Editing Nebular hypothesis (section)

=== Rocky planets ===
According to the solar nebular disk model, [[rocky planet]]s form in the inner part of the protoplanetary disk, within the [[frost line (astrophysics)|frost line]], where the temperature is high enough to prevent condensation of water ice and other substances into grains.<ref name=Raymond2007>{{cite journal|last=Raymond|first=Sean N.|author2=Quinn, Thomas |author3=Lunine, Jonathan I. |title=High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability|journal=Astrobiology|volume=7|pages=66–84|date=2007|doi=10.1089/ast.2006.06-0126|bibcode=2007AsBio...7...66R|pmid=17407404|issue=1|arxiv = astro-ph/0510285|s2cid=10257401}}</ref> This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals.{{Refn|group=lower-alpha|The [[planetesimal]]s near the outer edge of the terrestrial planet region—2.5 to 4&nbsp;AU from the Sun—may accumulate some amount of ice. However the rocks will still dominate, like in the [[asteroid belt|outer main belt]] in the Solar System.<ref name=Raymond2007 />}}<ref name=Raymond2007 /> Such conditions are thought to exist in the inner 3–4&nbsp;AU part of the disk of a Sun-like star.<ref name=Montmerle2006 />

After small planetesimals—about 1&nbsp;km in diameter—have formed by one way or another, ''runaway accretion'' begins.<ref name=Kokubo2002 /> It is called runaway because the mass growth rate is proportional to {{nowrap|R<sup>4</sup>~M<sup>4/3</sup>}}, where R and M are the radius and mass of the growing body, respectively.<ref name=Thommes2003 /> The specific (divided by mass) growth accelerates as the mass increases. This leads to the preferential growth of larger bodies at the expense of smaller ones.<ref name=Kokubo2002 /> The runaway accretion lasts between 10,000 and 100,000&nbsp;years and ends when the largest bodies exceed approximately 1,000&nbsp;km in diameter.<ref name=Kokubo2002>{{cite journal|last=Kokubo|first=Eiichiro|author2=Ida, Shigeru|title=Formation of protoplanet systems and diversity of planetary systems|journal=The Astrophysical Journal|volume=581|issue=1|pages=666–680|date=2002| doi=10.1086/344105|bibcode=2002ApJ...581..666K|doi-access=}}</ref> Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals.<ref name=Kokubo2002 /><ref name=Thommes2003 /> In addition, the influence of larger bodies stops further growth of smaller bodies.<ref name=Kokubo2002 />

The next stage is called ''oligarchic accretion''.<ref name=Kokubo2002 /> It is characterized by the dominance of several hundred of the largest bodies—oligarchs, which continue to slowly accrete planetesimals.<ref name=Kokubo2002 /> No body other than the oligarchs can grow.<ref name=Thommes2003 /> At this stage the rate of accretion is proportional to R<sup>2</sup>, which is derived from the geometrical [[Cross section (geometry)|cross-section]] of an oligarch.<ref name=Thommes2003 /> The specific accretion rate is proportional to {{nowrap|M<sup>−1/3</sup>}}; and it declines with the mass of the body. This allows smaller oligarchs to catch up to larger ones. The oligarchs are kept at the distance of about {{nowrap|10·H<sub>r</sub>}} ({{nowrap|H<sub>r</sub>}}={{nowrap|a(1-e)(M/3M<sub>s</sub>)<sup>1/3</sup>}} is the [[Hill radius]], where a is the [[semimajor axis]], e is the [[orbital eccentricity]], and M<sub>s</sub> is the mass of the central star) from each other by the influence of the remaining planetesimals.<ref name=Kokubo2002 /> Their orbital eccentricities and inclinations remain small. The oligarchs continue to accrete until planetesimals are exhausted in the disk around them.<ref name=Kokubo2002 /> Sometimes nearby oligarchs merge. The final mass of an oligarch depends on the distance from the star and surface density of planetesimals and is called the isolation mass.<ref name=Thommes2003 /> For the rocky planets it is up to {{Earth mass|0.1}}, or one [[Mars]] mass.<ref name=Montmerle2006 /> The final result of the oligarchic stage is the formation of about 100 [[Moon]]- to Mars-sized planetary embryos uniformly spaced at about {{nowrap|10·H<sub>r</sub>}}.<ref name=Raymond2006 /> They are thought to reside inside gaps in the disk and to be separated by rings of remaining planetesimals. This stage is thought to last a few hundred thousand years.<ref name=Montmerle2006 /><ref name=Kokubo2002 />

The last stage of rocky planet formation is the ''merger stage''.<ref name=Montmerle2006 /> It begins when only a small number of planetesimals remains and embryos become massive enough to perturb each other, which causes their orbits to become [[Chaos theory|chaotic]].<ref name=Raymond2006>{{cite journal|last=Raymond|first=Sean N.|author2=Quinn, Thomas |author3=Lunine, Jonathan I. |title=High-resolution simulations of the final assembly of earth-like planets 1: terrestrial accretion and dynamics|journal=Icarus|volume=183|issue=2|pages=265–282|date=2006| doi=10.1016/j.icarus.2006.03.011|bibcode=2006Icar..183..265R|arxiv = astro-ph/0510284|s2cid=119069411}}</ref> During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for 10 to 100&nbsp;million years, is the formation of a limited number of Earth-sized bodies. Simulations show that the number of surviving planets is on average from 2 to 5.<ref name=Montmerle2006 /><ref name=Raymond2006 /><ref name=Bottke2005 /><ref name=Petit2001 /> In the Solar System they may be represented by Earth and [[Venus]].<ref name=Raymond2006 /> Formation of both planets required merging of approximately 10–20 embryos, while an equal number of them were thrown out of the Solar System.<ref name=Bottke2005 /> Some of the embryos, which originated in the [[asteroid belt]], are thought to have brought water to Earth.<ref name=Raymond2007 /> Mars and [[Mercury (planet)|Mercury]] may be regarded as remaining embryos that survived that rivalry.<ref name=Bottke2005 /> Rocky planets which have managed to coalesce settle eventually into more or less stable orbits, explaining why planetary systems are generally packed to the limit; or, in other words, why they always appear to be at the brink of instability.<ref name=Raymond2006 />