Editing Nebular hypothesis (section)

=== Giant planets ===
[[File:Fomalhaut Circumstellar Disk.jpg|right|thumb|250px|The dust disk around [[Fomalhaut]]—the brightest star in Piscis Austrinus constellation. Asymmetry of the disk may be caused by a giant planet (or planets) orbiting the star.]]
The formation of [[giant planet]]s is an outstanding problem in the [[planetary science]]s.<ref name=Wurchterl2004 /> In the framework of the solar nebular model two theories for their formation exist. The first one is the ''disk instability model'', where giant planets form in the massive protoplanetary disks as a result of its [[gravity|gravitational]] fragmentation (see above).<ref name=Boss2003 /> The second possibility is the ''core accretion model'', which is also known as the ''nucleated instability model''.<ref name=Wurchterl2004 /><ref name="ddl2011"/> The latter scenario is thought to be the most promising one, because it can explain the formation of the giant planets in relatively low-mass disks (less than {{Solar mass|0.1}}).<ref name=ddl2011 /> In this model giant planet formation is divided into two stages: a) accretion of a core of approximately {{Earth mass|10}} and b) accretion of gas from the protoplanetary disk.<ref name=Montmerle2006 /><ref name=Wurchterl2004 /><ref name=dl2018>{{cite book|last=D'Angelo|first=G.|author2=Lissauer, J. J.|chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D| title=Handbook of Exoplanets |publisher=Springer International Publishing AG, part of Springer Nature| editor=Deeg H., Belmonte J. |pages= 2319–2343|date=2018|arxiv=1806.05649|doi=10.1007/978-3-319-55333-7_140|isbn=978-3-319-55332-0|s2cid=116913980}}</ref> Either method may also lead to the creation of [[brown dwarfs]].<ref name="bodenheimer2013"/><ref name=Janson2011>{{cite journal|last=Janson|first=M.|display-authors=4|author2=Bonavita, M. |author3=Klahr, H. |author4=Lafreniere, D. |author5=Jayawardhana, R. |author6= Zinnecker, H. |title=High-contrast Imaging Search for Planets and Brown Dwarfs around the Most Massive Stars in the Solar Neighborhood|journal=Astrophys. J.|date=2011|volume=736|issue=89|pages=89|arxiv=1105.2577|doi=10.1088/0004-637x/736/2/89 |bibcode = 2011ApJ...736...89J|s2cid=119217803}}</ref> Searches as of 2011 have found that core accretion is likely the dominant formation mechanism.<ref name=Janson2011 />

