Editing Nebular hypothesis

{{short description|Astronomical theory about the Solar System}}
{{Star formation}}
The '''nebular hypothesis''' is the most widely accepted model in the field of [[cosmogony]] to explain the [[formation and evolution of the Solar System]] (as well as other [[planetary system]]s). It suggests the Solar System is formed from gas and dust orbiting the [[Sun]] which clumped up together to form the planets. The theory was developed by [[Immanuel Kant]] and published in his ''[[Universal Natural History and Theory of the Heavens]]'' (1755) and then modified in 1796 by [[Pierre Laplace]]. Originally applied to the [[Solar System]], the process of planetary system formation is now thought to be at work throughout the [[universe]]. The widely accepted modern variant of the nebular theory is the '''solar nebular disk model''' ('''SNDM''') or '''solar nebular model'''.<ref name=Woolfson1993 /> It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular theory are echoed in modern theories of planetary formation, but most elements have been superseded.

According to the nebular theory, stars form in massive and dense clouds of [[molecular hydrogen]]—[[giant molecular cloud]]s (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous [[protoplanetary disk]] ([[proplyd]]) around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1&nbsp;million years to form, with the protoplanetary disk evolving into a planetary system over the next&nbsp;10–100 million years.<ref name=Montmerle2006 />

The protoplanetary disk is an [[accretion disk]] that feeds the central star.<ref name="NYT-20220810">{{cite news |last=Andrews |first=Robin George |title=Astronomers May Have Found the Galaxy's Youngest Planet - The Webb telescope soon will help measure the world, which may offer insights into how our own formed. |url=https://www.nytimes.com/2022/08/10/science/newest-youngest-exoplanet.html |date=10 August 2022 |work=[[The New York Times]] |accessdate=11 August 2022 }}</ref> Initially very hot, the disk later cools in what is known as the [[T Tauri star]] stage; here, formation of small [[dust]] grains made of [[rock (geology)|rocks]] and ice is possible. The grains eventually may coagulate into kilometer-sized [[planetesimal]]s. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000&nbsp;years—formation of Moon- to Mars-sized [[Protoplanet|planetary embryo]]s. Near the star, the planetary embryos go through a stage of violent mergers, producing a few [[terrestrial planet]]s. The last stage takes approximately 100&nbsp;million to a billion years.<ref name=Montmerle2006 />

The formation of [[giant planet]]s is a more complicated process. It is thought to occur beyond the [[Frost line (astrophysics)|frost line]], where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 [[Earth mass]]es—the threshold value, which is necessary to begin accretion of the [[hydrogen]]–[[helium]] gas from the disk.<ref name=dangelo_bodenheimer_2013>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=[[The Astrophysical Journal]]|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D |s2cid=118522228}}</ref> The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses ({{Earth mass|link=y}}) it accelerates and proceeds in a runaway manner. [[Jupiter]]- and [[Saturn]]-like planets are thought to accumulate the bulk of their mass during only 10,000&nbsp;years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. [[Ice giant]]s such as [[Uranus]] and [[Neptune]] are thought to be failed cores, which formed too late when the disk had almost disappeared.<ref name=Montmerle2006 />

== History ==
{{Main|History of Solar System formation and evolution hypotheses}}
There is evidence that [[Emanuel Swedenborg]] first proposed parts of the nebular theory in 1734.<ref name=Swedenborg1734>{{cite book |last= Swedenborg |first= Emanuel |date= 1734|title= (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works)|volume= I}}</ref><ref name="Swenborg">[http://www.newchurchhistory.org/articles/glb2007/baker.pdf Baker, Gregory L. "Emanuel Swenborg – an 18th century cosomologist".] ''The Physics Teacher''. October 1983, pp. 441–446.</ref> [[Immanuel Kant]], familiar with Swedenborg's work, developed the theory further in 1755, publishing his own ''[[Universal Natural History and Theory of the Heavens]]'', wherein he argued that gaseous clouds ([[nebulae]]) slowly rotate, gradually collapse and flatten due to [[gravity]], eventually forming [[star]]s and [[planet]]s.<ref name=Woolfson1993>{{cite journal|last= Woolfson|first= M.M.|title= Solar System&nbsp;– its origin and evolution|journal= Q. J. R. Astron. Soc.|volume= 34| pages= 1–20|date= 1993|bibcode= 1993QJRAS..34....1W}} For details of Kant's position, see Stephen Palmquist, "Kant's Cosmogony Re-Evaluated", ''Studies in History and Philosophy of Science'' 18:3 (September 1987), pp.255–269.</ref>

[[Pierre-Simon Laplace]] independently developed and proposed a similar model in 1796<ref name=Woolfson1993 /> in his ''Exposition du systeme du monde''. He envisioned that the Sun originally had an extended hot atmosphere throughout the volume of the Solar System. His theory featured a contracting and cooling protosolar cloud—the protosolar nebula. As this cooled and contracted, it flattened and spun more rapidly, throwing off (or shedding) a series of gaseous rings of material; and according to him, the planets condensed from this material. His model was similar to Kant's, except more detailed and on a smaller scale.<ref name=Woolfson1993 /> While the Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties. The main problem involved [[angular momentum]] distribution between the Sun and planets. The planets have 99% of the angular momentum, and this fact could not be explained by the nebular model.<ref name=Woolfson1993 /> As a result, astronomers largely abandoned this theory of planet formation at the beginning of the 20th century.

According to some, a major critique came during the 19th century from [[James Clerk Maxwell]] (1831–1879), who in some sources is claimed to have maintained that ''different rotation between the inner and outer parts of a ring'' could not allow condensation of material.<ref>George H. A. Cole (2013). Planetary Science: The Science of Planets around Stars, Second Edition, Michael M. Woolfson, p. 190</ref> However, both the critique and the attribution to Maxwell have been deemed to be incorrect upon further investigation, with the original error being made by [[George Gamow]] in some popular publications and propagated continually ever since.<ref name=CharlesPetzold>{{cite web |first = Charles |last = Petzold |title = Maxwell, Molecules, and Evolution | date = February 2005 |url = https://www.charlespetzold.com/etc/MaxwellMoleculesAndEvolution.html |access-date = 2023-01-03}}</ref> Astronomer [[Sir David Brewster]] also rejected Laplace, writing in 1876 that "those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process". He argued that under such view, "the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere".<ref>Brester, David (1876), "More Worlds Than One: The Creed of the Philosopher and the Hope of the Christian", Chatto and Windus, Piccadilly, p. 153</ref> Brewster claimed that [[Sir Isaac Newton]]'s religious beliefs had previously considered nebular ideas as tending to atheism, and quoted him as saying that "the growth of new systems out of old ones, without the mediation of a Divine power, seemed to him apparently absurd".<ref>As quoted by David Brewster, "More worlds than one : the creed of the philosopher and the hope of the Christian", Fixed stars and binary systems. p. 233</ref>

The perceived deficiencies of the Laplacian model stimulated scientists to find a replacement for it. During the 20th century many theories addressed the issue, including the ''planetesimal theory'' of [[Thomas Chrowder Chamberlin|Thomas Chamberlin]] and [[Forest Ray Moulton|Forest Moulton]] (1901), the ''tidal model'' of [[James Jeans]] (1917), the ''accretion model'' of [[Otto Schmidt]] (1944), the ''protoplanet theory'' of [[William McCrea (astronomer)|William McCrea]] (1960) and finally the ''capture theory'' of [[Michael Woolfson]].<ref name=Woolfson1993 /> In 1978 [[Andrew Prentice]] resurrected the initial Laplacian ideas about planet formation and developed the ''modern Laplacian theory''.<ref name=Woolfson1993 /> None of these attempts proved completely successful, and many of the proposed theories were descriptive.

