbit around it.<ref name=dyson />

==History==
{{Main article|History of Solar System formation and evolution hypotheses}}
[[File:Pierre-Simon Laplace.jpg|thumb|upright|right|[[Pierre-Simon Laplace]], one of the originators of the nebular hypothesis]]
Ideas concerning the origin and fate of the world date from the earliest known writings; however, for almost all of that time, there was no attempt to link such theories to the existence of a "Solar System", simply because it was not generally thought that the Solar System, in the sense we now understand it, existed. The first step toward a theory of Solar System formation and evolution was the general acceptance of [[heliocentrism]], which placed the Sun at the centre of the system and the [[Earth]] in orbit around it. This concept had been developed for millennia ([[Aristarchus of Samos]] had suggested it as early as 250 BC), but was not widely accepted until the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704.<ref>{{cite web | work=Merriam Webster Online Dictionary | title=Solar system | url = http://www.merriam-webster.com/dictionary/solar%20system | access-date=2008-04-15 |year=2008 }}</ref>

The current standard [[theory]] for Solar System formation, the [[nebular hypothesis]], has fallen into and out of favour since its formulation by [[Emanuel Swedenborg]], [[Immanuel Kant]], and [[Pierre-Simon Laplace]] in the 18th century. The most significant criticism of the hypothesis was its apparent inability to explain the Sun's relative lack of [[angular momentum]] when compared to the planets.<ref>{{cite journal | author=Michael Mark Woolfson | author-link=Michael Woolfson | journal=Philosophical Transactions of the Royal Society | volume=313 | title=Rotation in the Solar System | issue=1524 |year=1984 | pages=5–18 | bibcode=1984RSPTA.313....5W | doi=10.1098/rsta.1984.0078 | s2cid=120193937 }}</ref> However, since the early 1980s studies of young stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts, which has led to its re-acceptance.<ref>{{cite web | url = https://www.newscientist.com/channel/solar-system/comets-asteroids/mg13117837.100 | title=Birth of the planets: The Earth and its fellow planets may be survivors from a time when planets ricocheted around the Sun like ball bearings on a pinball table | work=New Scientist | author=Nigel Henbest |year=1991 | access-date=2008-04-18 }}</ref>

Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power. [[Arthur Stanley Eddington]]'s confirmation of [[Albert Einstein]]'s [[theory of relativity]] led to his realisation that the Sun's energy comes from [[nuclear fusion]] reactions in its core, fusing hydrogen into helium.<ref>{{cite book | title=The Sun: A Biography | author=David Whitehouse | date=2005 | publisher=John Wiley and Sons | isbn=978-0-470-09297-2 }}</ref> In 1935, Eddington went further and suggested that other elements also might form within stars.<ref name=Hoyle2005>{{cite book | title=Fred Hoyle: A Life in Science | author=Simon Mitton | publisher=Aurum|date=2005|chapter=Origin of the Chemical Elements|pages=197–222 | isbn=978-1-85410-961-3 }}</ref> [[Fred Hoyle]] [[B2FH paper|elaborated]] on this premise by arguing that evolved stars called [[red giant]]s [[Stellar nucleosynthesis|created many elements]] heavier than hydrogen and helium in their cores. When a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems.<ref name=Hoyle2005/>

==Formation==
{{See also|Nebular hypothesis}}

===Presolar nebula===
The nebular hypothesis says that the Solar System formed from the [[gravitational collapse]] of a fragment of a giant [[molecular cloud]],<ref name="Montmerle2006" /> most likely at the edge of a [[Wolf-Rayet star|Wolf-Rayet bubble]].<ref name="Wolf-Rayet">{{cite journal |last1=Dwarkadas |first1=Vikram V. |last2=Dauphas |first2=Nicolas |last3=Meyer |first3=Bradley |last4=Boyajian |first4=Peter |last5=Bojazi |first5=Michael |date=22 December 2017 |title=Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System |journal=The Astrophysical Journal |volume=852 |issue=2 |page=147 |doi=10.3847/1538-4357/aa992e |pmid=30905969 |pmc=6430574 |arxiv=1712.10053 |bibcode=2017ApJ...851..147D |doi-access=free }}</ref> The cloud was about 20&nbsp;[[parsecs]] (65 light years) across,<ref name="Montmerle2006" /> while the fragments were roughly 1&nbsp;parsec (three and a quarter [[light-year]]s) across.<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 | author=Ann Zabludoff | author-link=Ann Zabludoff | access-date=2006-12-27 | date=Spring 2003 | archive-date=2012-07-10 | archive-url=https://archive.today/20120710135114/http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html | url-status=dead }}</ref> The further collapse of the fragments led to the formation of dense cores 0.01–0.1&nbsp;parsec (2,000–20,000&nbsp;[[Astronomical unit|AU]]) in size.<ref group="lower-alpha">An astronomical unit, or AU, is the average distance between the Earth and the Sun, or about 150 million kilometres. It is the standard unit of measurement for interplanetary distances.</ref><ref name="Montmerle2006" /><ref>{{cite journal|journal=Earth, Moon, and Planets|publisher=Springer Netherlands|volume=34|year=1986|pages=93–100 |title= Further Considerations on Contracting Solar Nebula|author=J. J. Rawal|place=Nehru Planetarium, Bombay India| doi=10.1007/BF00054038|issue=1|bibcode=1986EM&P...34...93R |s2cid=121914773}}</ref> One of these collapsing fragments (known as the ''presolar nebula'') formed what became the Solar System.<ref name="composition">{{cite conference | author=W. M. Irvine | title=The chemical composition of the pre-solar nebula | book-title=Cometary Exploration |year=1983 | volume=1 | editor=T. I. Gombosi | pages=3–12 | bibcode=1983coex....1....3I }}</ref> The composition of this region with a mass just over that of the Sun ({{Solar mass|link=y}}) was about the same as that of the Sun today, with [[hydrogen]], along with [[helium]] and trace amounts of [[lithium]] produced by [[Big Bang nucleosynthesis]], forming about 98% of its mass. The remaining 2% of the mass consisted of [[Metallicity|heavier elements]] that were created by [[nucleosynthesis]] in earlier generations of stars.{{sfn|Zeilik|Gregory|1998|p=207}} Late in the life of these stars, they ejected heavier elements into the [[interstellar medium]].<ref name=Lineweaver2001 /> Some scientists have given the name [[Coatlicue (star)|Coatlicue]] to a hypothetical star that went supernova and created the presolar nebula.

[[File:M42proplyds.jpg|thumb|left|Hubble image of [[Protoplanetary disk|protoplanetary discs]] in the [[Orion Nebula]], a light-years-wide [[stellar nursery]] probably very similar to the primordial nebula from which the Sun formed]]

The [[Calcium–aluminium-rich inclusion|oldest inclusions found in meteorites]], thought to trace the first solid material to form in the presolar nebula, are 4,568.2&nbsp;million years old, which is one definition of the age of the Solar System.<ref name="Bouvier">{{cite journal |author1=Audrey Bouvier |author2=Meenakshi Wadhwa | author2-link=Meenakshi Wadhwa | title=The age of the solar system redefined by the oldest Pb-Pb age of a meteoritic inclusion | journal=Nature Geoscience |year=2010 | doi=10.1038/NGEO941 | volume=3 |issue=9 | pages=637–641|bibcode = 2010NatGe...3..637B }}</ref> Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as [[iron-60]], that only form in exploding, short-lived stars. This indicates that one or more [[supernova]]e occurred nearby. A [[shock wave]] from a supernova may have triggered the formation of the Sun by creating relatively dense regions within the cloud, causing these regions to collapse.<ref name="CameronTruran1977">{{cite journal |last1=Cameron |first1=A. G. W. |last2=Truran |first2=J. W. |date=March 1977 |title=The supernova trigger for formation of the solar system |url=https://www.sciencedirect.com/science/article/abs/pii/0019103577901014 |journal=[[Icarus (journal)|Icarus]] |volume=30 |issue=3 |pages=447–461 |doi=10.1016/0019-1035(77)90101-4 |bibcode=1977Icar...30..447C |access-date=12 November 2022}}</ref><ref>{{Cite journal| last1 = Williams | first1 = J.| title = The astrophysical environment of the solar birthplace| journal = Contemporary Physics| volume = 51| issue = 5| pages = 381–396| year = 2010| doi = 10.1080/00107511003764725|bibcode = 2010ConPh..51..381W |arxiv = 1008.2973 | s2cid = 118354201}}</ref> The highly homogeneous distribution of iron-60 in the Solar System points to the occurrence of this supernova and its injection of iron-60 being well before the accretion of nebular dust into planetary bodies.<ref>{{cite journal |last1=Dauphas |first1=Nicolas |last2=Cook |first2=D. L. |last3=Sacarabany |first3=A. |last4=Fröhlich |first4=C. |last5=Davis |first5=A. M. |last6=Wadhwa |first6=M. |last7=Pourmand |first7=A. |last8=Rauscher |first8=T. |last9=Gallino |first9=A. |date=10 October 2008 |title=Iron 60 Evidence for Early Injection and Efficient Mixing of Stellar Debris in the Protosolar Nebula |url=https://iopscience.iop.org/article/10.1086/589959/meta |journal=[[The Astrophysical Journal]] |volume=686 |issue=1 |pages=560–569 |doi=10.1086/589959 |bibcode=2008ApJ...686..560D |s2cid=15771704 |access-date=12 November 2022|arxiv=0805.2607 }}</ref> Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars, possibly similar to the [[Orion Nebula]].<ref name=cradle>{{cite journal|author1=J. Jeff Hester |author2=Steven J. Desch |author3=Kevin R. Healy |author4=Laurie A. Leshin | title= The Cradle of the Solar System|journal=Science| date=21 May 2004 | pages= 1116–1117 | volume=304 | doi=10.1126/science.1096808 | pmid=15155936| issue=5674|bibcode = 2004Sci...304.1116H |s2cid=117722734 |url=https://pdfs.semanticscholar.org/9beb/4bbfab32abbd1dd0cc34f3345a01e179bcab.pdf |archive-url=https://web.archive.org/web/20200213000042/https://pdfs.semanticscholar.org/9beb/4bbfab32abbd1dd0cc34f3345a01e179bcab.pdf |url-status=dead |archive-date=13 February 2020 }}</ref><ref name=iron>{{cite journal| journal=Science |year= 2007| volume= 316| issue= 5828| pages=1178–1181| doi=10.1126/science.1141040| title=Evidence for a Late Supernova Injection of <sup>60</sup>Fe into the Protoplanetary Disk|author1=Martin Bizzarro |author2=David Ulfbeck |author3=Anne Trinquier |author4=Kristine Thrane |author5=James N. Connelly |author6=Bradley S. Meyer | pmid=17525336|bibcode = 2007Sci...316.1178B |s2cid= 19242845}}</ref> Studies of the structure of the [[Kuiper belt]] and of anomalous materials within it suggest that the Sun formed within a cluster of between 1,000 and 10,000 stars with a diameter of between 6.5 and 19.5 light years and a collective mass of {{Solar mass|3,000|link=y}}. This cluster began to break apart between 135 million and 535 million years after formation.<ref>{{cite web|url=http://www.princeton.edu/main/news/archive/S34/82/42M30/|publisher=News at Princeton|access-date=Sep 24, 2012|title= Slow-Moving Rocks Better Odds That Life Crashed to Earth from Space|author=Morgan Kelly}}</ref><ref>{{cite journal|title=The Lost Siblings of the Sun|author= Simon F. Portegies Zwart|journal=Astrophysical Journal|year=2009|volume=696|issue= L13–L16| doi= 10.1088/0004-637X/696/1/L13|pages=L13–L16|bibcode=2009ApJ...696L..13P |arxiv = 0903.0237 |s2cid= 17168366}}</ref> Several simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produced anomalous orbits observed in the outer Solar System, such as [[detached object]]s.<ref>{{cite journal|title= The formation of the Oort cloud in open cluster environments|author1=Nathan A. Kaib |author2=Thomas Quinn |journal=Icarus|volume= 197|issue=1|year=2008|pages= 221–238|doi=10.1016/j.icarus.2008.03.020|bibcode=2008Icar..197..221K |arxiv = 0707.4515 |s2cid=14342946 }}</ref> A recent study suggests that such a passing star is not only responsible for the orbits of the detached objects but also the [[Kuiper belt|hot and cold Kuiper belt population]], the [[Sedna (dwarf planet)|Sedna]]-like objects, the [[Extreme trans-Neptunian object|extreme TNOs]] and the [[Retrograde and prograde motion|retrograde TNOs]].<ref>{{Cite journal |last1=Pfalzner |first1=Susanne |last2=Govind |first2=Amith |last3=Portegies Zwart |first3=Simon |date=2024-09-04 |title=Trajectory of the stellar flyby that shaped the outer Solar System |journal=Nature Astronomy |volume=8 |issue=11 |language=en |pages=1380–1386 |doi=10.1038/s41550-024-02349-x |issn=2397-3366|doi-access=free |arxiv=2409.03342 |bibcode=2024NatAs...8.1380P }}</ref>

