Editing Jupiter trojan (section)

== Origin and evolution ==

Two main theories have emerged to explain the formation and evolution of the Jupiter trojans. The first suggests that the Jupiter trojans formed in the same part of the [[Solar System]] as Jupiter and entered their orbits while it was forming.<ref name=Marzari2002/> The last stage of Jupiter's formation involved runaway growth of its mass through the accretion of large amounts of [[hydrogen]] and [[helium]] from the [[protoplanetary disk]]; during this growth, which lasted for only about 10,000 years, the mass of Jupiter increased by a factor of ten. The [[planetesimal]]s that had approximately the same orbits as Jupiter were caught by the increased gravity of the planet.<ref name=Marzari2002/> The capture mechanism was very efficient—about 50% of all remaining planetesimals were trapped. This hypothesis has two major problems: the number of trapped bodies exceeds the observed population of Jupiter trojans by four [[order of magnitude|orders of magnitude]], and the present Jupiter trojan asteroids have larger orbital inclinations than are predicted by the capture model.<ref name=Marzari2002/> Simulations of this scenario show that such a mode of formation also would inhibit the creation of similar trojans for [[Saturn]], and this has been borne out by observation: to date no trojans have been found near Saturn.<ref>{{cite journal|title=The growth of Jupiter and Saturn and the capture of Trojans |last1=Marzari|first1=F. |last2=Scholl|first2=H. |journal=Astronomy and Astrophysics|volume=339|pages=278–285|date=1998|bibcode=1998A&A...339..278M}}</ref> In a variation of this theory Jupiter captures trojans during its initial growth then migrates as it continues to grow. During Jupiter's migration the orbits of objects in horseshoe orbits are distorted causing the L4 side of these orbits to be over occupied. As a result, an excess of trojans is trapped on the L4 side when the horseshoe orbits shift to tadpole orbits as Jupiter grows. This model also leaves the Jupiter trojan population 3–4 orders of magnitude too large.<ref name="Pirani_etal_2019">{{cite journal |last1=Pirani |first1=S. |last2=Johansen |first2=A. |last3=Bitsch |first3=B. |last4=Mustill |first4=A. J. |last5=Turrini |first5=D. |title=Consequences of planetary migration on the minor bodies of the early solar system |journal=Astronomy & Astrophysics |date=2019 |volume=623 |page=A169 |doi=10.1051/0004-6361/201833713 |arxiv=1902.04591 |bibcode=2019A&A...623A.169P |s2cid=119546182 }}</ref>

The second theory proposes that the Jupiter trojans were captured during the migration of the giant planets described in the [[Nice model]]. In the Nice model the orbits of the giant planets became unstable {{Nowrap|500–600 million}} years after the Solar System's formation when Jupiter and Saturn crossed their 1:2 mean-motion [[orbital resonance|resonance]]. Encounters between planets resulted in [[Uranus]] and [[Neptune]] being scattered outward into the primordial [[Kuiper belt]], disrupting it and throwing millions of objects inward.<ref name=Levison2007/> When Jupiter and Saturn were near their 1:2 resonance the orbits of pre-existing Jupiter trojans became unstable during a secondary resonance with Jupiter and Saturn. This occurred when the period of the trojans' libration about their Lagrangian point had a 3:1 ratio to the period at which the position where Jupiter passes Saturn circulated relative to its perihelion. This process was also reversible allowing a fraction of the numerous objects scattered inward by Uranus and Neptune to enter this region and be captured as Jupiter's and Saturn's orbits separated. These new trojans had a wide range of inclinations, the result of multiple encounters with the giant planets before being captured.<ref name="Morbidelli">{{cite journal|last=Morbidelli|first=A.|author2=Levison, H. F.|author3=Tsiganis, K.|author4=Gomes, R.|date=26 May 2005|title=Chaotic capture of Jupiter's Trojan asteroids in the early Solar System|journal=Nature|volume=435|issue=7041|pages=462–465|oclc=112222497|url=http://www.oca.eu/michel/PubliGroupe/MorbyNature2005.pdf|doi=10.1038/nature03540|pmid=15917801|bibcode=2005Natur.435..462M|s2cid=4373366 |access-date=19 January 2009|archive-url=https://web.archive.org/web/20090731120551/http://www.oca.eu/michel/PubliGroupe/MorbyNature2005.pdf|archive-date=31 July 2009|url-status=dead}}</ref> This process can also occur later when Jupiter and Saturn cross weaker resonances.<ref name=Nesvorny_2013>{{cite journal|last=Nesvorný|first=David|author2=Vokrouhlický, David |author3=Morbidelli, Alessandro |title=Capture of Trojans by Jumping Jupiter|journal=The Astrophysical Journal|date=2013|volume=768|issue=1|page=45|doi=10.1088/0004-637X/768/1/45|arxiv=1303.2900|bibcode=2013ApJ...768...45N|s2cid=54198242 }}</ref>

In a [[Jumping-Jupiter scenario|revised version]] of the Nice model Jupiter trojans are captured when Jupiter encounters an ice giant during the instability. In this version of the Nice model one of the ice giants (Uranus, Neptune, or a lost [[Five-planet Nice model|fifth planet]]) is scattered inward onto a Jupiter-crossing orbit and is scattered outward by Jupiter causing the orbits of Jupiter and Saturn to quickly separate. When Jupiter's semi-major axis jumps during these encounters existing Jupiter trojans can escape and new objects with semi-major axes similar to Jupiter's new semi-major axis are captured. Following its last encounter the ice giant can pass through one of the libration points and perturb their orbits leaving this libration point depleted relative to the other. After the encounters end some of these Jupiter trojans are lost and others captured when Jupiter and Saturn are near weak mean motion resonances such as the 3:7 resonance via the mechanism of the original Nice model.<ref name="Nesvorny_2013"/>

The long-term future of the Jupiter trojans is open to question, because multiple weak resonances with Jupiter and Saturn cause them to behave chaotically over time.<ref name=Robutal2005/> Collisional shattering slowly depletes the Jupiter trojan population as fragments are ejected. Ejected Jupiter trojans could become temporary satellites of Jupiter or [[Jupiter-family comet]]s.<ref name=Jewitt2004/> Simulations show that the orbits of up to 17% of Jupiter trojans are unstable over the age of the Solar System.<ref>{{cite journal|title=Chaotic Diffusion And Effective Stability of Jupiter trojans |author=Kleomenis Tsiganis |author2=Harry Varvoglis |author3=Rudolf Dvorak |publisher=Springer|journal=Celestial Mechanics and Dynamical Astronomy|volume=92|date=April 2005|doi=10.1007/s10569-004-3975-7|pages=71–87|issue=1–3|bibcode = 2005CeMDA..92...71T |s2cid=123648472 }}</ref> Levison et al. believe that roughly 200 ejected Jupiter trojans greater than 1&nbsp;km in diameter might be travelling the Solar System, with a few possibly on Earth-crossing orbits.<ref name=Levison1997>{{cite journal|title=Dynamical evolution of Jupiter's Trojan asteroids|author=Levison, Harold F. |author2=Shoemaker, Eugene M. |author3=Shoemaker, Carolyn S.|journal=Nature|volume=385|issue=6611|pages=42–44|date=1997|doi=10.1038/385042a0|bibcode = 1997Natur.385...42L |s2cid=4323757 }}</ref> Some of the escaped Jupiter trojans may become Jupiter-family comets as they approach the Sun and their surface ice begins evaporating.<ref name=Levison1997/>