Editing Nebular hypothesis (section)

== 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>