The birth of the modern widely accepted theory of planetary formation—the solar nebular disk model (SNDM)—can be traced to the Soviet astronomer [[Victor Safronov]].<ref name=NewScientist>{{cite web |last= Henbest |first= Nigel |url= https://www.newscientist.com/channel/solar-system/comets-asteroids/mg13117837.100|title= Birth of the planets: The Earth and its fellow planets may be survivors from a time when planets ricocheted around the Sun like ball bearings on a pinball table|publisher= New Scientist|date= 1991|access-date= 2008-04-18}}</ref> His 1969 book ''Evolution of the protoplanetary cloud and formation of the Earth and the planets'',<ref name=Safronov1972>{{cite book |first= Viktor Sergeevich|last= Safronov|title= Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets|isbn= 978-0-7065-1225-0|date= 1972| publisher=Israel Program for Scientific Translations}}</ref> which was translated to English in 1972, had a long-lasting effect on the way scientists think about the formation of the planets.<ref name=Safronov>{{cite journal |last= Wetherill |first= George W. |bibcode= 1989Metic..24..347W|title= Leonard Medal Citation for Victor Sergeevich Safronov|journal= Meteoritics|date= 1989|volume= 24|issue= 4 |page= 347|doi= 10.1111/j.1945-5100.1989.tb00700.x|doi-access= free}}</ref> In this book almost all major problems of the planetary formation process were formulated and some of them solved. Safronov's ideas were further developed in the works of [[George Wetherill]], who discovered ''[[Accretion (astrophysics)#Runaway accretion|runaway accretion]]''.<ref name=Woolfson1993 /> While originally applied only to the [[Solar System]], the SNDM was subsequently thought by theorists to be at work throughout the Universe; as of {{Extrasolar planet counts|asof}} astronomers have discovered {{Extrasolar planet counts|planet_count}} [[extrasolar planet]]s in our [[galaxy]].<ref name="Encyclopaedia">
{{cite encyclopedia 
|last1= Schneider |first1= Jean 
|date= 10 September 2011 
|title= Interactive Extra-solar Planets Catalog 
|url= https://exoplanet.eu/catalog/
|encyclopedia= [[Extrasolar Planets Encyclopaedia]] |access-date= 2011-09-10
}}
</ref>

== Solar nebular model: achievements and problems ==

=== Achievements ===
[[File:SPHERE images a zoo of dusty discs around young stars.jpg|thumb|Dusty disks surrounding nearby young stars in greater detail.<ref>{{cite web|title=SPHERE Reveals Fascinating Zoo of Discs Around Young Stars|url=https://www.eso.org/public/news/eso1811/|website=www.eso.org|access-date=11 April 2018}}</ref>]]

The star formation process naturally results in the appearance of [[accretion disk]]s around young stellar objects.<ref name=Andre1994 /> At the age of about 1&nbsp;million years, 100% of stars may have such disks.<ref name=Haisch2001 /> This conclusion is supported by the discovery of the gaseous and dusty disks around [[protostar]]s and [[T Tauri star]]s as well as by theoretical considerations.<ref name=Padgett1999 /> Observations of these disks show that the [[dust]] grains inside them grow in size on short (thousand-year) time scales, producing 1&nbsp;centimeter sized particles.<ref name=Kessler-Silacci2006 />

The accretion process, by which 1&nbsp;km [[planetesimal]]s grow into 1,000&nbsp;km sized bodies, is well understood now.<ref name=Kokubo2002 /> This process develops inside any disk where the number density of planetesimals is sufficiently high, and proceeds in a runaway manner. Growth later slows and continues as oligarchic accretion. The end result is formation of [[planetary embryo]]s of varying sizes, which depend on the distance from the star.<ref name=Kokubo2002 /> Various simulations have demonstrated that the merger of embryos in the inner part of the protoplanetary disk leads to the formation of a few Earth-sized bodies. Thus the origin of [[terrestrial planet]]s is now considered to be an almost solved problem.<ref name=Raymond2006 />

=== Current issues ===
The physics of accretion disks encounters some problems.<ref name=Wurchterl2004 /> The most important one is how the material, which is accreted by the protostar, loses its [[angular momentum]]. One possible explanation suggested by [[Hannes Alfvén]] was that angular momentum was shed by the solar wind during its [[T Tauri star]] phase. The momentum is transported to the outer parts of the disk by viscous stresses.<ref name=lynden-bell_1974>{{cite journal|last=Lynden-Bell|first=D.|author2=Pringle, J. E.|title=The evolution of viscous discs and the origin of the nebular variables|journal=[[Monthly Notices of the Royal Astronomical Society]]|date=1974|volume=168|issue=3|pages=603–637|doi=10.1093/mnras/168.3.603|bibcode = 1974MNRAS.168..603L|doi-access=free}}</ref> Viscosity is generated by macroscopic turbulence, but the precise mechanism that produces this turbulence is not well understood. Another possible process for shedding angular momentum is [[magnetic braking (astronomy)|magnetic braking]], where the spin of the star is transferred into the surrounding disk via that star's magnetic field.<ref>{{cite news |first=Terry |last=Devitt |title=What Puts The Brakes On Madly Spinning Stars? |publisher=University of Wisconsin-Madison |date=January 31, 2001 |url=http://www.news.wisc.edu/5732 |access-date=2013-04-09}}</ref> The main processes responsible for the disappearance of the gas in disks are viscous diffusion and photo-evaporation.<ref name=dullemond_2007>{{cite book|last1=Dullemond|first1=C.|last2=Hollenbach|first2=D.|last3=Kamp|first3=I.|last4=D'Alessio|first4=P.|author4-link=Paola D'Alessio|editor1-last=Reipurth|editor1-first=B.|editor2-last=Jewitt|editor2-first=D.|editor3-last=Keil|editor3-first=K.|title=Protostars and Planets V|date=2007|publisher=University of Arizona Press|location=Tucson, AZ|isbn=978-0816526543|pages=555–572|arxiv=astro-ph/0602619|chapter=Models of the Structure and Evolution of Protoplanetary Disks|bibcode = 2007prpl.conf..555D }}</ref><ref name=clarke_2011>{{cite book|last1=Clarke|first1=C.|editor1-last=Garcia|editor1-first=P.|title=Physical Processes in Circumstellar Disks around Young Stars|url=https://archive.org/details/physicalprocesse00garc|url-access=limited|date=2011|publisher=University of Chicago Press|location=Chicago, IL|isbn=9780226282282|pages=[https://archive.org/details/physicalprocesse00garc/page/n369 355]–418|chapter=The Dispersal of Disks around Young Stars}}</ref>

