View source for Formation and evolution of the Solar System

[[File:Protoplanetary-disk.jpg|thumb|400px|Artist's conception of a [[protoplanetary disc]]]]

<!--Note: per [[WP:BOLDTITLE]], topic name should NOT be bold because the title of this article is descriptive-->
The formation and [[wikt:evolution|evolution]] of the [[Solar System]] is estimated to have begun 4.6&nbsp;[[1000000000 (number)|billion]] years ago with the [[gravitation]]al collapse of a small part of a giant [[molecular cloud]]. Most of the collapsing mass collected in the centre, forming the [[Sun]], while the rest flattened into a [[protoplanetary disc]] out of which the [[planet]]s, [[Natural satellite|moons]], [[asteroid]]s, and other [[small Solar System bodies]] formed.

This widely accepted 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]], [[physics]], [[geology]], and [[planetary science]]. Since the dawn of the [[space age]] in the 1950s and the discovery of [[extrasolar planets]] in the 1990s, the models have 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 believed to have formed independently and later been captured by their planets. Still others, as the [[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. The positions of the planets often shifted, and planets have switched places.<ref name="Gomes"/> This [[planetary migration]] now is believed to have been responsible for much of the Solar System's early evolution.

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 corpse known as a [[white dwarf]]. In the far distant future, the gravity of passing stars gradually will whittle away at the Sun's retinue of planets. Some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of [[10^12|trillions]] of years, it is likely that the Sun will be left alone with no bodies in orbit around it.<ref name=dyson />

== History ==
{{main|History of Solar System formation and evolution hypotheses}}
[[File:Pierre-Simon Laplace.jpg|thumb|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 believed 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]], the model which placed the Sun at the centre of the system and the Earth in orbit around it. This conception had been gestating for millennia, but was widely accepted only by 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 | accessdate=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=M. M. Woolfson | journal=Philosophical Transactions of the Royal Society of London | volume=313 | title=Rotation in the Solar System | year=1984 | pages=5 | url=http://adsabs.harvard.edu/abs/1984RSPTA.313....5W | doi=10.1098/rsta.1984.0078 }}</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=http://space.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 | publisher=New Scientist | author=Nigel Henbest | year=1991 | accessdate=2008-04-18}}</ref>

Understanding of how the Sun will 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.<ref>{{cite book | title=The Sun: A Biography | author=David Whitehouse | year=2005 | publisher=John Wiley and Sons | isbn=978-0470092972}}</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|year=2005|chapter=Origin of the Chemical Elements|pages=197–222 | isbn=978-1854109613}}</ref> [[Fred Hoyle]] elaborated on this premise by arguing that evolved stars called [[red giant]]s 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}}
=== Pre-solar nebula ===
The nebular hypothesis maintains that the Solar System formed from the gravitational collapse of a fragment of a giant [[molecular cloud]] which likely was several [[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 (University of Arizona) | accessdate=2006-12-27 | date=Spring 2003 }}</ref> Until a few decades ago, the conventional view was that the Sun formed in relative isolation, but studies of ancient [[meteorite]]s reveal traces of short-lived isotopes such as [[iron-60]] which only form in exploding, short-lived stars. This indicates that a number of [[supernova]]e occurred near the Sun while it was forming. A [[shock wave]] from one of these supernovae may have triggered the formation of the Sun by creating regions of over-density within the cloud, causing these regions to collapse. Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region which produced massive stars, possibly similar to the [[Orion nebula]].<ref name=cradle>{{cite journal| year=2004|author= J. Jeff Hester, Steven J. Desch, Kevin R. Healy, 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 | accessdate=2007-01-11}}</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| author=Martin Bizzarro, David Ulfbeck, Anne Trinquier, Kristine Thrane, James N. Connelly, Bradley S. Meyer| url= http://www.sciencemag.org/cgi/content/abstract/316/5828/1178| format=abstract page}}</ref>

[[File:M42proplyds.jpg|thumb|left|Hubble image of protoplanetary discs in the [[Orion nebula]], a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed]]
One of these regions of collapsing gas (known as the ''pre-solar nebula'')<ref name=composition>{{cite conference | author=W. M. Irvine | title=The chemical composition of the pre-solar nebula | booktitle=Cometary Exploration | year=1983 | volume=1 | editor=T. I. Gombosi (ed.) | pages=3–12 | url=http://adsabs.harvard.edu/abs/1983coex....1....3I}}</ref> would form what became the Solar System. This region had a diameter of between 7000 and 20,000&nbsp;[[astronomical unit]]s (AU)<ref name= "Arizona" /><ref>An astronomical unit, or AU, is the average distance between the Earth and the Sun, or ~150 million kilometres. It is the standard unit of measurement for interplanetary distances.</ref><ref>{{cite journal|journal=Earth, Moon, and Planets|publisher=Springer Netherlands|volume=34|number=1|year=1986|pages=93–100 |title= Further Considerations on Contracting Solar Nebula|author=J. J. Rawal|work=Nehru Planetarium, Bombay India|url= http://www.springerlink.com/content/r5825j48k66n8284/fulltext.pdf|accessdate=2006-12-27|format=PDF | doi=10.1007/BF00054038 }} </ref> and a mass just over that of the Sun. Its composition 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 [[Metalicity|heavier elements]] that were created by [[nucleosynthesis]] in earlier generations of stars.<ref>{{harvtxt|Zeilik|Gregory|1998|loc=p. 207}}</ref> Late in the life of these stars, they ejected heavier elements into the [[interstellar medium]].<ref name=Lineweaver2001 />

Because of the conservation of [[angular momentum]], the nebula spun faster as it collapsed. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, converting their [[kinetic energy]] into [[heat]]. The centre, 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|author=Thierry Montmerle, Jean-Charles Augereau, Marc Chaussidon |title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|publisher=Spinger|pages=39–95|year=2006|doi=10.1007/s11038-006-9087-5| url=http://adsabs.harvard.edu/abs/2006EM%26P...98...39M}}</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 ~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 | doi=10.1126/science.1101979 }} </ref>