Because of the [[Conservation of Angular Momentum|conservation of angular momentum]], the nebula spun faster as it collapsed. As the material within the nebula condensed, [[kinetic theory of gases|the temperature rose]]. The center, where most of the mass collected, became increasingly hotter than the surrounding disc.<ref name= "Arizona" /> Over about 100,000 years,<ref name=Montmerle2006>{{cite journal|author1=Thierry Montmerle |author2=Jean-Charles Augereau |author3=Marc Chaussidon |title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|issue=1–4|publisher=Springer|pages=39–95|year=2006|doi=10.1007/s11038-006-9087-5| bibcode=2006EM&P...98...39M |s2cid=120504344 }}</ref> the competing forces of [[gravity]], gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning [[protoplanetary disc]] with a diameter of about 200&nbsp;AU<ref name= "Arizona" /> and form a hot, dense [[protostar]] (a star in which hydrogen fusion has not yet begun) at the centre.<ref>{{cite journal |year= 2005|author=Jane S. Greaves | title= Disks Around Stars and the Growth of Planetary Systems| journal=Science | volume=307 | pages=68–71 | doi=10.1126/science.1101979 | pmid= 15637266 | issue= 5706 |bibcode = 2005Sci...307...68G |s2cid=27720602 }}</ref> Since about half of all known stars form [[star system|systems of multiple stars]], and because [[Jupiter]] is made of the same elements as the Sun (hydrogen and helium), it has been suggested that the Solar System might have been early in its formation a protostar system with Jupiter being the second but failed protostar, but Jupiter has far too little mass to trigger fusion in its core and so became a [[gas giant]]; it is in fact younger than the Sun and the oldest planet of the Solar System.<ref name="Scientific American 1999 g762">{{cite web | title=I have heard people call Jupiter a "failed star" that just did not get big enough to shine. Does that make our sun a kind of double star? And why didn't Jupiter become a real star? | website=Scientific American | date=1999-10-21 | url=https://www.scientificamerican.com/article/i-have-heard-people-call/ | access-date=2023-12-05}}</ref><ref name="DROBYSHEVSKI 1974 pp. 35–36">{{cite journal | last=DROBYSHEVSKI | first=E. M. | title=Was Jupiter the protosun's core? | journal=Nature | publisher=Springer Science and Business Media LLC | volume=250 | issue=5461 | year=1974 | issn=0028-0836 | doi=10.1038/250035a0 | pages=35–36| bibcode=1974Natur.250...35D }}</ref>

At this point in the Sun's [[stellar evolution|evolution]], the Sun is thought to have been a [[T Tauri star]].<ref name=apj2_313>{{cite journal |author1=Caffe, M. W. |author2=Hohenberg, C. M. |author3=Swindle, T. D. |author4=Goswami, J. N. | title=Evidence in meteorites for an active early sun | journal=Astrophysical Journal Letters | volume=313 | date=February 1, 1987 | pages=L31–L35 | doi=10.1086/184826 | bibcode=1987ApJ...313L..31C | hdl=2060/19850018239 | hdl-access=free }}</ref> Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of {{Solar mass|0.001–0.1}}.<ref name= "Kitamara">{{cite conference |author1=M. Momose |author2=Y. Kitamura |author3=S. Yokogawa |author4=R. Kawabe |author5=M. Tamura |author6=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 |journal=The Proceedings of the IAU 8th Asian-Pacific Regional Meeting | book-title=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. |editor2=Hearnshaw, J. |editor3= Hanawa, T. | pages=85 |bibcode=2003ASPC..289...85M }}</ref> These discs extend to several hundred&nbsp;[[Astronomical unit|AU]]—the [[Hubble Space Telescope]] has observed protoplanetary discs of up to 1000&nbsp;AU in diameter in [[Star formation|star-forming regions]] such as the Orion Nebula<ref>{{cite journal| journal=The Astronomical Journal|date=March 1999| volume= 117| issue=3| pages=1490–1504| doi=10.1086/300781| title=Hubble Space Telescope/NICMOS Imaging of Disks and Envelopes around Very Young Stars|author1=Deborah L. Padgett |author2=Wolfgang Brandner |author3=Karl R. Stapelfeldt |display-authors=etal | bibcode=1999AJ....117.1490P|arxiv = astro-ph/9902101 |s2cid=16498360}}</ref>—and are rather cool, reaching a surface temperature of only about {{cvt|1000|K|C F|||}} at their hottest.<ref>{{cite journal |author1=M. Küker |author2=T. Henning |author3=G. Rüdiger | title=Magnetic Star-Disk Coupling in Classical T Tauri Systems | journal=Astrophysical Journal |year=2003 | volume=589 | issue=1 | pages=397–409 | doi=10.1086/374408 | bibcode=2003ApJ...589..397K |s2cid=54039084 |url=http://pdfs.semanticscholar.org/8402/67bfa6887ea23cc1e4610c42cfe012fc8de6.pdf |archive-url=https://web.archive.org/web/20200412143753/http://pdfs.semanticscholar.org/8402/67bfa6887ea23cc1e4610c42cfe012fc8de6.pdf |url-status=dead |archive-date=2020-04-12 }}</ref>
Within 50&nbsp;million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until [[hydrostatic equilibrium]] was achieved.<ref name="Yi2001">{{cite journal |author1=Sukyoung Yi |author2=Pierre Demarque |author3=Yong-Cheol Kim |author4=Young-Wook Lee |author5=Chang H. Ree |author6=Thibault Lejeune |author7=Sydney Barnes | title=Toward Better Age Estimates for Stellar Populations: The <math>Y^{2}</math> Isochrones for Solar Mixture | journal=Astrophysical Journal Supplement | arxiv=astro-ph/0104292 | date=2001 | volume=136 |issue=2 | pages=417–437 | doi=10.1086/321795 | bibcode=2001ApJS..136..417Y |s2cid=118940644 }}</ref> This marked the Sun's entry into the prime phase of its life, known as the [[main sequence]]. Main-sequence stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a main-sequence star today.<ref name=sequence>{{harvnb|Zeilik|Gregory|1998|loc=p. 320 }}</ref>

As the early Solar System continued to evolve, it eventually drifted away from its siblings in the stellar nursery, and continued orbiting the [[Milky Way]]'s center on its own. The Sun likely drifted from its original orbital distance from the center of the galaxy. The chemical history of the Sun suggests it may have formed as much as 3&nbsp;kpc closer to the galaxy core.<ref>{{cite journal
 | title=Keeping It Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk
 | last1=Frankel | first1=Neige | last2=Sanders | first2=Jason
 | last3=Ting | first3=Yuan-Sen | last4=Rix | first4=Hans-Walter
 | journal=The Astrophysical Journal
 | volume=896 | issue=1 | id=15 | date=June 2020
 | page=15 | doi=10.3847/1538-4357/ab910c | arxiv=2002.04622
 | bibcode=2020ApJ...896...15F | s2cid=211082559 | doi-access=free }} See §6.4.</ref>

=== Solar System birth environment ===
Like most stars, the Sun likely formed not in isolation but as part of a young [[star cluster]].<ref>{{Cite journal |last=Adams |first=Fred C. |date=2010-08-01 |title=The Birth Environment of the Solar System |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-081309-130830 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=48 |issue=1 |pages=47–85 |doi=10.1146/annurev-astro-081309-130830 |issn=0066-4146|arxiv=1001.5444 |bibcode=2010ARA&A..48...47A |s2cid=119281082 }}</ref> There are several indications that hint at the cluster environment having had some influence over the young, still-forming Solar System. For example, the decline in mass beyond Neptune and the extreme eccentric-orbit of [[90377 Sedna|Sedna]] have been interpreted as a signature of the Solar System having been influenced by its birth environment. Whether the presence of the isotopes [[iron-60]] and [[aluminium-26]] can be interpreted as a sign of a birth cluster containing massive stars is still under debate.  If the Sun was part of a star cluster, it might have been influenced by close [[Flyby (astronomy)|flybys]] of other stars, the strong radiation of nearby [[List of most massive stars|massive stars]] and ejecta from [[supernova]]e occurring close by.

==={{anchor|Formation of planets}}Formation of the planets===
{{See also|Protoplanetary disk|#Terrestrial planets}}

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation.<ref>{{cite journal|title=Chondrule-forming Shock Fronts in the Solar Nebula: A Possible Unified Scenario for Planet and Chondrite Formation|author1=A. P. Boss |author2=R. H. Durisen |doi=10.1086/429160|year=2005|journal=The Astrophysical Journal|volume=621|issue=2|pages=L137–L140|bibcode=2005ApJ...621L.137B|arxiv = astro-ph/0501592 |s2cid=15244154 }}</ref> The currently accepted method by which the planets formed is [[Accretion (astrophysics)|accretion]], in which the planets began as dust grains in orbit around the central protostar. Through direct contact and [[self-organization]], these grains formed into clumps up to {{cvt|200|m||||}} in diameter, which in turn collided to form larger bodies ([[planetesimals]]) of ~{{cvt|10|km||||}} in size. These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.<ref name=Goldreich1973>{{cite journal |author1=P. Goldreich |author2=W. R. Ward | title=The Formation of Planetesimals | journal=Astrophysical Journal |year=1973 | volume=183 | pages=1051 | bibcode=1973ApJ...183.1051G | doi=10.1086/152291 | doi-access=free }}</ref>

The [[inner Solar System]], the region of the Solar System inside 4&nbsp;AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like [[iron]], [[nickel]], and [[aluminium]]) and rocky [[silicate]]s. These rocky bodies would become the [[terrestrial planet]]s ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]). These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large.<ref name=Arizona /> The terrestrial embryos grew to about 0.05 [[Earth mass]]es ({{Earth mass}}) and ceased accumulating matter about 100,000&nbsp;years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes.<ref name=sciam />

When terrestrial planets were forming, they remained immersed in a disk of gas and dust. Pressure partially supported the gas and so did not orbit the Sun as rapidly as the planets. The resulting [[drag (physics)|drag]] and, more importantly, gravitational interactions with the surrounding material caused a transfer of [[angular momentum]], and as a result the planets gradually migrated to new orbits. Models show that density and temperature variations in the disk governed this rate of migration,<ref name=dangelo_lubow_2010>{{cite journal|last=D'Angelo|first=G.|author2=Lubow, S. H. |title=Three-dimensional Disk-Planet Torques in a Locally Isothermal Disk|journal=The Astrophysical Journal|date=2010|volume=724|issue=1|pages=730–747|doi=10.1088/0004-637X/724/1/730|arxiv = 1009.4148 |bibcode = 2010ApJ...724..730D |s2cid=119204765}}</ref><ref name=li2011>{{cite book|last=Lubow|first=S. H.|author2=Ida, S. |chapter=Planet Migration |bibcode=2010exop.book..347L| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=347–371|year=2011|arxiv=1004.4137 }}</ref> but the net trend was for the inner planets to migrate inward as the disk dissipated, leaving the planets in their current orbits.<ref>{{cite web
 | author=Staff | title=How Earth Survived Birth
 | work=Astrobiology Magazine
 | date=12 January 2010
 | url = http://www.astrobio.net/pressrelease/3370/how-earth-survived-birth | access-date=2010-02-04
 |archive-url=https://web.archive.org/web/20100412232232/http://astrobio.net/pressrelease/3370/how-earth-survived-birth |archive-date=2010-04-12
 |url-status=usurped}}</ref>

The [[giant planet]]s ([[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]) formed further out, beyond the [[frost line (astrophysics)|frost line]], which is the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the [[Giant planet|Jovian]] planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most [[Abundance of the chemical elements|abundant]] elements.<ref name="Arizona" /> Planetesimals beyond the frost line accumulated up to {{Earth mass|4}} within about 3&nbsp;million years.<ref name=sciam /> Today, the four giant planets comprise just under 99% of all the mass orbiting the Sun.<ref group=lower-alpha>The combined mass of Jupiter, Saturn, Uranus and Neptune is 445.6 Earth masses. The mass of remaining material is ~5.26 Earth masses or 1.1% (see [[Solar System#Notes]] and [[List of Solar System objects by mass]])</ref> Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused the material to accumulate rapidly at ~5&nbsp;AU from the Sun. This excess material coalesced into a large embryo (or core) on the order of {{Earth mass|10}}, which began to accumulate an envelope via accretion of gas from the surrounding disc at an ever-increasing rate.<ref name=ayliffe_bate_2009>{{cite journal|last=Ayliffe|first=B.|author2= Bate, M. R.|title=Gas accretion on to planetary cores: three-dimensional self-gravitating radiation hydrodynamical calculations|journal=Monthly Notices of the Royal Astronomical Society|year=2009|volume=393|issue=1|pages=49–64|doi=10.1111/j.1365-2966.2008.14184.x|doi-access=free |arxiv = 0811.1259 |bibcode = 2009MNRAS.393...49A |s2cid=15124882}}</ref><ref name=dangelo_bodenheimer_2013>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=Three-dimensional Radiation-hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=The Astrophysical Journal|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D |s2cid=118522228}}</ref> Once the envelope mass became about equal to the solid core mass, growth proceeded very rapidly, reaching about 150&nbsp;Earth masses ~10<sup>5</sup>&nbsp;years thereafter and finally topping out at {{Earth mass|318}}.<ref name=lhdb2009>{{cite journal|last=Lissauer|first=J. J.|author2=Hubickyj, O. |author3=D'Angelo, G. |author4=Bodenheimer, P. |title=Models of Jupiter's growth incorporating thermal and hydrodynamic constraints| journal=Icarus|year=2009|volume=199|issue=2| pages=338–350|arxiv=0810.5186|doi=10.1016/j.icarus.2008.10.004|bibcode=2009Icar..199..338L |s2cid=18964068}}</ref> Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume.<ref name=sciam>{{cite journal|title=The Genesis of Planets|author=Douglas N. C. Lin | journal=Scientific American | issue=5|volume=298|date=May 2008 | pages=50–59 | url = http://www.sciam.com/article.cfm?id=the-genesis-of-planets | format=fee required|doi=10.1038/scientificamerican0508-50|doi-broken-date=1 November 2024 |pmid=18444325 |bibcode = 2008SciAm.298e..50C }}</ref><ref name="ddl2011">{{cite book |chapter=Giant Planet Formation |title=Exoplanets |publisher=University of Arizona Press |first1=Gennaro |last1=D'Angelo |first2=Richard H. |last2=Durisen |first3=Jack J. |last3=Lissauer |editor-first=Sara |editor-last=Seager |pages=319–346 |date=December 2010 |isbn=978-0-8165-2945-2 |bibcode=2010exop.book..319D |arxiv=1006.5486}}</ref>