Giant planet core formation is thought to proceed roughly along the lines of the terrestrial planet formation.<ref name=Kokubo2002 /> It starts with planetesimals that undergo runaway growth, followed by the slower oligarchic stage.<ref name=Thommes2003>{{cite journal|last=Thommes|first=E.W.|author2=Duncan, M.J. |author3=Levison, H.F. |title=Oligarchic growth of giant planets|journal=Icarus|volume=161|issue=2|pages=431–455|date=2003| doi=10.1016/S0019-1035(02)00043-X|bibcode=2003Icar..161..431T|arxiv = astro-ph/0303269|s2cid=16522991}}</ref> Hypotheses do not predict a merger stage, due to the low probability of collisions between planetary embryos in the outer part of planetary systems.<ref name=Thommes2003 /> An additional difference is the composition of the [[planetesimal]]s, which in the case of giant planets form beyond the so-called [[Frost line (astrophysics)|frost line]] and consist mainly of ice—the ice to rock ratio is about 4 to 1.<ref name=Inaba2003 /> This enhances the mass of planetesimals fourfold. However, the minimum mass nebula capable of terrestrial planet formation can only form {{Earth mass|1–2}} cores at the distance of Jupiter (5&nbsp;AU) within 10&nbsp;million years.<ref name=Thommes2003 /> The latter number represents the average lifetime of gaseous disks around Sun-like stars.<ref name=Haisch2001 /> The proposed solutions include enhanced mass of the disk—a tenfold increase would suffice;<ref name=Thommes2003 /> protoplanet migration, which allows the embryo to accrete more planetesimals;<ref name=Inaba2003 /> and finally accretion enhancement due to [[drag (physics)|gas drag]] in the gaseous envelopes of the embryos.<ref name=Inaba2003 /><ref name="dangelo2014"/><ref name=Fortier2007>{{cite journal|last=Fortier|first=A.|author2=Benvenuto, A.G.|title=Oligarchic planetesimal accretion and giant planet formation|journal=Astron. Astrophys.|volume=473|issue=1|pages=311–322|date=2007|doi=10.1051/0004-6361:20066729| bibcode=2007A&A...473..311F|arxiv = 0709.1454|s2cid=14812137}}</ref> Some combination of the above-mentioned ideas may explain the formation of the cores of gas giant planets such as [[Jupiter]] and perhaps even [[Saturn]].<ref name=Wurchterl2004 /> The formation of planets like [[Uranus]] and [[Neptune]] is more problematic, since no theory has been capable of providing for the in situ formation of their cores at the distance of 20–30&nbsp;AU from the central star.<ref name=Montmerle2006 /> One hypothesis is that they initially accreted in the Jupiter-Saturn region, then were scattered and migrated to their present location.<ref name=Thommes1999>{{cite journal|last=Thommes|first=Edward W.|author2=Duncan, Martin J. |author3=Levison, Harold F. |title=The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System|journal=Nature|volume=402|pages=635–638| url=http://www.boulder.swri.edu/~hal/PDF/un-scat_nature.pdf|date=1999|doi=10.1038/45185|pmid=10604469|issue=6762|bibcode = 1999Natur.402..635T|s2cid=4368864}}</ref> Another possible solution is the growth of the cores of the giant planets via [[pebble accretion]]. In pebble accretion objects between a cm and a meter in diameter falling toward a massive body are slowed enough by gas drag for them to spiral toward it and be accreted. Growth via pebble accretion may be as much as 1000 times faster than by the accretion of planetesimals.<ref name=Lambrechts_Johansen_2012>{{cite journal |title=Rapid growth of gas-giant cores by pebble accretion |journal=Astronomy & Astrophysics |last1=Lambrechts |first1=M. |last2=Johansen |first2=A. |volume=544 |page=A32 |date=August 2012 |doi=10.1051/0004-6361/201219127 |bibcode=2012A&A...544A..32L |arxiv=1205.3030|s2cid=53961588 }}</ref>

Once the cores are of sufficient mass ({{Earth mass|5–10}}), they begin to gather gas from the surrounding disk.<ref name=Montmerle2006 /> Initially it is a slow process, increasing the core masses up to {{Earth mass|30}} in a few million years.<ref name=Inaba2003>{{cite journal|last=Inaba |first=S. |author2=Wetherill, G.W. |author3=Ikoma, M. |title=Formation of gas giant planets: core accretion models with fragmentation and planetary envelope |journal=Icarus |volume=166 |issue=1 |pages=46–62 |date=2003 |doi=10.1016/j.icarus.2003.08.001 |url=http://isotope.colorado.edu/~astr5835/Inaba%20et%20al%202003.pdf |bibcode=2003Icar..166...46I |url-status=dead |archive-url=https://web.archive.org/web/20060912185426/http://isotope.colorado.edu/~astr5835/Inaba%20et%20al%202003.pdf |archive-date=2006-09-12 }}</ref><ref name=Fortier2007 /> After that, the accretion rates increase dramatically and the remaining 90% of the mass is accumulated in approximately 10,000&nbsp;years.<ref name=Fortier2007 /> The accretion of gas stops when the supply from the disk is exhausted.<ref name=dl2018/> This happens gradually, due to the formation of a density gap in the protoplanetary disk and to disk dispersal.<ref name=ddl2011 /><ref name=Papaloizou2007>{{cite encyclopedia |last1=Papaloizou |first1=J. C. B. |last2=Nelson |first2=R. P. |last3=Kley |first3=W. |last4=Masset |first4=F. S. |last5=Artymowicz |first5=P. |display-authors=3 |title=Disk-Planet Interactions During Planet Formation |encyclopedia=Protostars and Planets V |date=2007 |publisher=Arizona Press |editor1=Bo Reipurth |editor2=David Jewitt |editor3=Klaus Keil |bibcode=2007prpl.conf..655P |ref=Papaloizou2007 |pages=655|arxiv = astro-ph/0603196 }}</ref> In this model ice giants—Uranus and Neptune—are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post-runaway-gas-accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion.<ref name=Papaloizou2007 /> Migration is caused by the interaction of the planet sitting in the gap with the remaining disk. It stops when the protoplanetary disk disappears or when the end of the disk is attained. The latter case corresponds to the so-called [[hot Jupiters]], which are likely to have stopped their migration when they reached the inner hole in the protoplanetary disk.<ref name=Papaloizou2007 />