At this point in its [[stellar evolution|evolution]], the Sun is believed to have been a [[T Tauri star]]. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 [[solar mass]]es.<ref name= "Kitamara">{{cite conference | author=M. Momose, Y. Kitamura, S. Yokogawa, R. Kawabe, M. Tamura, S. Ida | title=Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a High-resolution Imaging Survey at lambda = 2 mm | booktitle=The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume I | year=2003 | publisher=Astronomical Society of the Pacific Conference Series | volume=289 | editor=Ikeuchi, S., Hearnshaw, J. and Hanawa, T. (eds.) | pages=85 | url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2003ASPC..289...85M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | format=PDF}}</ref> These discs extend to several hundred&nbsp;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| month= March| year= 1999| volume= 117| pages=1490–1504| doi=10.1086/300781| title=Hubble Space Telescope/NICMOS Imaging of Disks and Envelopes around Very Young Stars|author=Deborah L. Padgett, Wolfgang Brandner, Karl R. Stapelfeldt et al. | url=http://adsabs.harvard.edu/abs/1999AJ....117.1490P}}</ref>—and are rather cool, reaching only a thousand [[kelvin]]s at their hottest.<ref>{{cite journal | author=M. Küker, T. Henning, G. Rüdiger | title=Magnetic Star-Disk Coupling in Classical T Tauri Systems | journal=Astrophysical Journal | year=2003 | volume=589 | pages=397 | doi=10.1086/374408 | url=http://adsabs.harvard.edu/abs/2003ApJ...589..397K}}</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 which countered the force of gravitational contraction until [[hydrostatic equilibrium]] was achieved.<ref name=Yi2001>{{cite journal | author= Sukyoung Yi, Pierre Demarque, Yong-Cheol Kim, Young-Wook Lee, Chang H. Ree, Thibault Lejeune, Sydney  Barnes | title=Toward Better Age Estimates for Stellar Populations: The <math>Y^{2}</math> Isochrones for Solar Mixture | journal=Astrophysical Journal Supplement | id={{arXiv|astro-ph|0104292}} | year=2001 | volume=136 | pages=417 | doi=10.1086/321795 | url=http://adsabs.harvard.edu/abs/2001ApJS..136..417Y}}</ref> This marked the Sun's entry into the prime phase of its life, known as the [[main sequence]]. Main sequence stars are those which derive their energy from the fusion of hydrogen into helium in their cores. The Sun remains a main sequence star today.<ref name=sequence>{{harvtxt|Zeilik|Gregory|1998|loc=p. 320}}</ref>

=== Formation of planets ===
{{see also|Protoplanetary disc}}
[[File:solarnebula.jpg|thumb|Artist's conception of the solar nebula]]

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|author=A. P. Boss, R. H. Durisen|doi = 10.1086/429160|url=http://www.journals.uchicago.edu/doi/abs/10.1086/429160|year=2005|journal=The Astrophysical Journal|volume=621|pages=L137–L140|format=abstract page}}</ref> The currently accepted method by which the planets formed is known as [[Accretion (astrophysics)|accretion]], in which the planets began as dust grains in orbit around the central protostar. Through direct contact, these grains formed into clumps between one and ten [[kilometre]]s (km) in diameter, which in turn collided to form larger bodies ([[planetesimals]]) of ~5&nbsp;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>{{cite journal | author=P. Goldreich, W. R. Ward | title=The Formation of Planetesimals | journal=Astrophysical Journal | year=1973 | volume=183 | pages=1051 | url=http://adsabs.harvard.edu/abs/1973ApJ...183.1051G | accessdate=2006-11-16 | doi=10.1086/152291}}</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 which 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 masses 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 (see [[#Terrestrial planets|Terrestrial planets]] below).<ref name=sciam />

The [[Gas giant|gas giant planets]] ([[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]) formed further out, beyond the [[frost line (astrophysics)|frost line]], the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices which formed the Jovian planets were more abundant than the metals and silicates which formed the terrestrial planets, allowing the Jovian 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 four Earth masses within about 3&nbsp;million years.<ref name=sciam /> Today, the four gas giants comprise just under 99% of all the mass orbiting the Sun.<ref>Combined mass of Jupiter, Saturn, Uranus and Neptune= 445.6 Earth masses. Mass of remaining material= ~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 material to accumulate rapidly at ~5&nbsp;AU from the Sun. This excess material coalesced into a large embryo of about 10&nbsp;Earth masses, which then began to grow rapidly by swallowing hydrogen from the surrounding disc, reaching 150&nbsp;Earth masses in only another 1000&nbsp;years and finally topping out at 318&nbsp;Earth masses. 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}}</ref>

T Tauri stars like the young Sun have far stronger [[stellar wind]]s than more stable, older stars. Uranus and Neptune are believed to have formed after Jupiter and Saturn did, when the strong [[solar wind]] had blown away much of the disc material. As a result, the planets accumulated little hydrogen and helium—not more than 1&nbsp;Earth mass 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 a hundred million years for their cores to accrete. This means that Uranus and Neptune probably formed closer to the Sun—near or even between Jupiter and Saturn—and later migrated 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/| accessdate=2007-01-02}}</ref>

After between three and ten&nbsp;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 | pages=77 | url=http://adsabs.harvard.edu/abs/1979A%26A....80...77E | accessdate=2006-11-19}}</ref><ref>{{cite web|date=24 November 2004 |author=Heng Hao|work=Harvard University | title=Disc-Protoplanet interactions | url=http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf | accessdate=2006-11-19|format=PDF}}</ref>

== Subsequent evolution ==
The planets were originally believed to have formed in or near the orbits at which we see them now. However, this view has been undergoing radical change during the late 20th and early 21st centuries. Currently, it is believed that the Solar System looked very different after its initial formation: several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the [[Kuiper belt]] was much closer to the Sun.<ref>{{cite web|author=[[Michael E. Brown|Mike Brown]] (California Institute of Technology)|url= http://www.gps.caltech.edu/~mbrown/planetlila/moon/index.html|title=Dysnomia, the moon of Eris|work=Personal web site|accessdate=2008-02-01}}</ref>