[[T Tauri star]]s like the young Sun have far stronger [[stellar wind]]s than more stable, older stars. Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong [[solar wind]] had blown away much of the disc material. As a result, those planets accumulated little hydrogen and helium—not more than {{Earth mass|1}} each. Uranus and Neptune are sometimes referred to as failed cores.<ref name="thommes" /> The main problem with formation theories for these planets is the timescale of their formation. At the current locations it would have taken millions of years for their cores to accrete.<ref name="ddl2011" /> This means that Uranus and Neptune may have formed closer to the Sun—near or even between Jupiter and Saturn—later migrating or being ejected outward (see [[#Planetary migration|Planetary migration]] below).<ref name="thommes" /><ref name="Levison2007" /> Motion in the planetesimal era was not all inward toward the Sun; the ''[[Stardust (spacecraft)|Stardust]]'' sample return from [[Comet Wild 2]] has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt.<ref>{{cite web |year= 2006|author=Emily Lakdawalla|work=The Planetary Society|title=Stardust Results in a Nutshell: The Solar Nebula was Like a Blender | url = http://www.planetary.org/blog/article/00000735/| access-date=2007-01-02 }}</ref>

After between three and ten million years,<ref name=sciam/> the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets.<ref>{{cite journal | author=B. G. Elmegreen| title=On the disruption of a protoplanetary disc nebula by a T Tauri like solar wind | journal=Astronomy & Astrophysics |year=1979 | volume=80 | issue=1 | pages=77 | bibcode=1979A&A....80...77E }}</ref><ref>{{cite web |date=24 November 2004 |author=Heng Hao |publisher=Harvard University |title=Disc-Protoplanet interactions |url=http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |access-date=2006-11-19 |url-status=dead |archive-url=https://web.archive.org/web/20060907170907/http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |archive-date=7 September 2006 }}</ref>

==Subsequent evolution==
The planets were originally thought to have formed in or near their current orbits. This has been questioned during the last 20 years. Currently, many planetary scientists think that the Solar System might have looked very different after its initial formation: several objects at least as massive as Mercury may have been present in the inner Solar System, the outer Solar System may have been much more compact than it is now, and the [[Kuiper belt]] may have been much closer to the Sun.<ref>{{cite web|author=Mike Brown |url = http://www.gps.caltech.edu/~mbrown/planetlila/moon/index.html |title=Dysnomia, the moon of Eris |work=Personal web site|access-date=2008-02-01 |author-link = Michael E. Brown }}</ref>

===Terrestrial planets===
At the end of the planetary formation epoch, the inner Solar System was populated by 50–100 Moon-to-[[Mars]]-sized [[protoplanet]]s.<ref name="Petit2001" /><ref name="Kominami">{{cite journal|title=The Effect of Tidal Interaction with a Gas Disk on Formation of Terrestrial Planets|author1=Junko Kominami |author2=Shigeru Ida |place=Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo|year=2001 | doi=10.1006/icar.2001.6811|journal=Icarus
|volume=157|issue=1|pages=43–56|bibcode=2002Icar..157...43K }}</ref> Further growth was possible only because these bodies collided and merged, which took less than 100&nbsp;million years. These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape.<ref name=sciam /> One such giant collision is thought to have formed the Moon (see [[#Moons|Moons]] below), while another removed the outer envelope of the young [[Mercury (planet)|Mercury]].<ref name=Solomon2003>{{cite journal|author=Sean C. Solomon|title=Mercury: the enigmatic innermost planet|journal=Earth and Planetary Science Letters|volume=216|issue=4|year=2003|pages=441–455|doi=10.1016/S0012-821X(03)00546-6| bibcode=2003E&PSL.216..441S }}</ref>

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto-terrestrial planets, which would have needed to be highly eccentric in order to collide, produced the remarkably stable and nearly circular orbits they have today.<ref name="Petit2001" /> One hypothesis for this "eccentricity dumping" is that terrestrials formed in a disc of gas still not expelled by the Sun. The "[[Dynamical friction|gravitational drag]]" of this residual gas would have eventually lowered the planets' energy, smoothing out their orbits.<ref name= Kominami /> However, such gas, if it existed, would have prevented the terrestrial planets' orbits from becoming so eccentric in the first place.<ref name=sciam /> Another hypothesis is that gravitational drag occurred not between the planets and residual gas but between the planets and the remaining small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by the larger planets' gravity, formed a region of higher density, a "gravitational wake", in the larger objects' path. As they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits.<ref>{{cite journal|title=Final Stages of Planet Formation|author1=Peter Goldreich |author2=Yoram Lithwick |author3=Re'em Sari |journal=The Astrophysical Journal|volume=614|issue=1|pages=497–507|date=10 October 2004|doi=10.1086/423612|bibcode=2004ApJ...614..497G|arxiv = astro-ph/0404240 |s2cid=16419857 }}</ref>

===Asteroid belt===
The outer edge of the terrestrial region, between 2 and 4&nbsp;AU from the Sun, is called the [[asteroid belt]]. The asteroid belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of [[planetesimal]]s formed there. As with the terrestrials, planetesimals in this region later coalesced and formed 20–30 Moon- to Mars-sized [[planetary embryo]]s;<ref name="Bottke2005">{{cite journal |author1=Bottke, William F. |author2=Durda, Daniel D. |author3=Nesvorny, David |display-authors=et al. | title=Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion | journal=Icarus | volume=179|issue=1 | pages=63–94|year=2005 | doi=10.1016/j.icarus.2005.05.017 | url = http://www.boulder.swri.edu/~bottke/Reprints/Bottke_Icarus_2005_179_63-94_Linking_Collision_Dynamics_MB.pdf| bibcode=2005Icar..179...63B }}</ref> however, the proximity of Jupiter meant that after this planet formed, 3&nbsp;million years after the Sun, the region's history changed dramatically.<ref name="Petit2001">{{cite journal |first1=Jean-Marc |last1=Petit |first2=Alessandro |last2=Morbidelli |title=The Primordial Excitation and Clearing of the Asteroid Belt |journal=Icarus |volume=153 |issue=2 |pages=338–347 |year=2001 |doi=10.1006/icar.2001.6702 |url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |bibcode=2001Icar..153..338P |access-date=2006-11-19 |archive-date=2007-02-21 |archive-url=https://web.archive.org/web/20070221085835/http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |url-status=dead }}</ref> [[Orbital resonance]]s with Jupiter and Saturn are particularly strong in the asteroid belt, and gravitational interactions with more massive embryos scattered many planetesimals into those resonances. Jupiter's gravity increased the velocity of objects within these resonances, causing them to shatter upon collision with other bodies, rather than accrete.<ref>{{cite journal |author1=R. Edgar |author2=P. Artymowicz | title=Pumping of a Planetesimal Disc by a Rapidly Migrating Planet | journal=[[Monthly Notices of the Royal Astronomical Society]] |year=2004 | volume=354 | issue=3 | pages=769–772 | url = http://www.utsc.utoronto.ca/~pawel/edgar+artymowicz.pdf | access-date=2008-05-12 | doi = 10.1111/j.1365-2966.2004.08238.x |doi-access=free | bibcode=2004MNRAS.354..769E|arxiv = astro-ph/0409017 |s2cid=18355985 }}</ref>

As Jupiter migrated inward following its formation (see [[#Planetary migration|Planetary migration]] below), resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.<ref>{{cite conference | author=E. R. D. Scott | title=Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids | book-title = Proceedings 37th Annual Lunar and Planetary Science Conference | publisher = Lunar and Planetary Society |year=2006 | location = League City, Texas | bibcode =2006LPI....37.2367S }}</ref> The cumulative action of the resonances and the embryos either scattered the planetesimals away from the asteroid belt or excited their [[orbital inclination]]s and [[orbital eccentricity|eccentricities]].<ref name="Bottke2005" /><ref name="OBrien2007" /> Some of those massive embryos too were ejected by Jupiter, while others may have migrated to the inner Solar System and played a role in the final accretion of the terrestrial planets.<ref name="Bottke2005" /><ref name="Raymond2007" /><ref>{{cite web| author = Susan Watanabe| date = 20 July 2001| url = http://www.jpl.nasa.gov/news/features.cfm?feature=520| title = Mysteries of the Solar Nebula| publisher = NASA| access-date = 2007-04-02| archive-date = 2012-01-17| archive-url = https://web.archive.org/web/20120117093701/http://www.jpl.nasa.gov/news/features.cfm?feature=520| url-status = dead}}</ref> During this primary depletion period, the effects of the giant planets and planetary embryos left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed mainly of small planetesimals.<ref name=OBrien2007>{{cite journal|author1=O'Brien, David |author2=Morbidelli, Alessandro |author3=Bottke, William F. |title=The primordial excitation and clearing of the asteroid belt—Revisited |journal=Icarus |volume=191 |issue=2 |pages=434–452 |year=2007 |doi=10.1016/j.icarus.2007.05.005 |url=http://www.boulder.swri.edu/~bottke/Reprints/OBrien_2007_Icarus_191_434_Primordial_Excitation_Clearing_Asteroid_Belt.pdf|bibcode=2007Icar..191..434O }}</ref>
This is still 10–20 times more than the current mass in the main belt, which is now about {{Earth mass|0.0005}}.<ref name=Krasinsky2002>{{cite journal |author=Georgij A. Krasinsky |author2=Elena V. Pitjeva |author3=M. V. Vasilyev |author4=E. I. Yagudina | bibcode=2002Icar..158...98K |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |date=July 2002 |doi=10.1006/icar.2002.6837 |author2-link=Elena V. Pitjeva |author-link=Georgij A. Krasinsky }}</ref> A secondary depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below).

