Editing Nebular hypothesis (section)

=== 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>