[[File:Worlds with many suns AS 205.tif|thumb|Multiple star system AS 205.<ref>{{cite web |title=Worlds with many suns |url=https://www.eso.org/public/images/potw1906a/ |website=www.eso.org |access-date=11 February 2019 |language=en}}</ref>]]

The formation of planetesimals is the biggest unsolved problem in the nebular disk model. How 1&nbsp;cm sized particles coalesce into 1&nbsp;km planetesimals is a mystery. This mechanism appears to be the key to the question as to why some stars have planets, while others have nothing around them, not even [[debris disk|dust belts]].<ref name=Youdin2002 />

The formation timescale of [[giant planet]]s is also an important problem. Old theories were unable to explain how their cores could form fast enough to accumulate significant amounts of gas from the quickly disappearing protoplanetary disk.<ref name=Kokubo2002 /><ref name=Inaba2003 /> The mean lifetime of the disks, which is less than ten million (10<sup>7</sup>)&nbsp;years, appeared to be shorter than the time necessary for the core formation.<ref name=Haisch2001 /> Much progress has been done to solve this problem and current models of giant planet formation are now capable of forming [[Jupiter]] (or more massive planets) in about 4 million years or less, well within the average lifetime of gaseous disks.<ref name=lhdb2009>{{cite journal|last=Lissauer|first=J. J.|author2=Hubickyj, O. |author3=D'Angelo, G. |author4=Bodenheimer, P. |title=Models of Jupiter's growth incorporating thermal and hydrodynamic constraints| journal=Icarus|year=2009|volume=199|issue=2| pages=338–350|arxiv=0810.5186|doi=10.1016/j.icarus.2008.10.004|bibcode=2009Icar..199..338L|s2cid=18964068}}</ref><ref name=bodenheimer2013>{{cite journal|last=Bodenheimer|first=P.|display-authors=4|author2=D'Angelo, G.|author3=Lissauer, J. J.|author4=Fortney, J. J.|author5=Saumon, D. |title=Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion|journal=The Astrophysical Journal|date=2013|volume=770|issue=2|pages=120 (13 pp.)|doi=10.1088/0004-637X/770/2/120|arxiv = 1305.0980 |bibcode = 2013ApJ...770..120B|s2cid=118553341}}</ref><ref name=dangelo2014>{{cite journal|last=D'Angelo|first=G.|author2=Weidenschilling, S. J. |author3=Lissauer, J. J. |author4=Bodenheimer, P. |title=Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope|journal=Icarus|date=2014|volume=241|pages=298–312|arxiv=1405.7305|doi=10.1016/j.icarus.2014.06.029|bibcode=2014Icar..241..298D|s2cid=118572605}}</ref>

Another potential problem of giant planet formation is their [[Planetary migration|orbital migration]]. Some calculations show that interaction with the disk can cause rapid inward migration, which, if not stopped, results in the planet reaching the "central regions still as a sub-[[Jupiter mass|Jovian]] object."<ref>[[#Papaloizou2007|Papaloizou 2007]] page 10</ref> More recent calculations indicate that disk evolution during migration can mitigate this problem.<ref name=ddl2011>{{cite book|last=D'Angelo|first=G.|author2=Durisen, R. H. |author3=Lissauer, J. J.|chapter=Giant Planet Formation |bibcode=2010exop.book..319D| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=319–346|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486}}</ref>

== Formation of stars and protoplanetary disks ==

=== Protostars ===
{{Main|Protostar}}
[[File:Ssc2005-02b.jpg|right|thumb|300px|The visible-light (left) and infrared (right) views of the [[Trifid Nebula]]—a giant star-forming cloud of gas and dust located 5,400&nbsp;light-years away in the constellation Sagittarius]]
[[Star]]s are thought to form inside [[molecular cloud|giant clouds]] of cold [[molecular hydrogen]]—[[giant molecular cloud]]s roughly 300,000&nbsp;times the mass of the Sun ({{Solar mass|link=y}}) and 20&nbsp;[[parsec]]s in diameter.<ref name=Montmerle2006>{{cite journal|last=Montmerle|first=Thierry|author2=Augereau, Jean-Charles |author3=Chaussidon, Marc |display-authors=etal |title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|issue=1–4|pages=39–95|date=2006|doi=10.1007/s11038-006-9087-5| bibcode=2006EM&P...98...39M|s2cid=120504344}}</ref><ref name=Pudritz2002>{{cite journal|last=Pudritz|first=Ralph E.|title=Clustered Star Formation and the Origin of Stellar Masses|journal=Science|volume=295| pages=68–75|date=2002|doi=10.1126/science.1068298|url=http://www.sciencemag.org/cgi/content/full/295/5552/68|pmid=11778037|issue=5552|bibcode = 2002Sci...295...68P|s2cid=33585808}}</ref> Over millions of years, giant molecular clouds are prone to [[gravitational collapse|collapse]] and fragmentation.<ref name=Clark2005>{{cite journal|last=Clark|first=Paul C.|author2=Bonnell, Ian A.|title=The onset of collapse in turbulently supported molecular clouds|journal=Mon. Not. R. Astron. Soc.|volume=361|issue=1| pages=2–16|date=2005|doi=10.1111/j.1365-2966.2005.09105.x|bibcode=2005MNRAS.361....2C|doi-access=free}}</ref> These fragments then form small, dense cores, which in turn collapse into stars.<ref name=Pudritz2002 /> The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae.<ref name=Montmerle2006 /> They possess diameters of 0.01–0.1&nbsp;pc (2,000–20,000&nbsp;AU) and a [[particle number density]] of roughly 10,000 to 100,000&nbsp;cm<sup>−3</sup>.<ref group=lower-alpha>Compare it with the particle number density of the air at the sea level—{{val|2.8|e=19|u=cm<sup>−3</sup>}}.</ref><ref name=Pudritz2002 /><ref name=Motte1998>{{cite journal|last1=Motte|first1=F.|last2=Andre|first2=P.|last3=Neri|first3=R.|title=The initial conditions of star formation in the ρ Ophiuchi main cloud: wide-field millimeter continuum mapping|journal=Astron. Astrophys.|volume=336|pages=150–172|date=1998| bibcode=1998A&A...336..150M}}</ref>