The inner Solar System's period of giant impacts probably played a role in Earth acquiring its current water content (~6{{e|21}}&nbsp;kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently delivered from outer, colder parts of the Solar System.<ref name=Hsieh2006 /> The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter.<ref name=Raymond2007>{{cite journal |author1=Raymond, Sean N. |author2=Quinn, Thomas |author3=Lunine, Jonathan I. |title=High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability | journal=Astrobiology | volume=7 | pages=66–84 |year=2007 | doi=10.1089/ast.2006.06-0126 | bibcode=2007AsBio...7...66R | pmid=17407404 | issue=1|arxiv = astro-ph/0510285 |s2cid=10257401 }}</ref> A population of [[main-belt comet]]s discovered in 2006 has also been suggested as a possible source for Earth's water.<ref name=Hsieh2006>{{cite journal|title=A Population of Comets in the Main Asteroid Belt |author=Henry H. Hsieh |author2=David Jewitt |journal=Science|date=23 March 2006 | volume=312|pages=561–563 | doi=10.1126/science.1125150 |pmid=16556801|issue=5773|bibcode = 2006Sci...312..561H |s2cid=29242874 |author2-link=David Jewitt |url=http://pdfs.semanticscholar.org/27c8/90717985ed970189e4577179cb3ad6f2e8c9.pdf |archive-url=https://web.archive.org/web/20200412143808/http://pdfs.semanticscholar.org/27c8/90717985ed970189e4577179cb3ad6f2e8c9.pdf |url-status=dead |archive-date=12 April 2020 }}</ref><ref>{{cite web|title=New comet class in Earth's backyard|url = http://www.astronomy.com/asy/default.aspx?c=a&id=4100| publisher=astronomy.com|author=Francis Reddy|year=2006|access-date=2008-04-29 }}</ref> In contrast, [[comet]]s from the Kuiper belt or farther regions delivered not more than about 6% of Earth's water.<ref name="Gomes" /><ref>{{cite journal |author1=Morbidelli, Alessandro |author2=Chambers, J. |author3=Lunine, Jonathan I. |author4=Petit, Jean-Marc |author5=Robert, F. |author6=Valsecchi, Giovanni B. |author7=Cyr, K. E. | title= Source regions and timescales for the delivery of water to the Earth | journal= Meteoritics & Planetary Science | volume=35 | pages=1309–1320 | issn= 1086-9379 |year=2000 |bibcode = 2000M&PS...35.1309M |doi = 10.1111/j.1945-5100.2000.tb01518.x | issue= 6 |doi-access=free }}</ref> The [[panspermia]] hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely accepted.<ref>{{cite journal|title=From Panspermia to Bioastronomy, the Evolution of the Hypothesis of Universal Life|author1=Florence Raulin-Cerceau |author2=Marie-Christine Maurel |author3=Jean Schneider |publisher=Springer Netherlands|journal=Origins of Life and Evolution of Biospheres|year=1998|volume=28|issue=4/6|doi=10.1023/A:1006566518046 | pages=597–612|pmid=11536892 |bibcode=1998OLEB...28..597R |s2cid=7806411 }}</ref>

===Planetary migration===
{{Main article|Nice model|Grand tack hypothesis}}
{{See also|Five-planet Nice model}}
According to the nebular hypothesis, the outer two planets may be in the "wrong place". [[Uranus]] and [[Neptune]] (known as the "[[ice giant]]s") exist in a region where the reduced density of the solar nebula and longer orbital times render their formation there highly implausible.<ref name=Taylor2001>{{cite web|url = http://www.psrd.hawaii.edu/Aug01/bombardment.html|title= Uranus, Neptune, and the Mountains of the Moon|work= Planetary Science Research Discoveries | date=21 August 2001 | author=G. Jeffrey Taylor | publisher = Hawaii Institute of Geophysics & Planetology |access-date=2008-02-01 }}</ref> The two are instead thought to have formed in orbits near Jupiter and Saturn (known as the "[[gas giant]]s"), where more material was available, and to have [[Planetary migration|migrated outward]] to their current positions over hundreds of millions of years.<ref name="thommes">{{cite journal |author1=Thommes, E. W. |author2=Duncan, M. J. |author3=Levison, Harold F. | title=The Formation of Uranus and Neptune among Jupiter and Saturn | journal=Astronomical Journal | arxiv=astro-ph/0111290 |year=2002 | volume=123 | issue=5 | pages=2862–2883 | doi=10.1086/339975 | bibcode=2002AJ....123.2862T |s2cid=17510705 }}</ref>

[[File:Lhborbits.png|thumb|upright=2|Simulation showing outer planets and Kuiper belt:<ref name="Gomes">{{cite journal | title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets |author1=Gomes, R. |author2=Levison, Harold F. |author3=Tsiganis, K. |author4=Morbidelli, Alessandro | journal=Nature |year=2005 | volume=435 | pages=466–9 | doi=10.1038/nature03676| pmid=15917802 | issue=7041|bibcode = 2005Natur.435..466G | doi-access=free }}</ref><br />
a) Before Jupiter/Saturn 2:1 resonance<br />
b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune<br />
c) After ejection of Kuiper belt bodies by Jupiter<br />
{{legend-line|2px solid lime|Orbit of Jupiter}}
{{legend-line|2px solid goldenrod|Orbit of Saturn}}
{{legend-line|2px solid cyan|Orbit of Uranus}}
{{legend-line|2px solid blue|Orbit of Neptune}}]]
The migration of the outer planets is also necessary to account for the existence and properties of the Solar System's outermost regions.<ref name="Levison2007">{{cite journal | first1=Harold F. |last1=Levison |first2=Alessandro |last2=Morbidelli |first3=Christa |last3=Van Laerhoven | display-authors=etal | title=Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune|year=2007| bibcode=2008Icar..196..258L | arxiv=0712.0553 | doi=10.1016/j.icarus.2007.11.035 | journal=Icarus | volume=196 | issue=1 | pages=258–273 |s2cid=7035885 }}</ref> [[Trans-Neptunian object|Beyond Neptune]], the Solar System continues into the [[Kuiper belt]], the [[scattered disc]], and the [[Oort cloud]], three sparse populations of small icy bodies thought to be the points of origin for most observed [[comet]]s. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet.<ref name="Taylor2001" /> The Kuiper belt lies between 30 and 55&nbsp;AU from the Sun, while the farther scattered disc extends to over 100&nbsp;AU,<ref name="Levison2007" /> and the distant Oort cloud begins at about 50,000&nbsp;AU.<ref>{{cite arXiv
 | title=Origin and dynamical evolution of comets and their reservoirs
 | first=Alessandro |last=Morbidelli
 | eprint=astro-ph/0512256
 | date=3 February 2008
}}</ref> Originally, however, the Kuiper belt was much denser and closer to the Sun, with an outer edge at approximately 30&nbsp;AU. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20&nbsp;AU), and in 50% of simulations ended up in opposite locations, with Uranus farther from the Sun than Neptune.<ref name="Tsiganis05">
{{cite journal
  |last1=Tsiganis |first1=K.
  |first2=R. |last2=Gomes
  |first3=A. |last3=Morbidelli
  |first4=H. |last4=F. Levison
  |date=2005
  |title=Origin of the orbital architecture of the giant planets of the Solar System
  |journal=Nature
  |volume=435   |issue=7041 |pages=459–461
  |url= https://www-n.oca.eu/morby/papers/nature-papers-5-26-05.pdf
  |doi=10.1038/nature03539
  |pmid=15917800
  |bibcode = 2005Natur.435..459T |s2cid=4430973
 }}</ref><ref name="Gomes" /><ref name="Levison2007" />

According to the [[Nice model]], after the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600&nbsp;million years (about 4&nbsp;billion years ago) Jupiter and Saturn fell into a 2:1 resonance: Saturn orbited the Sun once for every two Jupiter orbits.<ref name="Levison2007" /> This resonance created a gravitational push against the outer planets, possibly causing Neptune to surge past Uranus and plough into the ancient Kuiper belt.<ref name="Tsiganis05" />
The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards.<ref name="Levison2007" /> This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward.<ref group=lower-alpha>The reason that Saturn, Uranus and Neptune all moved outward whereas Jupiter moved inward is that Jupiter is massive enough to eject planetesimals from the Solar System, while the other three outer planets are not. To eject an object from the Solar System, Jupiter transfers energy to it, and so loses some of its own orbital energy and moves inwards. When Neptune, Uranus and Saturn perturb planetesimals outwards, those planetesimals end up in highly eccentric but still bound orbits, and so can return to the perturbing planet and possibly return its lost energy. On the other hand, when Neptune, Uranus and Saturn perturb objects inwards, those planets gain energy by doing so and therefore move outwards. More importantly, an object being perturbed inwards stands a greater chance of encountering Jupiter and being [[Interstellar comet|ejected]] from the Solar System, in which case the energy gains of Neptune, Uranus and Saturn obtained from their inwards deflections of the ejected object become permanent.</ref> Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud;<ref name="Levison2007" /> those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc.<ref name="Levison2007" /> This scenario explains the Kuiper belt's and scattered disc's present low mass. Some of the scattered objects, including [[Pluto]], became gravitationally tied to Neptune's orbit, forcing them into [[mean-motion resonance#Types of resonance|mean-motion resonance]]s.<ref name=Malhorta1995>{{cite journal | author=R. Malhotra | title=The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune | journal=Astronomical Journal | arxiv=astro-ph/9504036 |year=1995 | volume=110 | pages=420 | doi=10.1086/117532 | bibcode=1995AJ....110..420M | s2cid=10622344 }}</ref> Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune near-circular again.<ref name="Levison2007" /><ref name="fogg_nelson">{{cite journal |author1=M. J. Fogg |author2=R. P. Nelson | title=On the formation of terrestrial planets in hot-Jupiter systems | journal=Astronomy & Astrophysics | arxiv=astro-ph/0610314 |year=2007 | volume = 461 | issue=3 | pages=1195–1208 | doi=10.1051/0004-6361:20066171 | bibcode=2007A&A...461.1195F |s2cid=119476713 }}</ref>

In contrast to the outer planets, the inner planets are not thought to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts.<ref name=sciam/>

Another question is why Mars came out so small compared with Earth. A study by Southwest Research Institute, San Antonio, Texas, published June 6, 2011 (called the [[Grand tack hypothesis]]), proposes that Jupiter had migrated inward to 1.5&nbsp;AU. After Saturn formed, migrated inward, and established the 2:3 mean motion resonance with Jupiter, the study assumes that both planets migrated back to their present positions. Jupiter thus would have consumed much of the material that would have created a bigger Mars. The same simulations also reproduce the characteristics of the modern asteroid belt, with dry asteroids and water-rich objects similar to comets.<ref>{{cite press release | url=http://www.astronomy.com/News-Observing/News/2011/06/Jupiter%20may%20have%20robbed%20Mars%20of%20mass%20new%20report%20indicates.aspx | title= Jupiter may have robbed Mars of mass, new report indicates | work= Southwest Research Institute, San Antonio, Texas | date=June 6, 2011}}</ref><ref>{{Cite journal |author1=Walsh, K. J. |author2=Morbidelli, Alessandro |author3=Raymond, S. N. |author4=O'Brien, D. P. |author5=Mandell, A. M. | title = A low mass for Mars from Jupiter's early gas-driven migration | doi = 10.1038/nature10201 | journal = Nature | volume = 475 | issue = 7355 | pages = 206–209  |year = 2011 | pmid =  21642961 | arxiv = 1201.5177 | bibcode = 2011Natur.475..206W |s2cid=4431823 }}</ref> However, it is unclear whether conditions in the solar nebula would have allowed Jupiter and Saturn to move back to their current positions, and according to current estimates this possibility appears unlikely.<ref name=dangelo_marzari_2012>{{cite journal|last=D'Angelo|first=G.|author2= Marzari, F. |title=Outward Migration of Jupiter and Saturn in Evolved Gaseous Disks|journal=The Astrophysical Journal|year=2012|volume=757|issue=1|pages=50 (23 pp.)|doi=10.1088/0004-637X/757/1/50|arxiv = 1207.2737 |bibcode = 2012ApJ...757...50D |s2cid=118587166}}</ref> Moreover, alternative explanations for the small mass of Mars exist.<ref name="Chambers2013">{{cite journal|last=Chambers|first=J. E.|title=Late-stage planetary accretion including hit-and-run collisions and fragmentation|journal=Icarus|year=2013|volume=224|issue=1|pages=43–56|doi=10.1016/j.icarus.2013.02.015|bibcode=2013Icar..224...43C }}</ref><ref name=Izidoro2014>{{cite journal|last=Izidoro|first=A.|author2=Haghighipour, N.|author3=Winter, O. C.|author4=Tsuchida, M.|title=Terrestrial Planet Formation in a Protoplanetary Disk with a Local Mass Depletion: A Successful Scenario for the Formation of Mars|journal=The Astrophysical Journal|year=2014|volume=782|issue=1|pages= 31, (20 pp.)|doi=10.1088/0004-637X/782/1/31|arxiv = 1312.3959|bibcode=2014ApJ...782...31I |s2cid=118419463}}</ref><ref name=fischer2014>{{cite journal|last=Fischer|first=R. A.|author2= Ciesla, F. J.|title=Dynamics of the terrestrial planets from a large number of N-body simulations|journal=Earth and Planetary Science Letters|year=2014|volume=392|pages= 28–38|doi=10.1016/j.epsl.2014.02.011|bibcode=2014E&PSL.392...28F }}</ref>

===Late Heavy Bombardment and after===
{{Main article|Late Heavy Bombardment}}
[[File:Barringer Crater aerial photo by USGS.jpg|thumb|left|[[Meteor Crater]] in Arizona. Created 50,000 years ago by an impactor about {{convert|50|m|ft}} across, it shows that the accretion of the Solar System is not over.]]
Gravitational disruption from the outer planets' migration would have sent large numbers of asteroids into the inner Solar System, severely depleting the original belt until it reached today's extremely low mass.<ref name=OBrien2007/> This event may have triggered the Late Heavy Bombardment that is hypothesised to have occurred approximately 4&nbsp;billion years ago, 500–600&nbsp;million years after the formation of the Solar System.<ref name="Gomes" /><ref name=shuffle>{{cite web |year= 2005| author= Kathryn Hansen | title=Orbital shuffle for early solar system | work=Geotimes | url = http://www.agiweb.org/geotimes/june05/WebExtra060705.html| access-date=2006-06-22 }}</ref> However, a recent re-appraisal of the cosmo-chemical constraints indicates that there was likely no late spike (“terminal cataclysm”) in the bombardment rate.<ref>{{Cite journal |last=Zellner |first=Nicolle E. B.|author-link= Nicolle Zellner |date=September 2017 |title=Cataclysm No More: New Views on the Timing and Delivery of Lunar Impactors |journal=Origins of Life and Evolution of Biospheres |language=en |volume=47 |issue=3 |pages=261–280 |doi=10.1007/s11084-017-9536-3 |issn=0169-6149 |pmc=5602003 |pmid=28470374|arxiv=1704.06694 |bibcode=2017OLEB...47..261Z }}</ref>