=== Terrestrial planets ===

At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized [[planetary embryo]]s.<ref name=Petit2001/><ref name= Kominami>{{cite journal|title=The Effect of Tidal Interaction with a Gas Disk on Formation of Terrestrial Planets|author=Junko Kominami, Shigeru Ida|work=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}}</ref> Further growth was possible only because these bodies collided and merged, a process which took up to 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 believed 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|year=2003|pages=441–455|doi=10.1016/S0012-821X(03)00546-6| url=http://adsabs.harvard.edu/abs/2003E%26PSL.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 near-circular orbits the terrestrial planets possess today.<ref name=Petit2001/> One hypothesis for this "eccentricity dumping" is that the terrestrials formed in a disc of gas still not expelled by the Sun. The "[[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 terrestrials' 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|author=Peter Goldreich, Yoram Lithwick, Re’em Sari|journal=The Astrophysical Journal|volume=614|pages=497|date=10 October 2004|doi=10.1086/423612}}</ref>

=== Asteroid belt ===
The outer edge of the terrestrial region, between 2 and 4&nbsp;AU from 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|author=William F. Bottke, Daniel D. Durda, David Nesvorny et al. | title=Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion | journal=Icarus | volume=179 | 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|format=PDF }}</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|author=Jean-Marc Petit, Alessandro Morbidelli|title=The Primordial Excitation and Clearing of the Asteroid Belt|journal=Icarus|volume=153| pages=338–347|year=2001|doi=10.1006/icar.2001.6702| url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf|format=PDF}}</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 | author=R. Edgar, 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 | pages=769–772 | url=http://www.utsc.utoronto.ca/~pawel/edgar+artymowicz.pdf | format=PDF | accessdate=2008-05-12 | doi = 10.1111/j.1365-2966.2004.08238.x}}</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 | booktitle = Proceedings 37th Annual Lunar and Planetary Science Conference | publisher = Lunar and Planetary Society | year=2006 | location = League City, Texas | url =http://adsabs.harvard.edu/abs/2006LPI....37.2367S | accessdate = 2007-04-16
}}</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| accessdate = 2007-04-02}}</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|author= David O’Brien, Alessandro Morbidelli, William F. Bottke|title=The primordial excitation and clearing of the asteroid belt—Revisited |journal=Icarus|volume=191| pages=434–452 |year=2007|doi= 10.1016/j.icarus.2007.05.005|format=PDF| url=http://www.boulder.swri.edu/~bottke/Reprints/OBrien_2007_Icarus_191_434_Primordial_Excitation_Clearing_Asteroid_Belt.pdf}}</ref>
This is still 10–20 times more than the current mass in the main belt, which is about 1/2,000 the Earth's mass.<ref name=Krasinsky2002>{{cite journal |author=[[Georgij A. Krasinsky]], [[Elena V. Pitjeva]], M. V. Vasilyev, E. I. Yagudina | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..158...98K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4326fb2cf906949 |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |month=July |year=2002 |doi=10.1006/icar.2002.6837}}</ref> A secondary depletion period that brought the asteroid belt down close to its present mass is believed 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 the 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 | author=Sean N. Raymond, Thomas Quinn, Jonathan I. Lunine |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 | url=http://adsabs.harvard.edu/abs/2007AsBio...7...66R}}</ref> A population of [[main-belt comet]]s discovered in 2006 has been also 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, [[David Jewitt]] | journal=Science|date=23 March 2006 | volume=312|number=5773|pages=561–563 | doi=10.1126/science.1125150| url=http://www.sciencemag.org/cgi/content/abstract/312/5773/561|accessdate=2008-04-05|format=abstract page}}</ref><ref>{{cite web|title=New comet class in Earth's backyard|url=http://www.astronomy.com/asy/default.aspx?c=a&id=4100| work=astronomy.com|author=Francis Reddy|year=2006|accessdate=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 | author= A. Morbidelli, J. Chambers, J. I. Lunine, J. M. Petit, F. Robert, G. B. Valsecchi, K. E. Cyr | title= Source regions and timescales for the delivery of water to the Earth | journal= Meteoritics & Planetary Science | volume=35 | pages=1309 | issn= 1086–9379 | year=2000 }}</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|author=Florence Raulin-Cerceau, Marie-Christine Maurel, Jean Schneider|publisher=Springer Netherlands|journal=Origins of Life and Evolution of Biospheres|year=1998|volume=28|doi=10.1023/A:1006566518046 | pages=597–612|url=http://www.springerlink.com/content/m1t14rtr7372tp22/|accessdate=2007-12-19}}</ref>

=== Planetary migration ===
{{main|Nice model}}
According to the nebular hypothesis, the outer two planets are 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 highly implausible. The two are instead believed to have formed in orbits near Jupiter and Saturn, where more material was available, but to have [[Planetary migration|migrated outward]] to their current positions over hundreds of millions of years.<ref name="thommes">{{cite journal | author=E. W. Thommes, M. J. Duncan, H. F. Levison | title=The Formation of Uranus and Neptune among Jupiter and Saturn | journal=Astronomical Journal | id={{arXiv|astro-ph|0111290}} | year=2002 | volume=123 | pages=2862 | doi=10.1086/339975 | url=http://adsabs.harvard.edu/abs/2002AJ....123.2862T}}</ref>