The initial collapse of a solar-mass protostellar nebula takes around 100,000&nbsp;years.<ref name=Montmerle2006 /><ref name=Pudritz2002 /> Every nebula begins with a certain amount of [[angular momentum]]. Gas in the central part of the nebula, with relatively low angular momentum, undergoes fast compression and forms a hot [[hydrostatic]] (not contracting) core containing a small fraction of the mass of the original nebula.<ref name=Stahler1980 /> This core forms the seed of what will become a star.<ref name=Montmerle2006 /><ref name=Stahler1980 /> As the collapse continues, conservation of angular momentum means that the rotation of the infalling envelope accelerates,<ref name=Nakamoto1995 /><ref name=Yorke1999 /> which largely prevents the gas from directly [[accretion (astrophysics)|accreting]] onto the central core. The gas is instead forced to spread outwards near its equatorial plane, forming a [[accretion disk|disk]], which in turn accretes onto the core.<ref name=Montmerle2006 /><ref name=Nakamoto1995>{{cite journal|last=Nakamoto|first=Taishi|author2=Nakagawa, Yushitsugu|title=Formation, early evolution, and gravitational stability of protoplanetary disks|journal=The Astrophysical Journal|volume=421|pages=640–650|date=1994|doi=10.1086/173678| bibcode=1994ApJ...421..640N|doi-access=}}</ref><ref name=Yorke1999>{{cite journal|last=Yorke|first=Harold W.|author2=Bodenheimer, Peter|title=The formation of protostellar disks. III. The influence of gravitationally induced angular momentum transport on disk structure and appearance|journal=The Astrophysical Journal|volume=525|issue=1|pages=330–342|date=1999|doi=10.1086/307867| bibcode=1999ApJ...525..330Y|doi-access=free}}</ref> The core gradually grows in mass until it becomes a young hot [[protostar]].<ref name=Stahler1980 /> At this stage, the protostar and its disk are heavily obscured by the infalling envelope and are not directly observable.<ref name=Andre1994 /> In fact the remaining envelope's [[opacity (optics)|opacity]] is so high that even [[millimeter-wave]] radiation has trouble escaping from inside it.<ref name=Montmerle2006 /><ref name=Andre1994 /> Such objects are observed as very bright condensations, which emit mainly millimeter-wave and [[Terahertz radiation|submillimeter-wave]] radiation.<ref name=Motte1998 /> They are classified as spectral Class&nbsp;0 protostars.<ref name=Andre1994>{{cite journal|last=Andre|first=Philippe|author2=Montmerle, Thierry|title=From T Tauri stars protostars: circumstellar material and young stellar objects in the ρ Ophiuchi cloud|journal=The Astrophysical Journal|volume=420| pages=837–862|date=1994|doi=10.1086/173608| bibcode=1994ApJ...420..837A|doi-access=free}}</ref> The collapse is often accompanied by [[bipolar outflow]]s—[[jet (gas)|jets]]—that emanate along the [[rotation]]al axis of the inferred disk. The jets are frequently observed in star-forming regions (see [[Herbig–Haro object|Herbig–Haro (HH) objects]]).<ref name=Lee2000>{{cite journal|last=Lee|first=Chin-Fei|author2=Mundy, Lee G. |author3=Reipurth, Bo |display-authors=etal |title=CO outflows from young stars: confronting the jet and wind models|journal=The Astrophysical Journal|volume=542|issue=2|pages=925–945|date=2000|doi=10.1086/317056|bibcode=2000ApJ...542..925L|s2cid=118351543 |doi-access=free}}</ref> The luminosity of the Class&nbsp;0 protostars is high&nbsp;— a solar-mass protostar may radiate at up to 100 solar luminosities.<ref name=Andre1994 /> The source of this energy is [[gravitational collapse]], as their cores are not yet hot enough to begin [[nuclear fusion]].<ref name=Stahler1980>{{cite journal|last=Stahler|first=Steven W.|author2=Shu, Frank H. |author3=Taam, Ronald E. |title=The evolution of protostars: II The hydrostatic core|journal=The Astrophysical Journal|volume=242|pages=226–241|date=1980|bibcode=1980ApJ...242..226S|doi=10.1086/158459|doi-access=free}}</ref><ref name=Stahler1988 />

[[File:Embedded Outflow in Herbig-Haro object HH 46 47.jpg|left|thumb|250px|Infrared image of the molecular outflow from an otherwise hidden newborn star HH 46/47]]
As the infall of its material onto the disk continues, the envelope eventually becomes thin and transparent and the [[young stellar object]] (YSO) becomes observable, initially in [[far-infrared]] light and later in the visible.<ref name=Motte1998 /> Around this time the protostar begins to [[nuclear fusion|fuse]] [[deuterium]]. If the protostar is sufficiently massive (above 80 Jupiter masses ({{Jupiter mass|link=y}})), hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a [[brown dwarf]].<ref name=Stahler1988>{{cite journal|last=Stahler|first=Steven W.|title=Deuterium and the Stellar Birthline|journal=The Astrophysical Journal|volume=332|pages=804–825|date=1988|bibcode=1988ApJ...332..804S|doi=10.1086/166694}}</ref> This birth of a new star occurs approximately 100,000&nbsp;years after the collapse begins.<ref name=Montmerle2006 /> Objects at this stage are known as Class I protostars,<ref name=Andre1994 /> which are also called young [[T Tauri star]]s, evolved protostars, or young stellar objects.<ref name=Andre1994 /> By this time the forming star has already accreted much of its mass: the total mass of the disk and remaining envelope does not exceed 10–20% of the mass of the central YSO.<ref name=Motte1998 />

At the next stage the envelope completely disappears, having been gathered up by the disk, and the protostar becomes a classical T Tauri star.{{Refn|group=lower-alpha|The T Tauri stars are young stars with mass less than about {{Solar mass|2.5}} showing a heightened level of activity. They are divided into two classes: weakly lined and classical T Tauri stars.<ref name=Mohanty2005>{{cite journal|last=Mohanty|first=Subhanjoy|author2=Jayawardhana, Ray |author3=Basri, Gibor |title=The T Tauri Phase down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs|journal=The Astrophysical Journal|volume=626|issue=1|pages=498–522|date=2005|doi=10.1086/429794|bibcode=2005ApJ...626..498M|arxiv = astro-ph/0502155|s2cid=8462683}}</ref> The latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars.<ref name=Martin1994>{{cite journal |last1=Martin |first1=E. L. |last2=Rebolo |first2=R. |last3=Magazzu |first3=A. |last4=Pavlenko |first4=Ya. V. |title=Pre-main sequence lithium burning |journal=Astron. Astrophys. |volume=282 |pages=503–517 |date=1994 |bibcode=1994A&A...282..503M |arxiv=astro-ph/9308047}}</ref>}} This happens after about 1&nbsp;million years.<ref name=Montmerle2006 /> The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, and it is accreted at a rate of 10<sup>−7</sup> to {{Solar mass|10<sup>−9</sup>}} per year.<ref name=Hartmann1998>{{cite journal|last=Hartmann|first=Lee|author2=Calvet, Nuria|author2-link=Nuria Calvet|author3=Gullbring, Eric|author4=D’Alessio, Paula|author4-link=Paola D'Alessio|title=Accretion and the evolution of T Tauri disks|journal=The Astrophysical Journal|volume=495|issue=1| pages=385–400|date=1998|doi=10.1086/305277|bibcode=1998ApJ...495..385H|doi-access=free}}</ref> A pair of bipolar jets is usually present as well.<ref name=Shu1997 /> The accretion explains all peculiar properties of classical T Tauri stars: strong [[flux]] in the [[emission line]]s (up to 100% of the intrinsic [[luminosity]] of the star), [[magnetic]] activity, [[photometry (astronomy)|photometric]] [[variable star|variability]] and jets.<ref name=Muzerolle2001>{{cite journal |last1=Muzerolle |first1=James |last2=Calvet |first2=Nuria|author2-link=Nuria Calvet |last3=Hartmann |first3=Lee |title= Emission-line diagnostics of T Tauri magnetospheric accretion. II. Improved model tests and insights into accretion physics |journal=The Astrophysical Journal |volume=550 |issue=2 |pages=944–961 |date=2001 |doi=10.1086/319779 |bibcode=2001ApJ...550..944M|doi-access=free }}</ref> The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its [[Poles of astronomical bodies#Magnetic poles|magnetic poles]].<ref name=Muzerolle2001 /> The jets are byproducts of accretion: they carry away excessive angular momentum. The classical T Tauri stage lasts about 10&nbsp;million years.<ref name=Montmerle2006 /> The disk eventually disappears due to accretion onto the central star, planet formation, ejection by jets and [[photoevaporation]] by UV-radiation from the central star and nearby stars.<ref name=Adams2004>{{cite journal|last=Adams|first=Fred C.|author2=Hollenbach, David |author3=Laughlin, Gregory |author4= Gorti, Uma |title=Photoevaporation of circumstellar disks due to external far-ultraviolet radiation in stellar aggregates|journal=The Astrophysical Journal|volume=611|issue=1|pages=360–379|date=2004|doi=10.1086/421989| bibcode=2004ApJ...611..360A|arxiv = astro-ph/0404383|s2cid=16093937}}</ref> As a result, the young star becomes a [[Weak-lined T Tauri star|weakly lined T Tauri star]], which slowly, over hundreds of millions of years, evolves into an ordinary Sun-like star.<ref name=Stahler1980 />