If it occurred, this period of heavy bombardment lasted several hundred million years and is evident in the cratering still visible on geologically dead bodies of the inner Solar System such as the Moon and Mercury.<ref name="Gomes" /><ref>{{cite web|url = https://history.nasa.gov/SP-467/ch3.htm | title=Chronology of Planetary surfaces|work=NASA History Division| date=January 1984|access-date=2008-03-13 | last1=Veverka| first1=J.}}</ref> The oldest known evidence for [[life|life on Earth]] dates to 3.8&nbsp;billion years ago—almost immediately after the end of the Late Heavy Bombardment.<ref name="life">{{cite press release | title=UCLA scientists strengthen case for life more than 3.8 billion years ago|url = http://www.eurekalert.org/pub_releases/2006-07/uoc--uss072006.php | date=21 July 2006 | publisher=University of California-Los Angeles | access-date=2008-04-29 }}</ref>

Impacts are thought to be a regular (if currently infrequent) part of the evolution of the Solar System. That they continue to happen is evidenced by the collision of [[Comet Shoemaker–Levy 9]] with [[Jupiter]] in 1994, the [[2009 Jupiter impact event]], the [[Tunguska event]], the [[Chelyabinsk meteor]] and the impact that created [[Meteor Crater]] in [[Arizona]]. The process of accretion, therefore, is not complete, and may still pose a threat to life on Earth.<ref>{{cite journal |journal=Abhandlungen der Geologischen Bundeanstalt, Wien |volume=53 |pages=51–54 |year=1996 |title=The Risk to Civilization From Extraterrestrial Objects and Implications of the Shoemaker-Levy 9 Comet Crash |author=Clark R. Chapman |url=http://www.geologie.ac.at/filestore/download/AB0053_051_A.pdf |access-date=2008-05-06 |issn=0016-7800 |url-status=dead |archive-url=https://web.archive.org/web/20080910084647/http://www.geologie.ac.at/filestore/download/AB0053_051_A.pdf |archive-date=2008-09-10 }}</ref><ref name=Agnor2006>{{cite journal|title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |author1=Craig B. Agnor |author2=Hamilton P. Douglas |journal=Nature |volume=441 |pages=192–194 |doi=10.1038/nature04792 |url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |year=2006 |pmid=16688170 |issue=7090 |bibcode=2006Natur.441..192A |s2cid=4420518 |url-status=dead |archive-url=https://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archive-date=2007-06-21 }}</ref>

Over the course of the Solar System's evolution, [[comet]]s were ejected out of the inner Solar System by the gravity of the giant planets and sent thousands of AU outward to form the [[Oort cloud]], a spherical outer swarm of cometary nuclei at the farthest extent of the Sun's gravitational pull. Eventually, after about 800 million years, the gravitational disruption caused by [[galactic tide]]s, passing stars and giant molecular clouds began to deplete the cloud, sending comets into the inner Solar System.<ref name="Morbidelli2006">{{cite arXiv
 | title=Origin and dynamical evolution of comets and their reservoirs
 | first=Alessandro |last=Morbidelli
 | eprint=astro-ph/0512256
 | date=2008-02-03
}}</ref> The evolution of the outer Solar System also appears to have been influenced by [[space weathering]] from the solar wind, micrometeorites, and the neutral components of the [[interstellar medium]].<ref>{{cite journal | url = http://www.agu.org/sci_soc/EISclark.html | title=Interplanetary Weathering: Surface Erosion in Outer Space|author1=Beth E. Clark |author2=Robert E. Johnson | journal= [[Eos (journal)|Eos, Transactions, American Geophysical Union]] | doi=10.1029/96EO00094 | volume=77 | issue=15 | pages=141 |year=1996 | access-date=2008-03-13| archive-url = https://web.archive.org/web/20080306012954/http://www.agu.org/sci_soc/EISclark.html| archive-date = March 6, 2008 | bibcode=1996EOSTr..77Q.141C }}</ref>

The evolution of the asteroid belt after Late Heavy Bombardment was mainly governed by collisions.<ref name="Bottke2005b">{{cite conference|author1=Bottke, William F. |author2=Durba, D. |author3=Nesvorny, D. |display-authors=etal|title=The origin and evolution of stony meteorites|conference=Dynamics of Populations of Planetary Systems|book-title=Proceedings of the International Astronomical Union|volume=197|pages=357–374|year=2005|doi=10.1017/S1743921304008865| url = http://www.boulder.swri.edu/~bottke/Reprints/Bottke_IAU197_Belgrade_Origin_Stony_Met.pdf |doi-access=free}}</ref> Objects with large mass have enough gravity to retain any material ejected by a violent collision. In the asteroid belt this usually is not the case. As a result, many larger objects have been broken apart, and sometimes newer objects have been forged from the remnants in less violent collisions.<ref name="Bottke2005b" /> Moons around some asteroids currently can only be explained as consolidations of material flung away from the parent object without enough energy to entirely escape its gravity.<ref>{{cite web
 |author1=H. Alfvén |author2=G. Arrhenius | date=1976
 | url = https://history.nasa.gov/SP-345/ch4.htm
 | title =The Small Bodies
 | work=SP–345 Evolution of the Solar System
 | publisher = NASA
 | access-date = 2007-04-12 }}</ref>

==Moons==
{{See also|Giant-impact hypothesis}}
Moons have come to exist around most planets and many other Solar System bodies. These [[natural satellite]]s originated by one of three possible mechanisms:
* Co-formation from a circumplanetary disc (only in the cases of the giant planets);
* Formation from impact debris (given a large enough impact at a shallow angle); and
* Capture of a passing object.
[[File:Artist's concept of collision at HD 172555.jpg|thumb|left|Artist's conception of the [[Giant impact hypothesis|giant impact]] thought to have formed the [[Moon]]]]

Jupiter and Saturn have several large moons, such as [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]] and [[Titan (moon)|Titan]], which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun.<ref name="arxiv0812">{{cite book |author1=Canup, Robin M. |author1-link=Robin Canup |author2=Ward, William R. |title=Origin of Europa and the Galilean Satellites |publisher=University of Arizona Press |date=2008-12-30 |arxiv=0812.4995|bibcode = 2009euro.book...59C |page=59|isbn=978-0-8165-2844-8}}</ref><ref name=dangelo_podolak_2015>{{cite journal|last=D'Angelo|first=G.|author2= Podolak, M.|title=Capture and Evolution of Planetesimals in Circumjovian Disks|journal=The Astrophysical Journal|date=2015|volume=806|issue=1|pages=29pp|doi=10.1088/0004-637X/806/2/203|arxiv = 1504.04364 |bibcode = 2015ApJ...806..203D |s2cid=119216797}}</ref><ref>{{cite journal |author1=N. Takato |author2=S. J. Bus | display-authors=etal | title=Detection of a Deep 3-<math>\mu</math>m Absorption Feature in the Spectrum of Amalthea (JV) | journal=Science |year=2004 | volume=306 | pages=2224–7 | doi=10.1126/science.1105427 | bibcode=2004Sci...306.2224T | pmid=15618511 | issue=5705 |s2cid=129845022 }}<br />
See also {{cite news | url = http://www.universetoday.com/2004/12/24/jovian-moon-was-probably-captured/ | work=Universe Today | date=24 December 2004 | title=Jovian Moon Was Probably Captured | author=Fraser Cain | access-date=2008-04-03 |archive-url = https://web.archive.org/web/20080130030816/http://www.universetoday.com/2004/12/24/jovian-moon-was-probably-captured/ <!-- Bot retrieved archive --> |archive-date = 2008-01-30 }}</ref> This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries also makes formation from collision debris unlikely. The outer moons of the giant planets tend to be small and have [[Orbital eccentricity|eccentric]] orbits with arbitrary inclinations. These are the characteristics expected of captured bodies.<ref>{{cite conference|author1=D. C. Jewitt |author2=S. Sheppard |author3=C. Porco |title=Jupiter's outer satellites and Trojans |book-title=Jupiter. The Planet, Satellites and Magnetosphere |year=2004 |pages=263–280 |url=http://www.ifa.hawaii.edu/~jewitt/papers/JUPITER/JSP.2003.pdf |editor=[[Frances Bagenal|Fran Bagenal]] |editor2=Timothy E. Dowling |editor3=William B. McKinnon |publisher=Cambridge University Press |isbn=0-521-81808-7 |url-status=dead |archive-url=https://web.archive.org/web/20070614045102/http://www.ifa.hawaii.edu/~jewitt/papers/JUPITER/JSP.2003.pdf |archive-date=2007-06-14 }}</ref><ref>{{cite web | url=http://www.dtm.ciw.edu/sheppard/satellites/ | title=The Giant Planet Satellite and Moon Page | author=Scott S. Sheppard | work=Personal web page | access-date=2008-03-13 | url-status=dead | archive-url=https://web.archive.org/web/20080311120653/http://www.dtm.ciw.edu/sheppard/satellites/ | archive-date=2008-03-11 }}</ref> Most such moons orbit in the direction opposite to the rotation of their primary. The largest irregular moon is Neptune's moon [[Triton (moon)|Triton]], which is thought to be a captured [[Kuiper belt object]].<ref name=Agnor2006/>

Moons of solid Solar System bodies have been created by both collisions and capture. [[Mars]]'s two small moons, [[Deimos (moon)|Deimos]] and [[Phobos (moon)|Phobos]], are thought to be captured [[asteroid]]s.{{sfn|Zeilik|Gregory|1998|pp=118–120}}
The Earth's [[moon]] is thought to have formed as a result of a [[Giant-impact hypothesis|single, large head-on collision]].<ref name=Canup2005/><ref>{{cite journal | author=D. J. Stevenson | title=Origin of the moon&nbsp;–&nbsp;The collision hypothesis | journal=Annual Review of Earth and Planetary Sciences |year=1987 | volume=15 | issue=1 | pages=271–315 | doi=10.1146/annurev.ea.15.050187.001415 | bibcode=1987AREPS..15..271S | s2cid=53516498 | url=http://pdfs.semanticscholar.org/6cd0/5a92552fe0b618abbb1dbb1a8dba79acbba5.pdf | archive-url=https://web.archive.org/web/20200412143819/http://pdfs.semanticscholar.org/6cd0/5a92552fe0b618abbb1dbb1a8dba79acbba5.pdf | url-status=dead | archive-date=2020-04-12 }}</ref>
The [[Theia (planet)|impacting object]] probably had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon.<ref name=Canup2005>{{cite journal |author1=R. M. Canup |author2=E. Asphaug | title=Origin of the Moon in a giant impact near the end of the Earth's formation | journal=Nature |year=2001 | volume=412 | pages=708–12 | bibcode=2001Natur.412..708C | doi=10.1038/35089010 | pmid=11507633 | issue=6848 |s2cid=4413525 }}</ref> The impact was probably the last in a series of mergers that formed the Earth.
It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth–Sun [[Lagrangian point]]s (either {{L4}} or {{L5}}) and drifted from its position.<ref>{{cite web | url = http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html | title = Origin of the Earth and Moon | date=31 December 1998 | author=G. Jeffrey Taylor | work = Planetary Science Research Discoveries | publisher = Hawaii Institute of Geophysics & Planetology | access-date = 2007-07-25 }}</ref> The moons of [[trans-Neptunian object]]s [[Pluto (dwarf planet)|Pluto]] ([[Charon (moon)|Charon]]) and [[90482 Orcus|Orcus]] ([[Vanth (moon)|Vanth]]) may also have formed by means of a large collision: the Pluto–Charon, Orcus–Vanth and Earth–Moon systems are unusual in the Solar System in that the satellite's mass is at least 1% that of the larger body.<ref name=impact_Pluto>{{cite journal|title=A Giant Impact Origin of Pluto-Charon|author=Robin M. Canup | journal=Science | date=28 January 2005 |volume=307 | pages=546–550 | doi=10.1126/science.1106818 |pmid=15681378|issue=5709|bibcode = 2005Sci...307..546C |s2cid=19558835 |url=https://authors.library.caltech.edu/51983/7/Canup.SOM.pdf }}</ref><ref>{{Cite journal | last1 = Brown | first1 = M. E. | last2 = Ragozzine | first2 = D. | last3 = Stansberry | first3 = J. | last4 = Fraser | first4 = W. C. | title = The Size, Density, and Formation of the Orcus-Vanth System in the Kuiper Belt | doi = 10.1088/0004-6256/139/6/2700 | journal = The Astronomical Journal | volume = 139 | issue = 6 | pages = 2700–2705 | year = 2010 | bibcode=2010AJ....139.2700B| arxiv= 0910.4784| s2cid = 8864460 }}</ref>