[[File:Lhborbits.png|thumb|400px|Simulation showing outer planets and Kuiper belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune c) After ejection of Kuiper belt bodies by Jupiter<ref name="Gomes">{{cite journal | url=http://www.nature.com/nature/journal/v435/n7041/pdf/nature03676.pdf | title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets | author=R. Gomes, H. F. Levison, K. Tsiganis, A. Morbidelli | journal=Nature | year=2005 | volume=435 | pages=466 | doi=10.1038/nature03676|format=PDF}}</ref>]]
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 | author=Harold F. Levison, Alessandro Morbidelli, Crista Van Laerhoven et al. | title=Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune|year=2007| url=http://adsabs.harvard.edu/abs/2007arXiv0712.0553L | id={{arxiv|0712.0553}} | doi=10.1016/j.icarus.2007.11.035 | journal=Icarus | volume=196 | pages=258 }}</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. 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 web
 |title=Origin and dynamical evolution of comets and their reservoirs
 |author=Alessandro Morbidelli
 |url=http://arxiv.org/abs/astro-ph/0512256
 |year=2006 |accessdate=2007-05-26
 |date=3 February 2008 |format=PDF |publisher=arxiv}}</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 opposite locations, with Uranus farther from the Sun than Neptune.<ref name="Gomes"/><ref name=Levison2007/>

After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with 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, causing Neptune to surge past Uranus and plough into the ancient Kuiper belt. 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>{{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 &amp; Planetology |accessdate=2008-02-01}}</ref> 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>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 tranfers 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 which is being perturbed inwards stands a greater chance of encountering Jupiter and being ejected from the Solar System, in which case the energy gains of Neptune, Uranus and Saturn obtained from their inwards deflections of the ejcted 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]]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 | id={{arXiv|astro-ph|9504036}} | year=1995 | volume=110 | pages=420 | doi=10.1086/117532 | url=http://adsabs.harvard.edu/abs/1995AJ....110..420M }}</ref> Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again.<ref name=Levison2007/><ref name="fogg_nelson">{{cite journal | author=M. J. Fogg, R. P. Nelson | title=On the formation of terrestrial planets in hot-Jupiter systems | journal=Astronomy & Astrophysics | id={{arXiv|astro-ph|0610314}} | year=2007 | volume = 461 | pages=1195 | doi=10.1051/0004-6361:20066171 }}</ref>

In contrast to the outer planets, the inner planets are not believed 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/>

=== Late Heavy Bombardment and after ===
{{main|Late Heavy Bombardment}}
[[File:Meteor.jpg|thumb|left|Meteor Crater in Arizona. Created 50,000 years ago by an impactor only 50m across, it is a stark reminder 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 which 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| accessdate=2006-06-22}} </ref> 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=http://history.nasa.gov/SP-467/ch3.htm | title=Chronology of Planetary surfaces|work=NASA History Division|accessdate=2008-03-13}}</ref> The oldest known evidence for [[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 | accessdate=2008-04-29}}</ref>

Impacts are believed 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, and the impact feature [[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 | format=PDF |accessdate=2008-05-06 | issn=00167800}}</ref><ref name=Agnor2006>{{cite journal|title=Neptune’s capture of its moon Triton in a binary-planet gravitational encounter|author=Craig B. Agnor, 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|format=PDF }}</ref>

The evolution of the outer Solar System appears to have been influenced by nearby [[supernova]]e and possibly also passage through [[interstellar cloud]]s. The surfaces of bodies in the outer Solar System would experience [[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|author=Beth E. Clark, Robert E. Johnson | journal= Eos, Transactions, American Geophysical Union | doi=10.1029/96EO00094 | volume=77 | pages=141 | year=1996 | accessdate=2008-03-13}}</ref>

The evolution of the asteroid belt after Late Heavy Bombardment was mainly governed by collisions.<ref name=Bottke2005b>{{cite conference|author=William F. Bottke, D. Durba, D. Nesvorny et al.|title=The origin and evolution of stony meteorites|conference=Dynamics of Populations of Planetary Systems|booktitle=Proceedings of the International Astronomical Union|volume=197|pages=357–374|year=2005|doi=10.1017/S1743921304008865|format=PDF| url=http://www.boulder.swri.edu/~bottke/Reprints/Bottke_IAU197_Belgrade_Origin_Stony_Met.pdf}}</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
  | author=H. Alfvén, G. Arrhenius
  | year=1976
  | url =http://history.nasa.gov/SP-345/ch4.htm
  | title =The Small Bodies
  | work=SP–345 Evolution of the Solar System
  | publisher = NASA
  | accessdate = 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 circum-planetary disc (only in the cases of the gas giants);
* formation from impact debris (given a large enough impact at a shallow angle); and
* capture of a passing object.

Jupiter and Saturn have a number of 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>{{cite journal | author=N. Takato, S. J. Bus et al. | 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 | doi=10.1126/science.1105427 | url=http://adsabs.harvard.edu/abs/2004Sci...306.2224T }}<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 | accessdate=2008-04-03}}</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 make formation from collision debris another impossibility. The outer moons of the gas giants tend to be small and have [[Orbital eccentricity|eccentric]] orbits with arbitrary inclinations. These are the characteristics expected of captured bodies.<ref>{{cite conference | author = D. C. Jewitt, S. Sheppard, C. Porco | title = Jupiter's outer satellites and Trojans | booktitle=Jupiter. The Planet, Satellites and Magnetosphere | year = 2004 | pages = 263–280 | url=http://www.ifa.hawaii.edu/~jewitt/papers/JUPITER/JSP.2003.pdf | format=PDF | editor= Fran Bagenal, Timothy E. Dowling, William B. McKinnon (eds.) | publisher=Cambridge University Press | id=ISBN 0-521-81808-7}} </ref><ref>{{cite web|url=http://www.dtm.ciw.edu/sheppard/satellites/| title= The Giant Planet Satellite and Moon Page |author=Scott S. Sheppard (Carnegie Institution of Washington) | work=Personal web page | accessdate=2008-03-13}}</ref> Most such moons orbit in the direction opposite the rotation of their primary. The largest irregular moon is Neptune's moon [[Triton (moon)|Triton]], which is believed 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 believed to be captured [[asteroid]]s.<ref>{{harvtxt|Zeilik|Gregory|1998|loc=pp. 118–120}}</ref>
The Earth's [[Moon]] is believed to have formed as a result of a single, large oblique 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 | pages=271 | doi=10.1146/annurev.ea.15.050187.001415 | url=http://adsabs.harvard.edu/abs/1987AREPS..15..271S }}</ref>
The impacting object likely 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 | author=R. M. Canup, 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 | url=http://adsabs.harvard.edu/abs/2001Natur.412..708C | doi=10.1038/35089010 }}</ref> The impact was probably the last in the 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 &amp; Planetology | accessdate = 2007-07-25 }}</ref> Pluto's moon Charon may also have formed by means of a large collision; the Pluto-Charon and Earth-Moon systems are the only two in the Solar System in which 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 | number=5709 | pages=546–550 | doi=10.1126/science.1106818 | url=http://www.sciencemag.org/cgi/content/abstract/307/5709/546 | accessdate=2008-05-01|format=abstract page}}</ref>