=== Protoplanetary disks ===
{{See also|Protoplanetary disk|planetesimal}}
[[File:NASA-14114-HubbleSpaceTelescope-DebrisDisks-20140424.jpg|thumb|right|250px|[[Debris disks]] detected in [[Hubble Space Telescope|HST]] archival images of young stars, HD 141943 and HD 191089, using improved imaging processes (24 April 2014).<ref name="NASA-20140424">{{cite web |last1=Harrington |first1=J.D. |last2=Villard |first2=Ray |title=RELEASE 14–114 Astronomical Forensics Uncover Planetary Disks in NASA's Hubble Archive |url=http://www.nasa.gov/press/2014/april/astronomical-forensics-uncover-planetary-disks-in-nasas-hubble-archive |date=24 April 2014 |work=[[NASA]] |url-status=live |archive-date=2014-04-25 |archive-url=https://web.archive.org/web/20140425125432/http://www.nasa.gov/press/2014/april/astronomical-forensics-uncover-planetary-disks-in-nasas-hubble-archive/ |access-date=2014-04-25}}</ref>]]
Under certain circumstances the disk, which can now be called protoplanetary, may give birth to a [[planetary system]].<ref name=Montmerle2006 /> Protoplanetary disks have been observed around a very high fraction of stars in young [[star clusters]].<ref name=Haisch2001>{{cite journal|last=Haisch|first=Karl E.|author2=Lada, Elizabeth A. |author3=Lada, Charles J. |title=Disk frequencies and lifetimes in young clusters|journal=The Astrophysical Journal|volume=553|issue=2|pages=L153–L156|date=2001|doi=10.1086/320685| bibcode=2001ApJ...553L.153H|arxiv = astro-ph/0104347|s2cid=16480998}}</ref><ref name=Megeath2005>{{cite journal|last=Megeath|first=S.T.|author2=Hartmann, L. |author3=Luhmann, K.L. |author4= Fazio, G.G. |title=Spitzer/IRAC photometry of the ρ Chameleontis association|journal=The Astrophysical Journal|volume=634|issue=1|pages=L113–L116|date=2005|doi=10.1086/498503| bibcode=2005ApJ...634L.113M|arxiv = astro-ph/0511314|s2cid=119007015}}</ref> They exist from the beginning of a star's formation, but at the earliest stages are unobservable due to the [[Opacity (optics)|opacity]] of the surrounding envelope.<ref name=Andre1994 /> The disk of a Class&nbsp;0 [[protostar]] is thought to be massive and hot. It is an [[accretion (astrophysics)|accretion disk]], which feeds the central protostar.<ref name=Nakamoto1995 /><ref name=Yorke1999 /> The temperature can easily exceed 400&nbsp;[[Kelvin|K]] inside 5&nbsp;AU and 1,000&nbsp;K inside 1&nbsp;AU.<ref name=Chick1997>{{cite journal|last=Chick|first=Kenneth M.|author2=Cassen, Patrick|title=Thermal processing of interstellar dust grains in the primitive solar environment|journal=The Astrophysical Journal|volume=477|issue=1|pages=398–409|date=1997|doi=10.1086/303700|bibcode=1997ApJ...477..398C|doi-access=free}}</ref> The heating of the disk is primarily caused by the [[viscosity|viscous]] [[dissipation]] of [[turbulence]] in it and by the infall of the gas from the nebula.<ref name=Nakamoto1995 /><ref name=Yorke1999 /> The high [[temperature]] in the inner disk causes most of the [[Volatile (astrogeology)|volatile]] material—water, organics, and some [[rock (geology)|rocks]]—to evaporate, leaving only the most [[refractory]] elements like [[iron]]. The ice can survive only in the outer part of the disk.<ref name=Chick1997 />
[[File:M42proplyds.jpg|thumb|left|250px|A protoplanetary disk forming in the [[Orion Nebula]]]]
The main problem in the physics of accretion disks is the generation of turbulence and the mechanism responsible for the high [[viscosity|effective viscosity]].<ref name=Montmerle2006 /> The turbulent viscosity is thought to be responsible for the [[transport phenomena|transport]] of the mass to the central protostar and momentum to the periphery of the disk. This is vital for accretion, because the gas can be accreted by the central protostar only if it loses most of its angular momentum, which must be carried away by the small part of the gas drifting outwards.<ref name=Nakamoto1995 /><ref name=Klahr2003>{{cite journal|last=Klahr|first=H.H.|author2=Bodenheimer, P.|title=Turbulence in accretion disks: vorticity generation and angular momentum transport via the global baroclinic instability|journal=The Astrophysical Journal|volume=582|issue=2|pages=869–892| date=2003|doi=10.1086/344743|bibcode=2003ApJ...582..869K|arxiv = astro-ph/0211629|s2cid=119362731}}</ref> The result of this process is the growth of both the protostar and of the disk [[radius]], which can reach 1,000&nbsp;AU if the initial angular momentum of the nebula is large enough.<ref name=Yorke1999 /> Large disks are routinely observed in many star-forming regions such as the [[Orion nebula]].<ref name=Padgett1999>{{cite journal|last=Padgett|first=Deborah L.|author2=Brandner, Wolfgang|author3=Stapelfeldt, Karl L.|display-authors=etal|title=Hubble space telescope/nicmos imaging of disks and envelopes around very young stars|journal=The Astronomical Journal|volume=117|issue=3|pages=1490–1504|date=1999|doi=10.1086/300781| bibcode=1999AJ....117.1490P|arxiv = astro-ph/9902101|s2cid=16498360}}</ref>
[[File:Artist’s impression of the disc and gas streams around HD 142527 (Animation).ogv|thumb|300px|Artist's impression of the disc and gas streams around young star [[HD 142527]].<ref>{{cite news|title=ALMA Sheds Light on Planet-Forming Gas Streams|url=http://www.eso.org/public/news/eso1301/|access-date=10 January 2013|newspaper=ESO Press Release}}</ref> ]]
The lifespan of the accretion disks is about 10&nbsp;million&nbsp;years.<ref name=Haisch2001 /> By the time the star reaches the classical T-Tauri stage, the disk becomes thinner and cools.<ref name=Hartmann1998 /> Less volatile materials start to [[condensation|condense]] close to its center, forming 0.1–1&nbsp;μm dust grains that contain [[crystalline]] [[silicate]]s.<ref name=Kessler-Silacci2006 /> The transport of the material from the outer disk can mix these newly formed [[cosmic dust|dust grains]] with [[primordial elements|primordial]] ones, which contain organic matter and other volatiles. This mixing can explain some peculiarities in the composition of Solar System bodies such as the presence of [[interstellar dust|interstellar]] grains in primitive [[meteorite]]s and refractory inclusions in comets.<ref name=Chick1997 />
[[File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg|thumb|350px|left|Various [[planet formation]] processes, including [[exocomets]] and other [[planetesimal]]s, around [[Beta Pictoris]], a very young type [[A V star]] ([[NASA]] artist's conception).]]
Dust particles tend to stick to each other in the dense disk environment, leading to the formation of larger particles up to several centimeters in size.<ref name=Michikoshi2006>{{cite journal|last=Michikoshi|first=Shugo|author2=Inutsuka, Shu-ichiro|title=A two-fluid analysis of the kelvin-helmholtz instability in the dusty layer of a protoplanetary disk: a possible path toward planetesimal formation through gravitational instability|journal=The Astrophysical Journal|volume=641|issue=2|pages=1131–1147|date=2006|doi=10.1086/499799| bibcode=2006ApJ...641.1131M|arxiv=astro-ph/0412643|s2cid=15477674}}</ref> The signatures of the dust processing and [[coagulation]] are observed in the infrared spectra of the young disks.<ref name=Kessler-Silacci2006>{{cite journal|last=Kessler-Silacci|first=Jacqueline|author2=Augereau, Jean-Charles|author3=Dullemond, Cornelis P.|display-authors=etal|title= c2d SPITZER IRS spectra of disks around T Tauri stars. I. Silicate emission and grain growth |journal=The Astrophysical Journal|volume=639|issue=3|pages=275–291|date=2006|doi=10.1086/499330 |arxiv = astro-ph/0511092 |bibcode = 2006ApJ...639..275K|s2cid=118938125}}</ref> Further aggregation can lead to the formation of [[planetesimal]]s measuring 1&nbsp;km across or larger, which are the building blocks of [[planet]]s.<ref name=Montmerle2006 /><ref name=Michikoshi2006 /> Planetesimal formation is another unsolved problem of disk physics, as simple sticking becomes ineffective as dust particles grow larger.<ref name=Youdin2002 />