During the accretion of gas via streams, a giant planet can be surrounded by a [[circumplanetary disk]]. This circumplanetary disk also carries solids and can form satellites. The [[Galilean moons]] are thought to have formed in such a circumplanetary disk.<ref name="dl2018" />
[[File:Planet formation.jpg|left|thumb|250px|In this artist's conception, a planet spins through a clearing (gap) in a nearby star's dusty, planet-forming disc.]]
Giant planets can significantly influence [[terrestrial planet]] formation. The presence of giants tends to increase [[Orbital eccentricity|eccentricities]] and [[orbital inclination|inclinations]] (see [[Kozai mechanism]]) of planetesimals and embryos in the terrestrial planet region (inside 4&nbsp;AU in the Solar System).<ref name=Bottke2005 /><ref name=Petit2001>{{cite journal|last=Petit|first=Jean-Marc|author2=Morbidelli, Alessandro|title=The Primordial Excitation and Clearing of the Asteroid Belt|journal=Icarus|volume=153|issue=2|pages=338–347|date=2001|doi=10.1006/icar.2001.6702|url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf|bibcode=2001Icar..153..338P|access-date=2008-03-18|archive-date=2007-02-21|archive-url=https://web.archive.org/web/20070221085835/http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf|url-status=dead}}</ref> If giant planets form too early, they can slow or prevent inner planet accretion. If they form near the end of the oligarchic stage, as is thought to have happened in the Solar System, they will influence the merges of planetary embryos, making them more violent.<ref name=Bottke2005 /> As a result, the number of terrestrial planets will decrease and they will be more massive.<ref name=Levinson2003>{{cite journal|last=Levison|first=Harold F.|author2=Agnor, Craig |title=The role of giant planets in terrestrial planet formation |journal=The Astronomical Journal|volume=125|issue=5|pages=2692–2713|date=2003|doi=10.1086/374625|url=http://www.boulder.swri.edu/~hal/PDF/tfakess.pdf|bibcode=2003AJ....125.2692L|s2cid=41888579 }}</ref> In addition, the size of the system will shrink, because terrestrial planets will form closer to the central star. The influence of giant planets in the Solar System, particularly that of [[Jupiter]], is thought to have been limited because they are relatively remote from the terrestrial planets.<ref name=Levinson2003 />

The region of a planetary system adjacent to the giant planets will be influenced in a different way.<ref name=Petit2001 /> In such a region, eccentricities of embryos may become so large that the embryos pass close to a giant planet, which may cause them to be ejected from the system.<ref group=lower-alpha>As a variant they may collide with the central star or a giant planet.</ref><ref name=Bottke2005>{{cite journal|last=Bottke|first=William F.|author2=Durda, Daniel D. |author3=Nesvorny, David |display-authors=etal |title=Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion |journal=Icarus|volume=179|issue=1| pages=63–94|date=2005|doi=10.1016/j.icarus.2005.05.017 |url=http://www.boulder.swri.edu/~bottke/Reprints/Bottke_Icarus_2005_179_63-94_Linking_Collision_Dynamics_MB.pdf|bibcode=2005Icar..179...63B}}</ref><ref name=Petit2001 /> If all embryos are removed, then no planets will form in this region.<ref name=Petit2001 /> An additional consequence is that a huge number of small planetesimals will remain, because giant planets are incapable of clearing them all out without the help of embryos. The total mass of remaining planetesimals will be small, because cumulative action of the embryos before their ejection and giant planets is still strong enough to remove 99% of the small bodies.<ref name=Bottke2005 /> Such a region will eventually evolve into an [[asteroid belt]], which is a full analog of the asteroid belt in the Solar System, located from 2 to 4&nbsp;AU from the Sun.<ref name=Bottke2005 /><ref name=Petit2001 />