== Future ==
Astronomers estimate that the Solar System as we know it today will not change drastically until the Sun has fused 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]] phase. Even so, the Solar System will continue to evolve until then.

=== Long-term stability ===
{{main|Stability of the Solar System}}
The Solar System is [[chaos theory|chaotic]],<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
|url=http://adsabs.harvard.edu/abs/1994A%26A...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 | author = Gerald Jay Sussman, 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
}}</ref> Another example is Earth's [[axial tilt]] which, thanks to friction raised within Earth's mantle by tidal interactions with the Moon ([[#Moon-ring systems|see below]]) will be rendered chaotic at 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, J. Laskar|journal=Astronomy and Astrophysics|date=February 1997|volume=318|pages=975–989|url=http://adsabs.harvard.edu/abs/1997A%26A...318..975N|accessdate=2008-06-08}}</ref>

The outer planets' orbits are chaotic over longer timescales, such that they possess 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 | id={{arXiv|astro-ph|0702179}} | year=2007 | volume=3 | pages=689–691 | doi=10.1038/nphys728 | url=http://adsabs.harvard.edu/abs/2007NatPh...3..689H}}</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 become uncertain), but 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>{{cite book
|author=Ian Stewart
|title=Does God Play Dice?
|publisher=Penguin Books
|edition=2nd
|pages=246–249
|year=1997
|isbn=0-14-025602-4}}</ref>

Ultimately, the Solar System is stable in that none of the planets will 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=http://space.newscientist.com/article/dn13757-solar-system-could-go-haywire-before-the-sun-dies.html?feedId=online-news_rss20 | author=David Shiga | work=NewScientist.com News Service | date=23 April 2008 | accessdate=2008-04-28}}</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 is equal to one of its rotations about its axis, which means that 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. In about 50&nbsp;billion years, if the two worlds survive the [[#The Sun and planetary environments|Sun's expansion]], they will become tidally locked to each other; each will be visible from only one hemisphere of the other.<ref>{{cite book|title=Solar System Dynamics|author=C.D. Murray & S.F. Dermott|publisher=Cambridge University Press|year=1999|page=184}}</ref> Other examples are the [[Galilean moons]] of [[Jupiter]] (as well as many of Jupiter's smaller moons)<ref>{{cite journal | url=http://adsabs.harvard.edu/full/1982MNRAS.201..415G%7D | 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 | pages=415 | year=1980|accessdate=2008-03-27}}</ref> and most of the larger moons of [[Saturn]].<ref>{{cite journal |author= R. Bevilacqua, O. Menchi, A. Milani et al. |year=1980 |month=April |title= Resonances and close approaches. I. The Titan-Hyperion case|journal=Earth, Moon, and Planets |volume=22 |issue=2 |pages=141–152  |url=http://www.springerlink.com/content/g627852062714784/ |accessdate= 2007-08-27 |doi= 10.1007/BF00898423}}</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>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 take place. It is the loss of dynamical energy through friction that makes 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 | author=Bruce G. Bills, Gregory A. Neumann, David E. Smith, and 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 |pages=E07004|doi=10.1029/2004JE002376| url=http://adsabs.harvard.edu/abs/2005JGRE..11007004B}}</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, D. G. Jankowski, P. D.  Nicholson | journal=Astronomy & Astrophysics | volume=219 | pages=23 | url=http://adsabs.harvard.edu/abs/1989A&A...219L..23C | year=1989 | accessdate=2007-03-03}}</ref> [[Metis (moon)|Metis]] and [[Adrastea (moon)|Adrastea]] of Jupiter,<ref name=Burns2004>{{cite conference | author=J. A. Burns, D. P. Simonelli, M. R. Showalter, D. P. Hamilton, C. C. Porco, L. W. Esposito, H. Throop | title=Jupiter’s Ring-Moon System | booktitle=Jupiter: The planet, Satellites and Magnetosphere | year=2004 | publisher=Cambridge University Press | editor=Fran Bagenal, Timothy E. Dowling, William B. McKinnon  (eds.) | url=http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf | format=PDF | accessdate=2008-05-14 | id=ISBN 0-521-81808-7 | pages=241}}</ref> and at least [[Tidal acceleration#Tidal deceleration|16 small satellites]] of Uranus and Neptune. Uranus' [[Desdemona (moon)|Desdemona]] may even collide with one of its neighboring moons.<ref name=Duncan1996>{{cite journal
 | author=Martin J. Duncan, Jack J. Lissauer
 |title=Orbital Stability of the Uranian Satellite System
 |journal=Icarus
 |volume=125 |issue=1 |pages=1–12 |year=1997
 |doi=10.1006/icar.1996.5568
 |accessdate=2008-05-09}}</ref>

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 transfer of angular momentum, and the orbital period will not change.  [[Pluto]] and [[Charon (moon)|Charon]] are an example of this type of configuration.<ref>{{cite journal | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AJ....132..290B&db_key=AST&data_type=HTML&format=&high=444b66a47d27727
| title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 | author=Marc Buie, William Grundy, Eliot Young, Leslie Young, Alan Stern | journal=The Astronomical Journal | volume=132 | pages=290 | year=2006 | doi=10.1086/504422 | id={{arXiv|astro-ph|0512491}}}}</ref>