==Future==
Astronomers estimate that the current state of the Solar System will not change drastically until the Sun has fused almost all the hydrogen fuel in its core into helium, beginning [[stellar evolution|its evolution]] from the [[main sequence]] of the [[Hertzsprung–Russell diagram]] and into its [[red giant|red-giant]] phase. The Solar System will continue to evolve until then. Eventually, the Sun will likely expand sufficiently to overwhelm the inner planets (Mercury, Venus, and possibly Earth) but not the outer planets, including Jupiter and Saturn. Afterward, the Sun would be reduced to the size of a [[white dwarf]], and the outer planets and their moons would continue orbiting this diminutive solar remnant. This future development may be similar to the observed detection of [[MOA-2010-BLG-477L#Planetary system|MOA-2010-BLG-477L b]], a Jupiter-sized exoplanet orbiting its host white dwarf star [[MOA-2010-BLG-477L]].<ref name="NAT-20211013">{{cite journal |author=Blackman, J. W. |display-authors=et al. |title=A Jovian analogue orbiting a white dwarf star |url=https://www.nature.com/articles/s41586-021-03869-6 |date=13 October 2021 |journal=[[Nature (journal)|Nature]] |volume=598 |issue=7880 |pages=272–275 |doi=10.1038/s41586-021-03869-6 |pmid=34646001 |arxiv=2110.07934 |bibcode=2021Natur.598..272B |s2cid=238860454 |access-date=14 October 2021 }}</ref><ref name="KO-20211013">{{cite news |last1=Blackman |first1=Joshua |last2=Bennett |first2=David |last3= Beaulieu |first3=Jean-Philippe |title=A Crystal Ball Into Our Solar System's Future - Giant Gas Planet Orbiting a Dead Star Gives Glimpse Into the Predicted Aftermath of our Sun's Demise |url=https://keckobservatory.org/white-dwarf-system/ |date=13 October 2021 |work=[[Keck Observatory]] |access-date=14 October 2021 }}</ref><ref name="NYT-20211013">{{cite news |last=Ferreira |first=Becky |title=Astronomers Found a Planet That Survived Its Star's Death - The Jupiter-size planet orbits a type of star called a white dwarf, and hints at what our solar system could be like when the sun burns out. |url=https://www.nytimes.com/2021/10/13/science/white-dwarf-planet.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/10/13/science/white-dwarf-planet.html |archive-date=2021-12-28 |url-access=limited |date=13 October 2021 |work=[[The New York Times]] |access-date=14 October 2021 }}{{cbignore}}</ref>

===Long-term stability===
{{Main|Stability of the Solar System}}
The Solar System is [[chaos theory|chaotic]] over million- and billion-year timescales,<ref name=laskar94>{{cite journal
|title=Large-scale chaos in the solar system
|author=J. Laskar
|journal=Astronomy and Astrophysics
|volume=287
|pages=L9–L12
|year=1994
|bibcode=1994A&A...287L...9L }}</ref> with the orbits of the planets open to long-term variations. One notable example of this chaos is the Neptune–Pluto system, which lies in a 3:2 [[orbital resonance]]. Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10–20&nbsp;million years (the [[Lyapunov time]]) into the future.<ref>{{cite journal | title = Numerical evidence that the motion of Pluto is chaotic |author1=Gerald Jay Sussman |author2=Jack Wisdom | journal = Science | volume = 241 | pages = 433–437 |year = 1988 | url = http://groups.csail.mit.edu/mac/users/wisdom/pluto-chaos.pdf | doi = 10.1126/science.241.4864.433 | pmid = 17792606 | issue = 4864 | bibcode=1988Sci...241..433S
 |hdl=1721.1/6038 |s2cid=1398095 | hdl-access = free }}</ref> Another example is Earth's [[axial tilt]], which, due to friction raised within Earth's mantle by tidal interactions with the Moon ([[#Moon–ring systems|see below]]), is incomputable from some point between 1.5 and 4.5 billion years from now.<ref>{{cite journal|title=On the long term evolution of the spin of the Earth|author=O. Neron de Surgy |author2=J. Laskar|journal=Astronomy and Astrophysics|date=February 1997|volume=318|pages=975–989 |bibcode=1997A&A...318..975N }}</ref>

The outer planets' orbits are chaotic over longer timescales, with a Lyapunov time in the range of 2–230&nbsp;million years.<ref name=hayes07>{{cite journal | author=Wayne B. Hayes | title=Is the outer Solar System chaotic? | journal=Nature Physics | arxiv=astro-ph/0702179 |year=2007 | volume=3 | issue=10 | pages=689–691 | doi=10.1038/nphys728 | bibcode=2007NatPh...3..689H | s2cid=18705038 }}</ref>
In all cases, this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer becomes uncertain). Still, in some cases, the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in [[Orbital eccentricity|eccentricity]], with some planets' orbits becoming significantly more—or less—[[ellipse|elliptical]].<ref name="Stewart1997">{{cite book
| first = Ian | last = Stewart
| title = Does God Play Dice?
| publisher = Penguin Books
| edition = 2nd
| pages = 246–249
| date = 1997
| isbn = 0-14-025602-4 }}</ref>

Ultimately, the Solar System is stable in that none of the planets are likely to collide with each other or be ejected from the system in the next few billion years.<ref name=hayes07/> Beyond this, within five&nbsp;billion years or so, Mars's eccentricity may grow to around 0.2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the Solar System altogether<ref name=laskar94/> or send it on a collision course with [[Venus]] or [[Earth]].<ref>{{cite news|title=The solar system could go haywire before the sun dies|url = https://www.newscientist.com/article/dn13757-solar-system-could-go-haywire-before-the-sun-dies.html | author=David Shiga | work=NewScientist.com News Service | date=23 April 2008 | access-date=2008-04-28 }}</ref> This could happen within a billion years, according to numerical simulations in which Mercury's orbit is perturbed.<ref name="Batygin">{{Cite journal | last1 = Batygin | first1 = K. | last2 = Laughlin | first2 = G. | doi = 10.1086/589232 | title = On the Dynamical Stability of the Solar System | journal = The Astrophysical Journal | volume = 683 | issue = 2 | pages = 1207–1216 | year = 2008 |arxiv = 0804.1946 |bibcode = 2008ApJ...683.1207B | s2cid = 5999697 }}</ref>

===Moon–ring systems===
The evolution of moon systems is driven by [[tide|tidal forces]]. A moon will raise a [[tidal bulge]] in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary. If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In this situation, [[angular momentum]] is transferred from the rotation of the primary to the revolution of the satellite. The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time.

The Earth and its Moon are one example of this configuration. Today, the Moon is [[tidally locked]] to the Earth; one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows one face to the Earth. The Moon will continue to recede from Earth, and Earth's spin will continue to slow gradually. Other examples are the [[Galilean moons]] of [[Jupiter]] (as well as many of Jupiter's smaller moons)<ref>{{cite journal | bibcode=1982MNRAS.201..415G | title=Tidal Heating of Io and orbital evolution of the Jovian satellites | journal=Monthly Notices of the Royal Astronomical Society | author=A. Gailitis | volume=201 | issue=2 | pages=415–420 |year=1980 | doi = 10.1093/mnras/201.2.415 | doi-access=free }}</ref> and most of the larger moons of [[Saturn]].<ref>{{cite journal |author1=R. Bevilacqua |author2=O. Menchi |author3=A. Milani |display-authors= etal |date=April 1980 |title= Resonances and close approaches. I. The Titan-Hyperion case|journal=Earth, Moon, and Planets |volume=22 |issue=2 |pages=141–152 |doi= 10.1007/BF00898423 |bibcode=1980M&P....22..141B |s2cid=119442634 }}</ref>
[[File:Voyager 2 Neptune and Triton.jpg|thumb|Neptune and its moon [[Triton (moon)|Triton]], taken by ''[[Voyager 2]]''. Triton's orbit will eventually take it within Neptune's [[Roche limit]], tearing it apart and possibly forming a new ring system.]]
A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates or is revolving in the direction opposite the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks. In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out).<ref group=lower-alpha>In all of these cases of transfer of angular momentum and energy, the angular momentum of the two-body system is conserved. In contrast, the summed energy of the moon's revolution plus the primary's rotation is not conserved but decreases over time due to dissipation via frictional heat generated by the movement of the tidal bulge through the body of the primary. If the primary were a frictionless ideal fluid, the tidal bulge would be centered under the satellite, and no transfer would occur. It is the loss of dynamical energy through friction that makes the transfer of angular momentum possible.</ref> In both cases, [[Tidal acceleration#Tidal deceleration|tidal deceleration]] causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a [[planetary ring]] system, or crashes into the planet's surface or atmosphere. Such a fate awaits the moons [[Phobos (moon)|Phobos]] of Mars (within 30&nbsp;to&nbsp;50&nbsp;million years),<ref name=Bills2006>{{cite journal |author1=Bruce G. Bills |author2=Gregory A. Neumann |author3=David E. Smith |author4=Maria T. Zuber |year=2006 |title=Improved estimate of tidal dissipation within Mars from MOLA observations of the shadow of Phobos|journal=Journal of Geophysical Research|volume=110 | issue=E7 |pages=E07004|doi=10.1029/2004JE002376| bibcode=2005JGRE..110.7004B |s2cid=6125538 |doi-access=free }}</ref> [[Triton (moon)|Triton]] of Neptune (in 3.6 billion years),<ref>{{cite journal |title=Tidal evolution in the Neptune-Triton system |author=C. F. Chyba |author2=D. G. Jankowski |author3=P. D. Nicholson |journal=Astronomy & Astrophysics |volume=219 |issue=1–2 |page=23 |bibcode=1989A&A...219L..23C |year=1989 }}</ref> and at least [[Tidal acceleration#Tidal deceleration|16 small satellites]] of Uranus and Neptune. Uranus's [[Desdemona (moon)|Desdemona]] may even collide with one of its neighboring moons.{{sfn|Duncan|Lissauer|1997}}

A third possibility is where the primary and moon are [[tidally locked]] to each other. In that case, the tidal bulge stays directly under the moon, there is no angular momentum transfer, and the orbital period will not change. Pluto and Charon are an example of this type of configuration.<ref>{{cite journal | bibcode=2006AJ....132..290B | title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 |author1=Marc Buie |author2=William Grundy |author3=Eliot Young |author4=Leslie Young |author5=Alan Stern | journal=The Astronomical Journal | volume=132 | issue=1 | pages=290–298 |year=2006 | doi=10.1086/504422 | arxiv=astro-ph/0512491 | s2cid=119386667 }}</ref>

There is no consensus on the mechanism of the formation of the rings of Saturn. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history,<ref name="Tiscareno">{{cite book |first=M. S. |last=Tiscareno | editor-last=Kalas | editor-first=P. | editor2-last=French | editor2-first=L. | contribution=Planetary Rings | arxiv=1112.3305v2 | title=Planets, Stars and Stellar Systems | pages=61–63 | date= 2012-07-04 | publisher=[[Springer Science+Business Media|Springer]] |url=http://refworks.springer.com/mrw/index.php?id=1654 |doi=10.1007/978-94-007-5606-9_7 |isbn=978-94-007-5605-2 |s2cid=118494597 |access-date=2012-10-05 }}</ref> data from the ''[[Cassini–Huygens]]'' spacecraft suggests they formed relatively late.<ref name="IessMilitzer2019">{{cite journal|last1= Iess|first1= L.|last2= Militzer|first2= B.|last3= Kaspi|first3= Y.|last4= Nicholson|first4= P.|last5= Durante|first5= D.|last6= Racioppa|first6= P.|last7= Anabtawi|first7= A.|last8= Galanti|first8= E.|last9= Hubbard|first9= W.|last10= Mariani|first10=M. J.|last11= Tortora|first11= P.|last12= Wahl|first12= S.|last13= Zannoni|first13= M.|title= Measurement and implications of Saturn's gravity field and ring mass|journal= Science|volume= 364|issue= 6445|year= 2019|pages= eaat2965|doi= 10.1126/science.aat2965|pmid= 30655447|bibcode= 2019Sci...364.2965I|hdl= 10150/633328|s2cid= 58631177|url= https://repository.arizona.edu/bitstream/10150/633328/1/aat2965_CombinedPDF_v4.pdf|doi-access= free}}</ref>