One hypothesis is formation by [[Jeans instability|gravitational instability]]. Particles several centimeters in size or larger slowly settle near the middle plane of the disk, forming a very thin—less than 100&nbsp;km—and dense layer. This layer is gravitationally unstable and may fragment into numerous clumps, which in turn collapse into planetesimals.<ref name=Montmerle2006 /><ref name=Youdin2002>{{cite journal|last=Youdin|first=Andrew N.|author2=Shu, Frank N.|title=Planetesimal formation by gravitational instability|journal=The Astrophysical Journal|volume=580|issue=1|pages=494–505|date=2002|doi=10.1086/343109| bibcode=2002ApJ...580..494Y|arxiv = astro-ph/0207536|s2cid=299829}}</ref> However, the differing velocities of the gas disk and the solids near the mid-plane can generate turbulence which prevents the layer from becoming thin enough to fragment due to gravitational instability.<ref name="Johansen_etal_2006">{{cite journal|last1=Johansen|first1=Anders|last2=Henning|first2=Thomas|last3=Klahr|first3=Hubert|title=Dust Sedimentation and Self-sustained Kelvin-Helmholtz Turbulence in Protoplanetary Disk Midplanes|journal=The Astrophysical Journal|date=2006|volume=643|issue=2|pages=1219–1232|doi=10.1086/502968|arxiv=astro-ph/0512272|bibcode = 2006ApJ...643.1219J|s2cid=15999094}}</ref> This may limit the formation of planetesimals via gravitational instabilities to specific locations in the disk where the concentration of solids is enhanced.<ref name="Protostars_and Planets_2014">{{cite book |last1=Johansen |first1=A. |last2=Blum |first2=J. |last3=Tanaka |first3=H. |last4=Ormel |first4=C. |last5=Bizzarro |first5=M. |last6=Rickman |first6=H. |title=Protostars and Planets VI |date=2014 |chapter=The Multifaceted Planetesimal Formation Process |editor1-last=Beuther |editor1-first=H. |editor2-last=Klessen |editor2-first=R. S. |editor3-last=Dullemond |editor3-first=C. P. |editor4-last=Henning |editor4-first=T. |pages=547–570 |publisher=University of Arizona Press |arxiv=1402.1344 |bibcode=2014prpl.conf..547J |doi=10.2458/azu_uapress_9780816531240-ch024 |isbn=978-0-8165-3124-0|s2cid=119300087 }}</ref>

Another possible mechanism for the formation of planetesimals is the [[streaming instability]] in which the drag felt by particles orbiting through gas creates a feedback effect causing the growth of local concentrations. These local concentrations push back on the gas creating a region where the headwind felt by the particles is smaller. The concentration is thus able to orbit faster and undergoes less radial drift. Isolated particles join these concentrations as they are overtaken or as they drift inward causing it to grow in mass. Eventually these concentrations form massive filaments which fragment and undergo gravitational collapse forming planetesimals the size of the larger asteroids.<ref name=Johansen_Jacquet_2015>{{cite book |last1=Johansen |first1=A. |last2=Jacquet |first2=E. |last3=Cuzzi |first3=J. N. |last4=Morbidelli |first4=A. |last5=Gounelle |first5=M. |date=2015 |chapter=New Paradigms For Asteroid Formation |editor1-last=Michel |editor1-first=P. |editor2-last=DeMeo |editor2-first=F. |editor3-last=Bottke |editor3-first=W. |title=Asteroids IV |pages=471 |publisher=University of Arizona Press |series=Space Science Series |arxiv=1505.02941 |bibcode=2015aste.book..471J |isbn=978-0-8165-3213-1|doi=10.2458/azu_uapress_9780816532131-ch025 |s2cid=118709894 }}</ref>