Prior to the 2004 arrival of the ''[[Cassini–Huygens]]'' spacecraft, the [[rings of Saturn]] were widely thought to be much younger than the Solar System and were not expected to survive beyond another 300&nbsp;million years. Gravitational interactions with Saturn's moons were expected to gradually sweep the rings' outer edge toward the planet, with abrasion by meteorites and Saturn's gravity eventually taking the rest, leaving Saturn unadorned.<ref>{{cite web|title=Saturn Rings Still A Mystery|work=Popular Mechanics|author= Stefano Coledan|url=http://www.popularmechanics.com/technology/industry/1285531.html|year=2002|accessdate=2007-03-03}}</ref> However, data from the ''Cassini'' mission led scientists to revise that early view. Observations revealed 10&nbsp;km-wide icy clumps of material that repeatedly break apart and reform, keeping the rings fresh. Saturn's rings are far more massive than the rings of the other gas giants. This large mass is believed to have preserved Saturn's rings since the planet first formed 4.5&nbsp;billion years ago, and is likely to preserve them for billions of years to come.<ref>{{cite journal | title=Saturn's recycled rings | journal=Astronomy Now| pages=9 |date=February 2008}}</ref>

===The Sun and planetary environments===<!--Note: this subsection linked to from [[1 E19 s and more]] as of 2008-08-07-->
{{seealso|Stellar evolution}}
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 supply of hydrogen fuel, 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=http://www.newscientist.com/article/mg14219191.900.html | date=2 April 1994 | issue=1919 | page=14 |accessdate=2007-10-29}}</ref> In one billion years' time, as the Sun's radiation output increases, its [[Habitable zone#Circumstellar habitable zone|circumstellar habitable zone]] will move outwards, making the Earth's surface hot enough that liquid water can no longer exist there naturally. At this point, all life on land will become extinct.<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|author=Knut Jørgen, Røed Ødegaard|work=Centre for International Climate and Environmental Research|year=2004|accessdate=2008-03-27}}</ref> During this time it is possible that as [[Mars]]'s surface temperature gradually rises, carbon dioxide and water currently frozen under the surface soil will be liberated into the atmosphere, creating a greenhouse effect which will heat up the planet until it achieves conditions parallel to those on 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=http://books.google.com/books?id=0QY0U6qJKFUC&pg=PA509&lpg=PA509&dq=mars+future+%22billion+years%22+sun&source=web&ots=d1qKC28btQ&sig=dYP1gA4LkGgJczuAFiBH1axxodw#PPA509,M1 | isbn=1852335688 | year=2004 | publisher=Springer |accessdate=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 our 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]].<ref>{{harvtxt|Zeilik|Gregory|1998|loc=p. 320–321}}</ref><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 |accessdate = 2006-12-29}}</ref> Within 7.5&nbsp;billion years, the Sun will have expanded to a radius of 1.2&nbsp;AU—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 2600&nbsp;K) 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]] which will carry away around 33% of its mass.<ref name=Schroder2008>{{cite journal|author= K. P. Schroder, Robert Cannon Smith|title= Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society | volume=386 |pages=155–163 | year=2008 |doi=10.1111/j.1365-2966.2008.13022.x |url=http://adsabs.harvard.edu/abs/2008MNRAS.386..155S }}</ref><ref name="sun_future">{{cite journal | author=I. J. Sackmann, A. I. Boothroyd, K. E. Kraemer | title=Our Sun. III. Present and Future | pages=457 | journal=Astrophysical Journal | year=1993 | volume=418 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S | doi=10.1086/173407 }}</ref><ref>{{harvtxt|Zeilik|Gregory|1998|loc=p. 322}}</ref> During these times, it is possible that [[Saturn]]'s moon [[Titan (moon)|Titan]] could achieve surface temperatures necessary to support life.<ref>{{cite journal|title=Titan under a red giant sun: A new kind of "habitable" moon|author=Ralph D. Lorenz, Jonathan I. Lunine, Christopher P. McKay | journal=Geophysical Research Letters | year=1997 | volume=24 | pages=2905 | url=http://www.lpl.arizona.edu/~rlorenz/redgiant.pdf | accessdate=2008-03-21|format=PDF|doi=10.1029/97GL52843 }}</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| accessdate=2006-06-23}}</ref>

As the Sun expands, it will swallow the planets [[Mercury (planet)|Mercury]] and, most likely, [[Venus]].<ref name=Rybicki2001>{{cite journal
  |author=K. R. Rybicki, C. Denis
  |title=On the Final Destiny of the Earth and the Solar System
  |journal=Icarus
  |volume=151 | issue=1 |pages=130–137 |year=2001
  |doi=10.1006/icar.2001.6591 }}</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 [[Roche limit|tidal interactions]] with the Sun's weakly bound outer envelope.<ref name=Schroder2008/>

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 4770&nbsp;K. The Sun will become a [[horizontal branch]] star, 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 and will expand a second time, turning into what is known as an [[asymptotic giant branch]] star. Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about 3500&nbsp;K.<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.<ref name=nebula> {{cite web | author=Bruce Balick (Department of Astronomy, University of Washington) | title=Planetary nebulae and the future of the Solar System | work=Personal web site | url=http://www.astro.washington.edu/balick/WFPC2/| accessdate=2006-06-23}} </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 our 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 still others to be torn apart by tidal interactions.<ref>{{cite journal|title=A Gaseous Metal Disk Around a White Dwarf|author=B. T. Gänsicke, T. R. Marsh, J. Southworth, A. Rebassa-Mansergas|journal=Science| year=2006 | volume=314 | number= 5807 | pages=1908–1910 | doi=10.1126/science.1135033}}</ref> Afterwards, all that will remain of the Sun is a [[white dwarf]], an extraordinarily dense object, 54% its original mass but only the size of the 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=[http://www.astronomy.ohio-state.edu/Vistas/ New Vistas in Astronomy]|accessdate=2005-12-07}}</ref>