===The Sun and planetary environments===<!--Note: this subsection linked to from [[1 E19 s and more]] as of 2008-08-07-->
{{See also|Stellar evolution|Future of Earth}}
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|Description = Formation of the Solar System after gas and dust coalesced into a protoplanetary disk. The vast majority of this material was sourced from a past [[supernova]].}}
In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. As the Sun burns through its hydrogen fuel supply, it gets hotter and burns the remaining fuel even faster. As a result, the Sun is growing brighter at a rate of ten percent every 1.1&nbsp;billion years.<ref name=scientist>{{cite news|title=Science: Fiery future for planet Earth |author=Jeff Hecht | work=New Scientist |url = https://www.newscientist.com/article/mg14219191.900.html | date=2 April 1994 | issue=1919 | page=14 |access-date=2007-10-29 }}</ref> In about 600 million years, the Sun's brightness will have disrupted the Earth's [[carbon cycle]] to the point where trees and forests (C3 photosynthetic plant life) will no longer be able to survive; and in around 800 million years, the Sun will have killed all complex life on the Earth's surface and in the oceans. In 1.1 billion years, the Sun's increased radiation output will cause its [[Habitable zone#Circumstellar habitable zone|circumstellar habitable zone]] to move outwards, making the Earth's surface too hot for liquid water to exist there naturally. At this point, all life will be reduced to single-celled organisms.<ref name=Schroder2008/> Evaporation of water, a potent [[greenhouse gas]], from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner.<ref>{{cite web|url=http://www.cicero.uio.no/fulltext/index_e.aspx?id=2737|title=Our changing solar system|author1=Knut Jørgen|author2=Røed Ødegaard|work=Centre for International Climate and Environmental Research|year=2004|access-date=2008-03-27|url-status=dead|archive-url=https://web.archive.org/web/20081009015241/http://www.cicero.uio.no/fulltext/index_e.aspx?id=2737|archive-date=2008-10-09}}</ref> During this time, it is possible that as [[Mars]]'s surface temperature gradually rises, carbon dioxide and water currently frozen under the surface [[regolith]] will release into the atmosphere, creating a [[greenhouse effect]] that will heat the planet until it achieves conditions parallel to Earth today, providing a potential future abode for life.<ref name=mars>{{cite book|title=Mars: A Warmer, Wetter Planet|author=Jeffrey Stuart Kargel | url = https://books.google.com/books?id=0QY0U6qJKFUC&q=mars+future+%22billion+years%22+sun&pg=PA509 | isbn=1-85233-568-8 | date=2004 | publisher=Springer |access-date=2007-10-29 }}</ref> By 3.5&nbsp;billion years from now, Earth's surface conditions will be similar to those of Venus today.<ref name=scientist />

[[File:Sun red giant.svg|thumb|left|Relative size of the Sun as it is now (inset) compared to its estimated future size as a red giant]]
Around 5.4&nbsp;billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell.<ref name=Schroder2008/> This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a [[red giant]].{{sfn|Zeilik|Gregory|1998|p=320–321}}<ref>{{cite web |title=Introduction to Cataclysmic Variables (CVs)|work=NASA Goddard Space Center|year=2006|url = http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |access-date = 2006-12-29 }}</ref> Within 7.5&nbsp;billion years, the Sun will have expanded to a radius of {{convert|1.2|AU|e6km e6mi|abbr=on}}—256 times its current size. At the tip of the [[red-giant branch]], as a result of the vastly increased surface area, the Sun's surface will be much cooler (about {{convert|2,600|K|C F}}) than now, and its [[luminosity]] much higher—up to 2,700&nbsp;current solar luminosities. For part of its red-giant life, the Sun will have a strong [[stellar wind]] that will carry away around 33% of its mass.<ref name=Schroder2008>{{cite journal|author1=K. P. Schroder |author2=Robert Connon Smith |title= Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society | volume=386|issue= 1 |pages=155–163 |year=2008 |doi=10.1111/j.1365-2966.2008.13022.x |doi-access=free |bibcode=2008MNRAS.386..155S |arxiv = 0801.4031 |s2cid=10073988 }}</ref><ref name="sun_future">{{cite journal |author1=I. J. Sackmann |author2=A. I. Boothroyd |author3=K. E. Kraemer | title=Our Sun. III. Present and Future | pages=457 | journal=Astrophysical Journal |year=1993 | volume=418 | bibcode=1993ApJ...418..457S | doi=10.1086/173407 | doi-access=free }}</ref>{{sfn|Zeilik|Gregory|1998|p=322}} During these times, it is possible that [[Saturn]]'s moon [[Titan (moon)|Titan]] could achieve surface temperatures necessary to support life.<ref name=Titan>{{cite journal |title=Titan under a red giant sun: A new kind of "habitable" moon |author1=Ralph D. Lorenz |author2=Jonathan I. Lunine |author3=Christopher P. McKay |journal=Geophysical Research Letters |year=1997 |volume=24 |pages=2905–8 |url=http://www.lpl.arizona.edu/~rlorenz/redgiant.pdf |access-date=2008-03-21 |doi=10.1029/97GL52843 |pmid=11542268 |issue=22 |bibcode=1997GeoRL..24.2905L |citeseerx=10.1.1.683.8827 |s2cid=14172341 |archive-date=2011-07-24 |archive-url=https://web.archive.org/web/20110724173621/http://www.lpl.arizona.edu/~rlorenz/redgiant.pdf |url-status=dead }}</ref><ref>{{cite web | author=Marc Delehanty | title= Sun, the solar system's only star | work=Astronomy Today|url = http://www.astronomytoday.com/astronomy/sun.html| access-date=2006-06-23 }}</ref>

As the Sun expands, it will swallow the planets [[Mercury (planet)|Mercury]] and [[Venus]].<ref name=Rybicki2001>{{cite journal
 |author1=K. R. Rybicki |author2=C. Denis |title=On the Final Destiny of the Earth and the Solar System
 |journal=Icarus
 |volume=151 | issue=1 |pages=130–137 |date=2001
 |doi=10.1006/icar.2001.6591 |bibcode=2001Icar..151..130R }}</ref> [[Earth]]'s fate is less clear; although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out.<ref name=Schroder2008/> If it were only for this, Venus and Earth would probably escape incineration,<ref name=sun_future /> but a 2008 study suggests that Earth will likely be swallowed up as a result of [[Tidal acceleration#Tidal deceleration|tidal interactions]] with the Sun's weakly-bound outer envelope.<ref name=Schroder2008/>

Additionally, the Sun's habitable zone will move into the outer Solar System and eventually beyond the Kuiper belt at the end of the red-giant phase, causing icy bodies such as [[Enceladus]] and Pluto to thaw. During this time, these worlds could support a water-based [[hydrologic cycle]], but as they were too small to hold a dense atmosphere like Earth, they would experience extreme day–night temperature differences. When the Sun leaves the red-giant branch and enters the [[asymptotic giant branch]], the habitable zone will abruptly shrink to roughly the space between Jupiter and Saturn's present-day orbits, but toward the end of the 200&nbsp;million-year duration of the asymptotic giant phase, it will expand outward to about the same distance as before.<ref>{{cite journal |last1=Ramirez |first1=Ramses M. |last2=Kaltenegger |first2=Lisa |title=Habitable Zones of Post-Main Sequence Stars |journal=The Astrophysical Journal |date=16 May 2016 |volume=823 |issue=1 |pages=6 |doi=10.3847/0004-637X/823/1/6|s2cid=119225201 |arxiv=1605.04924 |bibcode=2016ApJ...823....6R |doi-access=free }}</ref>

Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. At this point, the density and temperature will become so high that the fusion of helium into [[carbon]] will begin, leading to a [[helium flash]]; the Sun will shrink from around 250 to 11&nbsp;times its present (main-sequence) radius. Consequently, its luminosity will decrease from around 3,000 to 54 times its current level, and its surface temperature will increase to about {{convert|4770|K|C F}}. The Sun will become a [[Horizontal branch|horizontal giant]], burning helium in its core in a stable fashion, much like it burns hydrogen today. The helium-fusing stage will last only 100&nbsp;million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers. It will expand a second time, becoming what is known as an [[asymptotic giant branch|asymptotic giant]]. Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about {{convert|3,500|K|C F}}.<ref name=Schroder2008/> This phase lasts about 30&nbsp;million years, after which, over the course of a further 100,000&nbsp;years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a [[planetary nebula]]. The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars and planets.<ref name=nebula>{{cite web | author=Bruce Balick | title=Planetary nebulae and the future of the Solar System | work=Personal web site | url=http://www.astro.washington.edu/balick/WFPC2/ | access-date=2006-06-23 | archive-url=https://web.archive.org/web/20081219010229/http://www.astro.washington.edu/balick/WFPC2/ | archive-date=2008-12-19 | url-status=dead }}</ref>

[[File:M57 The Ring Nebula.JPG|thumb|The [[Ring nebula]], a planetary nebula similar to what the Sun will become]]
This is a relatively peaceful event, nothing akin to a [[supernova]], which the Sun is too small to undergo as part of its evolution. Any observer present to witness this occurrence would see a massive increase in the speed of the solar wind, but not enough to destroy a planet completely. However, the star's loss of mass could send the orbits of the surviving planets into chaos, causing some to collide, others to be ejected from the Solar System, and others to be torn apart by tidal interactions.<ref>{{cite journal|title=A Gaseous Metal Disk Around a White Dwarf|author1=B. T. Gänsicke |author2=T. R. Marsh |author3=J. Southworth |author4=A. Rebassa-Mansergas |journal=Science|year=2006 | volume=314 | pages=1908–1910 | doi=10.1126/science.1135033|pmid=17185598|issue=5807|arxiv = astro-ph/0612697 |bibcode = 2006Sci...314.1908G |s2cid=8066922 }}</ref> Afterwards, all that will remain of the Sun is a [[white dwarf]], an extraordinarily dense object, 54% of its original mass but only the size of Earth. Initially, this white dwarf may be 100&nbsp;times as luminous as the Sun is now. It will consist entirely of [[degenerate matter|degenerate]] [[carbon]] and [[oxygen]] but will never reach temperatures hot enough to fuse these elements. Thus, the white dwarf Sun will gradually cool, growing dimmer and dimmer.<ref name="future-sun">{{cite web|author=Richard W. Pogge |year=1997 |url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html |title=The Once & Future Sun |format=lecture notes |work=New Vistas in Astronomy |access-date=2005-12-07 |url-status=dead |archive-url=https://web.archive.org/web/20050527094435/http://www-astronomy.mps.ohio-state.edu/Vistas/ |archive-date=2005-05-27 }}</ref>

As the Sun dies, its gravitational pull on orbiting bodies, such as planets, comets, and asteroids, will weaken due to its mass loss. All remaining planets' orbits will expand; if Venus, Earth, and Mars still exist, their orbits will lie roughly at {{convert|1.4|AU|e6km e6mi| lk=on | abbr=unit}}, {{convert|1.9|AU|e6km e6mi| lk=on | abbr=unit}}, and {{convert|2.8|AU|e6km e6mi| lk=on | abbr=unit}}, respectively. They and the other remaining planets will become dark, frigid husks, completely devoid of life.<ref name="sun_future" /> They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Two&nbsp;billion years later, when the Sun has cooled to the {{convert|6000|–|8000|K|C F}} range, the carbon and oxygen in the Sun's core will freeze, with over 90% of its remaining mass assuming a crystalline structure.<ref>{{cite journal |author1=T. S. Metcalfe |author2=M. H. Montgomery |author3=A. Kanaan | title=Testing White Dwarf Crystallization Theory with Asteroseismology of the Massive Pulsating DA Star BPM 37093 | journal=Astrophysical Journal | arxiv=astro-ph/0402046 |year=2004 | volume=605 |issue=2 | pages=L133 | doi=10.1086/420884 | bibcode=2004ApJ...605L.133M |s2cid=119378552 }}</ref> Eventually, after roughly one&nbsp;quadrillion years, the Sun will finally cease to shine altogether, becoming a [[black dwarf]].<ref name=Fontaine2001>{{cite journal|title=The Potential of White Dwarf Cosmochronology |author1=G. Fontaine |author2=P. Brassard |author3=P. Bergeron | journal=Publications of the Astronomical Society of the Pacific | volume=113|issue=782 | pages=409–435 |year=2001 | doi=10.1086/319535 | bibcode=2001PASP..113..409F |doi-access=free }}</ref>

==Galactic interaction==
[[File:Milky_Way_Spiral_Arms_EN.svg|left|thumb|Location of the Solar System within the Milky Way]]
The Solar System travels alone through the Milky Way in a circular orbit approximately 30,000&nbsp;light years from the [[Galactic Center]]. Its speed is about 220&nbsp;km/s. The period required for the Solar System to complete one revolution around the Galactic Center, the [[galactic year]], is in the range of 220–250 million years. Since its formation, the Solar System has completed at least 20 such revolutions.<ref name="biblio">{{cite web | url = http://hypertextbook.com/facts/2002/StacyLeong.shtml|work=The Physics Factbook (self-published)| author=Stacy Leong| editor=Glenn Elert |year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year) | access-date=2008-06-26 }}</ref>

Various scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of [[mass extinction]]s observed in the Earth's [[fossil record]]. One hypothesis supposes that vertical oscillations made by the Sun as it orbits the Galactic Centre cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the [[galactic tide]] is weaker; as it re-enters the galactic disc, as it does every 20–25&nbsp;million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of [[Oort cloud]] comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact.<ref>{{cite web
| title = Perturbing the Oort Cloud
| first = Michael
| last = Szpir
| work = American Scientist
| url = http://www.americanscientist.org/issues/pub/perturbing-the-oort-cloud
| access-date = 2008-03-25
| publisher = The Scientific Research Society
| archive-date = 2012-04-02
| archive-url = https://web.archive.org/web/20120402163230/http://www.americanscientist.org/issues/pub/perturbing-the-oort-cloud
| url-status = dead
}}</ref>