Planetary formation can also be triggered by gravitational instability within the disk itself, which leads to its fragmentation into clumps. Some of them, if they are dense enough, will [[gravitational collapse|collapse]],<ref name=Klahr2003 /> which can lead to rapid formation of [[gas giant]] planets and even [[brown dwarf]]s on the timescale of 1,000&nbsp;years.<ref name=Boss2003>{{cite journal|last=Boss|first=Alan P.|title=Rapid formation of outer giant planets by disk instability|journal=The Astrophysical Journal|volume=599|issue=1|pages=577–581|date=2003|doi=10.1086/379163|bibcode=2003ApJ...599..577B|doi-access=free}}</ref> If these clumps migrate inward as the collapse proceeds tidal forces from the star can result in a significant [[tidal downsizing|mass loss]] leaving behind a smaller body.<ref name=Nayaksin_2010>{{cite journal|last1=Nayakshin|first1=Sergie|title=Formation of planets by tidal downsizing of giant planet embryos|journal=Monthly Notices of the Royal Astronomical Society Letters |date=2010|volume=408|issue=1|page=L36–L40|doi=10.1111/j.1745-3933.2010.00923.x|doi-access=free |arxiv=1007.4159|bibcode=2010MNRAS.408L..36N|s2cid=53409577}}</ref> However it is only possible in massive disks—more massive than {{Solar mass|0.3}}. In comparison, typical disk masses are {{Solar mass|0.01–0.03}}. Because the massive disks are rare, this mechanism of planet formation is thought to be infrequent.<ref name=Montmerle2006 /><ref name=Wurchterl2004>{{cite encyclopedia|last=Wurchterl|first=G.|title=Planet Formation Towards Estimating Galactic Habitability|encyclopedia=Astrobiology:Future Perspectives|date=2004|publisher=Kluwer Academic Publishers|editor=P. Ehrenfreund |display-editors=etal|pages=67–96|isbn=9781402023040 |doi=10.1007/1-4020-2305-7|chapter=Planet Formation|series=Astrophysics and Space Science Library|chapter-url=https://cds.cern.ch/record/1338806}}</ref> On the other hand, it may play a major role in the formation of [[brown dwarf]]s.<ref name=Stamatellosetal2007>{{cite journal|last=Stamatellos|first=Dimitris|author2=Hubber, David A. |author3=Whitworth, Anthony P. |title=Brown dwarf formation by gravitational fragmentation of massive, extended protostellar discs|journal=[[Monthly Notices of the Royal Astronomical Society Letters]]|volume=382|issue=1|pages=L30–L34|date=2007|doi=10.1111/j.1745-3933.2007.00383.x |doi-access=free |bibcode = 2007MNRAS.382L..30S |arxiv = 0708.2827|s2cid=17139868}}</ref>
[[File:PIA18469-AsteroidCollision-NearStarNGC2547-ID8-2013.jpg|thumb|right|300px|Asteroid collision—building planets (artist concept).]]
The ultimate [[dissipation]] of protoplanetary disks is triggered by a number of different mechanisms. The inner part of the disk is either accreted by the star or ejected by the [[bipolar outflow|bipolar jets]],<ref name=Hartmann1998 /><ref name=Shu1997>{{cite journal|last=Shu|first=Frank H.|author2=Shang, Hsian |author3=Glassgold, Alfred E. |author4= Lee, Typhoon |title=X-rays and Fluctuating X-Winds from Protostars|journal=Science |volume=277|issue=5331|pages=1475–1479|date=1997|doi=10.1126/science.277.5331.1475 |url=http://www.sciencemag.org/cgi/content/full/277/5331/1475|bibcode = 1997Sci...277.1475S}}</ref> whereas the outer part can [[photoevaporation|evaporate]] under the star's powerful [[ultraviolet|UV]] [[radiation]] during the T Tauri stage<ref name=Font2004>{{cite journal|last=Font|first=Andreea S.|author2=McCarthy, Ian G. |author3=Johnstone, Doug |author4= Ballantyne, David R. |title=Photoevaporation of circumstellar disks around young stars|journal=The Astrophysical Journal|volume=607|issue=2|pages=890–903|date=2004|doi=10.1086/383518| bibcode=2004ApJ...607..890F|arxiv = astro-ph/0402241|s2cid=15928892}}</ref> or by nearby stars.<ref name=Adams2004 /> The gas in the central part can either be accreted or ejected by the growing planets, while the small dust particles are ejected by the [[radiation pressure]] of the central star. What is finally left is either a planetary system, a remnant disk of dust without planets, or nothing, if planetesimals failed to form.<ref name=Montmerle2006 />

Because planetesimals are so numerous, and spread throughout the protoplanetary disk, some survive the formation of a planetary system. [[Asteroid]]s are understood to be left-over planetesimals, gradually grinding each other down into smaller and smaller bits, while comets are typically planetesimals from the farther reaches of a planetary system. Meteorites are samples of planetesimals that reach a planetary surface, and provide a great deal of information about the formation of the Solar System. Primitive-type meteorites are chunks of shattered low-mass planetesimals, where no thermal [[Planetary differentiation|differentiation]] took place, while processed-type meteorites are chunks from shattered massive planetesimals.<ref name=Bottke2005 /> Interstellar objects could have been captured, and become part of the young Solar system.<ref>{{Cite journal|last1=Grishin|first1=Evgeni|last2=Perets|first2=Hagai B.|last3=Avni|first3=Yael|date=2019-08-11|title=Planet seeding through gas-assisted capture of interstellar objects|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=487|issue=3|pages=3324–3332|doi=10.1093/mnras/stz1505|doi-access=free |issn=0035-8711|arxiv=1804.09716|s2cid=119066860}}</ref>

== Formation of planets{{Anchor|Formation of planets}} ==

=== 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 />

=== 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 />

===Exoplanets===

Thousands of exoplanets have been identified in the last twenty years, with, at the very least, billions more, within our observable universe, yet to be discovered.<ref>{{Cite web|url=https://scitechdaily.com/are-we-alone-discovery-of-billions-of-earth-like-planets-may-hold-the-answer/|title=Are We Alone? Discovery of Billions of Earth-Like Planets May Hold the Answer|date=July 8, 2020|website=SciTechDaily}}</ref> The orbits of many of these planets and systems of planets differ significantly from the planets in the Solar System. The exoplanets discovered include hot-Jupiters, warm-Jupiters, super-Earths, and systems of tightly packed inner planets.