As the Sun dies, its gravitational pull on the 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|km| lk=on | abbr=on}}, {{convert|1.9|AU|km| lk=on | abbr=on}}, and {{convert|2.8|AU|km| lk=on | abbr=on}}. They and the other remaining planets will become dark, frigid hulks, completely devoid of any form 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 billion years later, when the Sun has cooled to the 6000–8000K 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 |author=T. S. Metcalfe, M. H. Montgomery, A. Kanaan| title=Testing White Dwarf Crystallization Theory with Asteroseismology of the Massive Pulsating DA Star BPM 37093 | journal=Astrophysical Journal | id={{arXiv|astro-ph|0402046}} | year=2004 | volume=605 | pages=L133 | doi=10.1086/420884 | url=http://adsabs.harvard.edu/abs/2004ApJ...605L.133M}}</ref> Eventually, after trillions more 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 | author=G. Fontaine, P. Brassard, P. Bergeron | journal=Publications of the Astronomical Society of the Pacific | volume=113 | pages=409–435 | year=2001 | doi=10.1086/319535 | url=http://www.journals.uchicago.edu/doi/full/10.1086/319535|accessdate=2008-05-11}}</ref>

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

The Solar System travels alone through the [[Milky Way]] galaxy in a circular orbit approximately 30,000&nbsp;light years from the [[galactic centre]]. Its speed is about 220&nbsp;km/s. The period required for the Solar System to complete one revolution around the galactic centre, 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=[http://hypertextbook.com/facts/ The Physics Factbook] (self-published)| author=Stacy Leong| editor=Glenn Elert (ed.) | year=2002|title=Period of the Sun's Orbit around the Galaxy (Cosmic Year) | accessdate=2008-06-26}}</ref>

A number of 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
|author=Michael Szpir
|work=American Scientist
|url=http://www.americanscientist.org/template/AssetDetail/assetid/24618
|accessdate=2008-03-25
|publisher=The Scientific Research Society}}</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]] stars, 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|author=Erik M. Leitch, Gautam Vasisht|year=1998|pages=51–56|journal=New Astronomy|volume= 3|url=http://arxiv.org/abs/astro-ph/9802174v1|accessdate=2008-04-09|doi=10.1016/S1384-1076(97)00044-4}}</ref>

=== Galactic collision and planetary disruption ===
{{main|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 our [[Local Group]] of galaxies, is heading towards it at about 120&nbsp;km/s.<ref name=cain /> In 2&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]]. When 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 [[gravity|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%, the galaxies' [[supermassive black hole]]s will merge. Eventually, in roughly 7&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|accessdate=2007-05-16}}</ref><ref>{{cite journal|title=The Collision Between The Milky Way And Andromeda | author=J. T. Cox, Abraham Loeb | journal=Monthly Notices of the Royal Astronomical Society |url=http://cfa-www.harvard.edu/~tcox/localgroup/|id={{arxiv|0705.1170}} | year=2007 | doi=10.1111/j.1365-2966.2008.13048.x|accessdate=2008-04-02|volume=386|pages=461}}</ref>

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. While 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. While the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed.<ref>{{cite journal|title=Colliding molecular clouds in head-on galaxy collisions|author=J. Braine, U. Lisenfeld, P. A. Duc, E. Brinks, V. Charmandaris, S. Leon|journal=Astronomy and Astrophysics|volume= 418|pages= 419–428 |year=2004|doi = 10.1051/0004-6361:20035732|url=http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361:20035732|accessdate=2008-04-02}}</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. While 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|work=Institute for Advanced Study, Princeton New Jersey|journal=Reviews of Modern Physics|volume=51|number=3 | pages=447|date=July 1979| url=http://www.think-aboutit.com/Misc/time_without_end.htm |accessdate=2008-04-02 | doi=10.1103/RevModPhys.51.447}}</ref>

== Chronology ==
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 | title= Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago | author= Simon A. Wilde, John W. Valley, William H. Peck, Colin M. Graham | doi=10.1038/35051550 | url=http://www.geology.wisc.edu/%7Evalley/zircons/Wilde2001Nature.pdf | format=PDF | year= 2001 }}</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 | year=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=0521478952}}</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 possess discs rich in gas, whereas discs around stars more than 10&nbsp;million years old have little to no gas, suggesting that gas giant planets within them have ceased forming.<ref name=sciam />

=== Timeline of Solar System evolution ===
Note: All dates and times in this chronology are approximate and should be taken as an [[order of magnitude]] indicator only.