However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore, the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's [[spiral arms]]. Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright [[blue giant]]s, which live for relatively short periods and then explode violently as [[supernovae]].<ref>{{cite journal|title=Mass Extinctions and The Sun's Encounters with Spiral Arms|author1=Erik M. Leitch |author2=Gautam Vasisht |year=1998|pages=51–56|issue=1|journal=New Astronomy|volume= 3|arxiv=astro-ph/9802174|doi=10.1016/S1384-1076(97)00044-4|bibcode = 1998NewA....3...51L |s2cid=17625755 }}</ref>

===Galactic collision and planetary disruption===
{{Main article|Andromeda–Milky Way collision}}
Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of the [[Local Group]] of galaxies, is heading toward it at about 120&nbsp;km/s.<ref name=cain /> In 4&nbsp;billion years, Andromeda and the Milky Way will collide, causing both to deform as [[tidal force]]s distort their outer arms into vast [[tidal tails]]. If this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy.<ref name=cain /> After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%,<ref name=cox /> the galaxies' [[supermassive black hole]]s will merge. Eventually, in roughly 6&nbsp;billion years, the Milky Way and Andromeda will complete their merger into a giant [[elliptical galaxy]]. During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy. This may lead to a short period of intensive star formation called a [[Starburst galaxy|starburst]].<ref name=cain /> In addition, the infalling gas will feed the newly formed black hole, transforming it into an [[active galactic nucleus]]. The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions.<ref name=cain>{{cite web|title=When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?|author=Fraser Cain|work=Universe Today|url = http://www.universetoday.com/2007/05/10/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/|year=2007|access-date=2007-05-16 }}</ref><ref name=cox>{{cite journal|title=The Collision Between The Milky Way And Andromeda |author1=J. T. Cox |author2=Abraham Loeb | journal=Monthly Notices of the Royal Astronomical Society |arxiv=0705.1170 |year=2007 | doi=10.1111/j.1365-2966.2008.13048.x|volume=386|issue=1|pages=461–474 |doi-access=free | bibcode=2008MNRAS.386..461C |s2cid=14964036 }}</ref>

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. Although it is true that the gravity of passing stars can detach planets into interstellar space, distances between stars are so great that the likelihood of the Milky Way–Andromeda collision causing such disruption to any individual star system is negligible. Although the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html |author=NASA|title=NASA's Hubble Shows Milky Way is Destined for Head-On Collision |work=NASA |date=2012-05-31 |access-date=2012-10-13}}</ref>

However, over time, the [[Cumulative distribution function|cumulative probability]] of a chance encounter with a star increases, and disruption of the planets becomes all but inevitable. Assuming that the [[Big Crunch]] or [[Big Rip]] scenarios for the end of the Universe do not occur, calculations suggest that the gravity of passing stars will have completely stripped the dead Sun of its remaining planets within 1&nbsp;quadrillion (10<sup>15</sup>) years. This point marks the end of the Solar System. Although the Sun and planets may survive, the Solar System, in any meaningful sense, will cease to exist.<ref name=dyson>{{cite journal|title=Time Without End: Physics and Biology in an open universe|author=[[Freeman Dyson]]|place=Institute for Advanced Study, Princeton New Jersey|journal=Reviews of Modern Physics|volume=51 | pages=447–460|date=July 1979 | doi=10.1103/RevModPhys.51.447|issue=3 | bibcode=1979RvMP...51..447D }}</ref>

==Chronology==
{{wide image|Solar Life Cycle.svg|600px|alt=Projected timeline of the Sun's life. From Formation To 14Gy}}
The time frame of the Solar System's formation has been determined using [[radiometric dating]]. Scientists estimate that the Solar System is 4.6&nbsp;billion years old. The [[oldest rock|oldest known mineral grains]] on [[Earth]] are approximately 4.4&nbsp;billion years old.<ref name=Wilde>{{cite journal | journal=Nature | volume=409 | pages=175–8 | title= Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago |author1=Simon A. Wilde |author2=John W. Valley |author3=William H. Peck |author4=Colin M. Graham | doi=10.1038/35051550 | url = http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf |year= 2001 | pmid=11196637 | issue=6817 | bibcode=2001Natur.409..175W | s2cid=4319774 }}</ref> Rocks this old are rare, as Earth's surface is constantly being reshaped by [[erosion]], [[volcanism]], and [[plate tectonics]]. To estimate the age of the Solar System, scientists use [[meteorite]]s, which were formed during the early condensation of the solar nebula. Almost all meteorites (see the [[Canyon Diablo meteorite]]) are found to have an age of 4.6&nbsp;billion years, suggesting that the Solar System must be at least this old.<ref>{{cite book | date=2000 |author=Gary Ernst Wallace|publisher=Cambridge University Press|chapter=Earth's Place in the Solar System|title=Earth Systems: Processes and Issues|pages=45–58|isbn=0-521-47895-2 }}</ref>

Studies of discs around other stars have also done much to establish a time frame for Solar System formation. Stars between one and three million years old have discs rich in gas, whereas discs around stars more than 10&nbsp;million years old have little to no gas, suggesting that giant planets within them have ceased forming.<ref name=sciam />

===Timeline of Solar System evolution===
{{External Timeline|Graphical timeline of the universe|Graphical timeline of Earth and Sun}}
Note: All dates and times in this chronology are approximate and should be taken as an [[order of magnitude]] indicator only.
{{Clear}}
{| class="wikitable sortable"
|+ Chronology of the formation and evolution of the Solar System
|-
! Phase !! Time since formation of the Sun !! Time from present (approximate) !! Event
|- bgcolor="#ffffff"
! rowspan="2" | Pre-Solar System
| Billions of years before the formation of the Solar System
| Over 4.6&nbsp;[[bya|billion years ago (bya)]]
| Previous generations of stars live and die, injecting [[metallicity|heavy elements]] into the [[interstellar medium]] out of which the Solar System formed.<ref name=Lineweaver2001>{{cite journal |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |author=Charles H. Lineweaver | journal=Icarus | volume=151 |issue=2 | pages=307–313 |year=2001 | doi=10.1006/icar.2001.6607 | arxiv=astro-ph/0012399 | bibcode=2001Icar..151..307L |s2cid=14077895 }}</ref>
|- bgcolor="#ffffff"
| ~ 50 million years before formation of the Solar System
| 4.6&nbsp;[[bya]]
| If the Solar System formed in an [[Orion Nebula]]-like star-forming region, the most massive stars are formed, live their lives, die, and explode in supernova. One particular supernova, called the ''primal supernova'', possibly triggers the formation of the Solar System.<ref name=cradle /><ref name=iron />
|- bgcolor="#ccffff"
! rowspan="4" | Formation of Sun
| 0–100,000 years
| 4.6&nbsp;[[bya]]
| Pre-solar nebula forms and begins to collapse. Sun begins to form.<ref name=sciam />
|- bgcolor="#ccffff"
| 100,000&nbsp;– 50 million years
| 4.6&nbsp;[[bya]]
| Sun is a [[T Tauri star|T Tauri]] [[protostar]].<ref name=Montmerle2006 />
|- bgcolor="#ccffff"
| 100,000 – 10 million years
| 4.6&nbsp;[[bya]]
| By 10 million years, gas in the [[protoplanetary disc]] has been blown away, and outer planet formation is likely complete.<ref name=sciam />
|- bgcolor="#ccffff"
| 10 million – 100 million years
| 4.5–4.6&nbsp;[[bya]]
| Terrestrial planets and the Moon form. Giant impacts occur. Water delivered to Earth.<ref name="Gomes" />
|- bgcolor="#ffffcc"
! rowspan="7" | Main sequence
| 50 million years
| 4.5&nbsp;[[bya]]
| Sun becomes a main-sequence star.<ref name=Yi2001/>
|- bgcolor="#ffffcc"
| 200 million years
| 4.4&nbsp;[[bya]]
| [[Late Heavy Bombardment#Geological consequences on Earth|Oldest known rocks]] on the Earth formed.<ref name=Wilde /><ref name="NS-20080702">{{cite web |last=Courtland |first=Rachel |title=Did newborn Earth harbour life? |url=https://www.newscientist.com/article/dn14245-did-newborn-earth-harbour-life.html |date=July 2, 2008 |work=[[New Scientist]] |access-date=April 13, 2014 }}</ref>
|- bgcolor="#ffffcc"
| 500 million&nbsp;– 600 million years
| 4.0–4.1&nbsp;[[bya]]
| Resonance in Jupiter and Saturn's orbits moves Neptune out into the Kuiper belt. [[Late Heavy Bombardment]] occurs in the inner Solar System.<ref name="Gomes" />
|- bgcolor="#ffffcc"
| 800 million years
| 3.8&nbsp;[[bya]]
| [[Abiogenesis|Oldest known life]] on Earth.<ref name=life /><ref name="NS-20080702" /> [[Oort cloud]] reaches maximum mass.<ref name="Morbidelli2006" />
|- bgcolor="#ccffcc"
| 4.6 billion years
| '''Today'''
| Sun remains a main-sequence star.<ref name=scientist />
|- bgcolor="#ffffcc"
| 6 billion years
| 1.4 billion years in&nbsp;the future
| Sun's [[habitable zone]] moves outside of the Earth's orbit, possibly shifting onto Mars's orbit.<ref name=mars />
|- bgcolor="#ffffcc"
| 7 billion years
| 2.4 billion years in&nbsp;the future
| The [[Milky Way]] and [[Andromeda Galaxy]] begin to [[Andromeda–Milky Way collision|collide]]. Slight chance the Solar System could be captured by Andromeda before the two galaxies fuse completely.<ref name=cain />
|- bgcolor="#ffcccc"
! rowspan="2" | Post–main sequence
| 10 billion&nbsp;– 12 billion years
| 5–7 billion years in&nbsp;the future
| Sun has fused all of the hydrogen in the core and starts to burn hydrogen in a shell surrounding its core, thus ending its main sequence life. Sun begins to ascend the [[red-giant branch]] of the [[Hertzsprung–Russell diagram]], growing dramatically more luminous (by a factor of up to 2,700), larger (by a factor of up to 250 in radius), and cooler (down to 2600 K): Sun is now a [[red giant]]. Mercury, Venus and possibly Earth are swallowed.<ref name=Schroder2008/><ref name=sun_future /> During this time Saturn's moon Titan may become habitable.<ref name=Titan/>
|- bgcolor="#ffcccc"
| ~ 12 billion years
| ~ 7 billion years in&nbsp;the future
| Sun passes through helium-burning [[horizontal branch|horizontal-branch]] and [[asymptotic giant branch|asymptotic-giant-branch]] phases, losing a total of ~30% of its mass in all post-main-sequence phases. The asymptotic-giant-branch phase ends with the ejection of its outer layers as a [[planetary nebula]], leaving the dense core of the Sun behind as a [[white dwarf]].<ref name=Schroder2008/><ref name=nebula/>
|-
! Remnant Sun
| ~ 1 quadrillion years (10<sup>15</sup> years)
| ~ 1 quadrillion years in&nbsp;the future
| Sun cools to 5 K.<ref>{{BarrowTipler1986 }}</ref> Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.<ref name=dyson />
|}

==See also==
{{div col|colwidth=20em}}
* {{annotated link|Accretion (astrophysics)|Accretion}}
* {{annotated link|Age of Earth}}
* {{annotated link|Big Bang}}
* {{annotated link|Chronology of the universe}}
* {{annotated link|Circumplanetary disk}}
* {{annotated link|Cosmology}}
* {{annotated link|Future of Earth}}
* {{annotated link|Galaxy formation and evolution}}
* {{annotated link|History of Earth}}
* {{annotated link|Magma ocean}}
* {{annotated link|Scale height}}
* {{annotated link|Space and survival}}
* {{annotated link|Stellar evolution}}
* {{annotated link|Structure formation}}
* {{annotated link|Tidal locking}}
* {{annotated link|Timeline of the far future}}
* {{annotated link|Timeline of the universe}}
{{div col end}}

==Notes==
{{notelist|1}}

==References==
{{reflist
| colwidth = 30em
| refs =
}}

==Bibliography==
* {{cite journal| doi = 10.1006/icar.1996.5568| last1 = Duncan| first1 = Martin J.| last2 = Lissauer| first2 = Jack J.| year = 1997| title = Orbital Stability of the Uranian Satellite System| journal = Icarus| volume = 125| issue = 1| pages = 1–12| bibcode = 1997Icar..125....1D}}
* {{cite book
  | last1 = Zeilik
  | first1 = Michael A.
  | last2 = Gregory
  | first2 = Stephen A.
  | date = 1998
  | title = Introductory Astronomy & Astrophysics
  | edition = 4th
  | publisher = Saunders College Publishing
  | isbn = 0-03-006228-4
  }}

==External links==
* [http://arquivo.pt/wayback/20160520023943/http://media.skyandtelescope.com/video/Solar_System_Sim.mov 7M animation] from [http://www.skyandtelescope.com skyandtelescope.com] showing the early evolution of the outer Solar System.
* [http://www.cfa.harvard.edu/seuforum/animations/animations/galaxycollision.mpg QuickTime animation of the future collision between the Milky Way and Andromeda]
* [http://www.space.com/common/media/video/player.php?videoRef=mm32_SunDeath How the Sun Will Die: And What Happens to Earth] (Video at Space.com)

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