The hot-Jupiters and warm-Jupiters are thought to have migrated to their current orbits during or following their formation. A number of possible mechanisms for this migration have been proposed. Type I or Type II migration could smoothly decrease the semimajor axis of the planet's orbit resulting in a warm- or hot-Jupiter. Gravitational scattering by other planets onto eccentric orbits with a perihelion near the star followed by the circularization of its orbit due to tidal interactions with the star can leave a planet on a close orbit. If a massive companion planet or star on an inclined orbit was present an exchange of inclination for eccentricity via the [[Kozai mechanism]] raising eccentricities and lowering perihelion followed by circularization can also result in a close orbit. Many of the Jupiter-sized planets have eccentric orbits which may indicate that gravitational encounters occurred between the planets, although migration while in resonance can also excite eccentricities.<ref name="Baruteau_etal_2014">{{cite book|last1=Baruteau|first1=C.|last2=Crida|first2=A.|last3=Paardekooper|first3=S.-J.|last4=Masset|first4=F.|last5=Guilet|first5=J.|last6=Bitsch|first6=B.|last7=Nelson|first7=R.|last8=Kley|first8=W.|last9=Papaloizou|first9=J.|chapter=Planet-Disk Interactions and Early Evolution of Planetary Systems|title=Protostars and Planets VI|date=2014|pages=667–689|arxiv=1312.4293|bibcode=2014prpl.conf..667B|doi=10.2458/azu_uapress_9780816531240-ch029|isbn=9780816531240|s2cid=67790867}}</ref> The in situ growth of hot Jupiters from closely orbiting super Earths has also been proposed. The cores in this hypothesis could have formed locally or at a greater distance and migrated close to the star.<ref name="Batygin_etal_2016">{{cite journal|last1=Batygin|first1=Konstantin|last2=Bodenheimer|first2=Peter H.|last3=Laughlin|first3=Gregory P.|title=In Situ Formation and Dynamical Evolution of Hot Jupiter Systems|journal=The Astrophysical Journal|date=2016|volume=829|issue=2|page=114|doi=10.3847/0004-637X/829/2/114|arxiv=1511.09157|bibcode = 2016ApJ...829..114B |s2cid=25105765 |doi-access=free }}</ref>

Super-Earths and other closely orbiting planets are thought to have either formed in situ or ex situ, that is, to have migrated inward from their initial locations.<ref name=dangelo_bodenheimer_2016>{{Cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |s2cid=119203398|title=In Situ and Ex Situ Formation Models of Kepler 11 Planets|journal=The Astrophysical Journal|year=2016|volume=828|issue=1|pages=id. 33 (32 pp.)|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D |doi-access=free }}</ref>  The in situ formation of closely orbiting super-Earths would require a massive disk, the migration of planetary embryos followed by collisions and mergers, or the radial drift of small solids from farther out in the disk. The migration of the super-Earths, or the embryos that collided to form them, is likely to have been Type I due to their smaller mass. The resonant orbits of some of the exoplanet systems indicates that some migration occurred in these systems, while the spacing of the orbits in many of the other systems not in resonance indicates that an instability likely occurred in those systems after the dissipation of the gas disk. The absence of Super-Earths and closely orbiting planets in the Solar System may be due to the previous formation of Jupiter blocking their inward migration.<ref name="Morbidelli_Raymond_2016">{{cite journal |last1= Morbidelli |first1= Alessandro |last2= Raymond |first2= Sean |title= Challenges in planet formation |journal= Journal of Geophysical Research: Planets |date=2016 |volume= 121 |issue=10 |pages= 1962–1980 |doi= 10.1002/2016JE005088 |arxiv= 1610.07202 |bibcode = 2016JGRE..121.1962M |s2cid= 119122001 }}</ref>

The amount of gas a super-Earth that formed in situ acquires may depend on when the planetary embryos merged due to giant impacts relative to the dissipation of the gas disk. If the mergers happen after the gas disk dissipates terrestrial planets can form, if in a transition disk a super-Earth with a gas envelope containing a few percent of its mass may form. If the mergers happen too early runaway gas accretion may occur leading to the formation of a gas giant. The mergers begin when the dynamical friction due to the gas disk becomes insufficient to prevent collisions, a process that will begin earlier in a higher metallicity disk.<ref name="Lee_Chiang_2016">{{cite journal|last1=Lee|first1=Eve J.|author1-link=Eve Lee|last2=Chiang|first2=Eugene|title=Breeding Super-Earths and Birthing Super-puffs in Transitional Disks|journal=The Astrophysical Journal|date=2016|volume=817|issue=2|page=90|doi=10.3847/0004-637X/817/2/90|arxiv=1510.08855|bibcode = 2016ApJ...817...90L |s2cid=118456061 |doi-access=free }}</ref> Alternatively gas accretion may be limited due to the envelopes not being in hydrostatic equilibrium, instead gas may flow through the envelope slowing its growth and delaying the onset of runaway gas accretion until the mass of the core reaches 15 Earth masses.<ref name="Lambrechts_Lega">{{cite journal|last1=Lambrechts|first1=Michiel|last2=Lega|first2=Elana|title=Reduced gas accretion on super-Earths and ice giants|journal=Astronomy and Astrophysics|volume=606|pages=A146|date=2017|arxiv=1708.00767|bibcode = 2017A&A...606A.146L |doi=10.1051/0004-6361/201731014|s2cid=118979289}}</ref>

== Meaning of ''accretion'' ==
Use of the term "[[accretion disk]]" for the [[protoplanetary disk]] leads to confusion over the [[planetary accretion]] process.
The protoplanetary disk is sometimes referred to as an accretion disk, because while the young [[T Tauri star|T Tauri]]-like protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge.<ref name=Yorke1999 /> In an accretion disk, there is a net flux of mass from larger radii toward smaller radii.<ref name=lynden-bell_1974 />

However, that meaning should not be confused with the process of accretion forming the planets. In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the [[protostar]] in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high-energy collisions between sizable [[planetesimal]]s.<ref name=Kokubo2002 />

In addition, the [[giant planet]]s probably had accretion disks of their own, in the first meaning of the word.<ref name=dangelo_podolak_2015>{{cite journal |last=D'Angelo |first=G. |author2= Podolak, M. |title= Capture and Evolution of Planetesimals in Circumjovian Disks |journal= The Astrophysical Journal |date= 2015 |volume=806 |issue=1 |pages=29pp |doi= 10.1088/0004-637X/806/2/203|arxiv = 1504.04364 |bibcode = 2015ApJ...806..203D|s2cid=119216797 }}</ref> The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant [[protoplanet]], while solid bodies within that disk accreted into the giant planet's regular moons.<ref name=Canup2002>{{cite journal |last=Canup |first=Robin M. |author1-link=Robin Canup |author2= Ward, William R. |title= Formation of the Galilean Satellites: Conditions of Accretion |journal= The Astronomical Journal |date=2002 |volume=124 |issue=6 |pages=3404–3423 |doi= 10.1086/344684 |url= http://www.boulder.swri.edu/~robin/cw02final.pdf |bibcode= 2002AJ....124.3404C|s2cid=47631608 }}</ref>

== See also ==
* [[Asteroid belt]]
* [[Bok globule]]
* [[Comet]]
* [[Exocomet]]
* [[Formation and evolution of the Solar System]]
* [[Herbig–Haro object]]
* [[History of Earth]]
* [[Kuiper belt]]
* [[Oort cloud]]
* [[T Tauri star]]

== Notes ==
{{Reflist|30em|group=lower-alpha}}

== References ==
{{Reflist|30em}}

== External links ==
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