{| class=wikitable |+Chronology of the formation and evolution of the Solar System
 |-
 ! Phase !! Time since formation of the Sun !! Event
 |-
 |-bgcolor=#FFFFFF
 ! rowspan=2 | Pre-Solar System
 | Billions of years before the formation of the Solar System
 | 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 | pages=307 | year=2001 | doi=10.1006/icar.2001.6607 | id={{arxiv|astro-ph|0012399}} }}</ref>
 |-
 |-bgcolor=#FFFFFF
 | ~5{{e|7}} years before formation of the Solar System
 | 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 supernovae. One supernova possibly triggers the formation of the Solar System.<ref name=cradle /><ref name=iron /> 
 |-bgcolor=#E0FFFF
 ! rowspan=4 | Formation of Sun
 | '''0'''–1{{e|5}}&nbsp;years
 | Pre-solar nebula forms and begins to collapse. Sun begins to form.<ref name=sciam />
 |-bgcolor=#E0FFFF
 | 1{{e|5}}–5{{e|7}}&nbsp;years
 | Sun is a [[T Tauri star|T Tauri]] [[protostar]].<ref name=Montmerle2006 />
 |-bgcolor=#E0FFFF
 | 1{{e|5–7}}&nbsp;years
 | Outer planets form. By 10<sup>7</sup>&nbsp;years, gas in the [[protoplanetary disc]] has been blown away, and outer planet formation is likely complete.<ref name=sciam />
 |-bgcolor=#E0FFFF
 | 1{{e|7–8}}&nbsp;years
 | Terrestrial planets and the Moon form. Giant impacts occur. Water delivered to Earth.<ref name=Gomes />
 |-
 |-bgcolor=#FFFF00
 ! rowspan=7 | Main sequence
 | 5{{e|7}}&nbsp;years
 | Sun becomes a [[main sequence]] star.<ref name=Yi2001/>
 |-
 |-bgcolor=#FFFF00
 | 2{{e|8}}&nbsp;years
 | Oldest known rocks on the Earth formed.<ref name=Wilde />
 |-bgcolor=#FFFF00
 | 5–6{{e|8}}&nbsp;years
 | 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=#FFFF00 
 |-
 |-bgcolor=#FFFF00 
 |-
 |-bgcolor=#FFFF00
 | 8{{e|8}}&nbsp;years
 | Oldest known life on Earth.<ref name=life />
 |- bgcolor=#A7FC00
 | 4.6{{e|9}}&nbsp;years
 | '''Today'''. Sun remains a main sequence star, continually growing warmer and brighter by ~10% every 10<sup>9</sup> years.<ref name=scientist />
 |-
 |-bgcolor=#FFFF00 
 | 6{{e|9}}&nbsp;years
 | Sun's [[habitable zone]] moves outside of the Earth's orbit, possibly shifting onto Mars' orbit.<ref name=mars />
 |-
 |-bgcolor=#FFFF00 
 | 7{{e|9}}&nbsp;years
 | 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= #FFC0CB
 ! rowspan=2 | Post-main sequence
 | 10–12{{e|9}}&nbsp;years
 | Sun starts burning hydrogen in a shell surrounding its core, 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 2700), larger (by a factor of up to 250 in radius), and cooler (down to 2600&nbsp;K): Sun is now a [[red giant]]. Mercury and possibly Venus and Earth are swallowed.<ref name=Schroder2008/><ref name=sun_future />
 |-bgcolor= #FFC0CB
 | ~12{{e|9}}&nbsp;years
 | Sun passes through helium-burning [[horizontal branch]] and [[asymptotic giant branch]] phases, losing a total of ~30% of its mass in all post-main sequence phases. Asymptotic giant branch phase ends with the ejection of a [[planetary nebula]], leaving the core of the Sun behind as a [[white dwarf]].<ref name=Schroder2008/><ref name=nebula/> 
 |-
 ! rowspan=2 | Remnant Sun
 | >12{{e|9}}&nbsp;years
 | The white dwarf Sun, no longer producing energy, begins to cool and dim continuously, eventually reaching a [[black dwarf]] state.<ref name=future-sun /><ref name=Fontaine2001/>
 |-
 | 10<sup>15</sup>&nbsp;years
 | Sun cools to 5&nbsp;K.<ref>{{cite book|title=The Anthropic Cosmological Principle|author=John D. Barrow, Frank J. Tipler|publisher= Oxford University Press|year= 1986|isbn=0-19-282147-4}}</ref> Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.<ref name=dyson /> 
 |}
{{External_Timeline|Graphical timeline of our universe|Graphical timeline of Earth and Sun}}

== See also ==
{{portal|Solar System|Solar system.jpg}}
*[[Age of the Earth]]
*[[History of Earth]]
*[[Tidal locking]]

== Notes and references ==
{{reflist|2}}

=== Bibliography ===
* {{anchor|CITEREFZeilikGregory1998}}{{cite book | author=Michael A. Zeilik, Stephen A. Gregory | title=Introductory Astronomy & Astrophysics | edition=4th | year=1998 | publisher=Saunders College Publishing | isbn=0030062284 }}

== External links ==
*[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://cfa-www.harvard.edu/~tcox/localgroup/localgroup.mov Quicktime animation of the future collision between the Milky Way and Andromeda]

{{featured article}}

{{DEFAULTSORT:Formation And Evolution Of The Solar System}}
{{Link FA|ko}}

[[Category:Stellar astronomy]]
[[Category:Planetary science]]
[[Category:Solar System]]
[[Category:Planets]]
[[Category:Solar System dynamic theories]]

[[be-x-old:Паходжаньне Сонечнай сыстэмы]]
[[ca:Formació i evolució del sistema solar]]
[[da:Solsystemets dannelse og udvikling]]
[[es:Formación y evolución del Sistema Solar]]
[[eu:Eguzki sistemaren sorrera eta garapena]]
[[fa:تشکیل و تکامل منظومه شمسی]]
[[fr:Formation et évolution du système solaire]]
[[gl:Formación e evolución do Sistema Solar]]
[[ko:태양계의 형성과 진화]]
[[id:Pembentukan dan evolusi Tata Surya]]
[[it:Origine ed evoluzione del sistema solare]]
[[he:ההיווצרות וההתפתחות של מערכת השמש]]
[[lv:Saules sistēmas veidošanās un evolūcija]]
[[lt:Saulės sistemos susidarymas]]
[[hu:A Naprendszer keletkezése és története]]
[[ja:太陽系の形成と進化]]
[[pl:Powstanie i ewolucja Układu Słonecznego]]
[[pt:Formação e evolução do Sistema Solar]]
[[ru:Формирование и эволюция Солнечной системы]]
[[simple:Formation and evolution of the Solar System]]
[[sk:Vznik a vývoj slnečnej sústavy]]
[[fi:Aurinkokunnan muodostuminen]]
[[sv:Solsystemets uppkomst och utveckling]]
[[th:กำเนิดและวิวัฒนาการของระบบสุริยะ]]
[[tr:Güneş sistemi'nin oluşumu ve evrimi]]
[[uk:Формування та еволюція Сонячної системи]]
[[zh:太阳系的形成与演化]]