Editing Protoplanetary disk (section)

==Formation==
[[File:The evolutionary sequence of protoplanetary disks with substructures.png|thumb|The evolutionary sequence of protoplanetary disks with substructures<ref>{{cite web |title=Early Evolution of Planetary Disk Structures Seen for the First Time |url=https://public.nrao.edu/news/early-evolution-of-planetary-disk-structures-seen-for-the-first-time/ |website=National Radio Astronomy Observatory |access-date=18 February 2024}}</ref>]]
[[Image:Mamajek09 diskfraction.jpg|thumb|A 2009 image showing fractions of stars that suggest some evidence of having a protoplanetary disk as a function of their stellar age in millions of years; The samples are nearby young clusters and associations.<ref>{{cite journal|title=Initial Conditions of Planet Formation: Lifetimes of Primordial Disks|author=Mamajek, E.E.|date=2009|journal=AIP Conference Proceedings|volume=1158|pages=3–10|bibcode=2009AIPC.1158....3M|doi=10.1063/1.3215910|last2=Usuda|first2=Tomonori|last3=Tamura|first3=Motohide|last4=Ishii|first4=Miki|arxiv = 0906.5011 |s2cid=16660243}}</ref>]]
[[Protostar]]s form from [[molecular cloud]]s consisting primarily of [[molecular hydrogen]]. When a portion of a molecular cloud reaches a critical size, [[Jeans mass|mass]], or density, it begins to collapse under its own [[gravity]]. As this collapsing cloud, called a [[solar nebula]], becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. [[Conservation of angular momentum]] causes the rotation to increase as the nebula radius decreases. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk. This occurs because [[centripetal acceleration]] from the orbital motion resists the gravitational pull of the star only in the radial direction, but the cloud remains free to collapse in the axial direction. The outcome is the formation of a thin disc supported by gas pressure in the axial direction.<ref>{{cite journal|title=Accretion discs in astrophysics|author=Pringle, J.E.|date=1981|journal=[[Annual Review of Astronomy and Astrophysics]]|volume=19|pages=137–162|bibcode=1981ARA&A..19..137P|doi=10.1146/annurev.aa.19.090181.001033}}</ref> The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible.

It is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years,<ref>{{cite journal | bibcode=2004ApJ...612..496M | author=Mamajek, E.E. | author2=Meyer, M.R. | author3=Hinz, P.M. | author4=Hoffmann, W.F. | author5=Cohen, M. | author6=Hora, J.L. | name-list-style=amp | title= Constraining the Lifetime of Circumstellar Disks in the Terrestrial Planet Zone: A Mid-Infrared Survey of the 30 Myr old Tucana-Horologium Association | journal= The Astrophysical Journal|volume=612 | issue=1 | date=2004 | pages= 496–510 | doi= 10.1086/422550 |arxiv = astro-ph/0405271 | s2cid=16366683 }}</ref> before the disk disappears, perhaps being blown away by the young star's [[stellar wind]], or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet discovered is 25 million years old.<ref>{{cite journal | bibcode=2005ApJ...621L..65W | author=White, R.J. | author2=Hillenbrand, L.A. | name-list-style=amp| title= A Long-lived Accretion Disk around a Lithium-depleted Binary T Tauri Star | journal= The Astrophysical Journal|volume=621 | issue=1 | date=2005 | pages= L65–L68| doi= 10.1086/428752 |arxiv = astro-ph/0501307 | s2cid=17532904 }}</ref><ref>{{cite web|url=http://www.universetoday.com/10795/audio-planetary-disk-that-refuses-to-grow-up/|title=Planetary Disk That Refuses to Grow Up (Interview with Lee Hartmann about the discovery)|last1=Cain|first1=Fraser|last2=Hartmann|first2=Lee|publisher=[[Universe Today]]|date=3 August 2005|access-date=1 June 2013}}</ref>

[[File:Protoplanetary Disk Simulated Spiral Arm vs Observational Data.jpg|thumb|Protoplanetary disk. Simulated spiral arm vs observational data.<ref>{{cite web|title=Protoplanetary Disk: Simulated Spiral Arm vs. Observational Data|url=http://www.spacetelescope.org/images/opo1540a/|access-date=30 October 2015}}</ref>]]

Protoplanetary disks around T Tauri stars differ from the disks surrounding the primary components of close binary systems with respect to their size and temperature. Protoplanetary disks have radii up to 1000 [[astronomical unit|AU]], and only their innermost parts reach temperatures above 1000 [[kelvin|K]]. They are very often accompanied by [[stellar jet|jets]].

Protoplanetary disks have been observed around several young stars in our galaxy. Observations by the [[Hubble Space Telescope]] have shown proplyds and planetary disks to be forming within the [[Orion Nebula]].<ref name="RicciRobberto2008">{{cite journal|last1=Ricci|first1=L.|last2=Robberto|first2=M.|last3=Soderblom|first3=D. R.|title=Thehubble Space Telescope/Advanced Camera for Surveys Atlas of Protoplanetary Disks in the Great Orion Nebula|journal=The Astronomical Journal|volume=136|issue=5|year=2008|pages=2136–2151|issn=0004-6256|doi=10.1088/0004-6256/136/5/2136|bibcode=2008AJ....136.2136R|s2cid=123470043 |doi-access=}}</ref><ref name="O'dellWong1996">{{cite journal|last1=O'dell|first1=C. R.|last2=Wong|first2=Kwan|title=Hubble Space Telescope Mapping of the Orion Nebula. I. A Survey of Stars and Compact Objects|journal=The Astronomical Journal|volume=111|year=1996|pages=846|issn=0004-6256|doi=10.1086/117832|bibcode=1996AJ....111..846O|doi-access=free}}</ref>

Protoplanetary disks are thought to be thin structures, with a typical vertical height much smaller than the radius, and a typical mass much smaller than the central young star.<ref>{{cite journal|last1=Armitage|first1=Philip J.|title=Dynamics of Protoplanetary Disks|journal=[[Annual Review of Astronomy and Astrophysics]]|date=2011|volume=49|issue=1|pages=195–236|doi=10.1146/annurev-astro-081710-102521|arxiv = 1011.1496 |bibcode = 2011ARA&A..49..195A |s2cid=55900935}}</ref>

The mass of a typical proto-planetary disk is dominated by its gas, however, the presence of dust grains has a major role in its evolution. Dust grains shield the mid-plane of the disk from energetic radiation from outer space that creates a dead zone in which the [[magnetorotational instability]] (MRI) no longer operates.<ref>{{cite journal|last1=Balbus|first1=Steven A.|last2=Hawley|first2=John F.|title=A powerful local shear instability in weakly magnetized disks. I - Linear analysis. II - Nonlinear evolution|journal=Astrophysical Journal|date=1991|volume=376|pages=214–233|doi=10.1086/170270|bibcode=1991ApJ...376..214B|url=https://articles.adsabs.harvard.edu/pdf/1991ApJ...376..223H|url-status=live|archiveurl=
https://web.archive.org/web/20201202075258/https://articles.adsabs.harvard.edu/pdf/1991ApJ...376..214B|archivedate=2020-12-02}}</ref><ref name="Layered Accretion In T Tauri Disks">{{cite journal|last1=Gammie|first1=Charles|title=Layered Accretion In T Tauri Disks|journal=Astrophysical Journal|date=1996|volume=457|page=355|doi=10.1086/176735|bibcode=1996ApJ...457..355G|url=https://articles.adsabs.harvard.edu/pdf/1996ApJ...457..355G|url-status=live|archiveurl=https://web.archive.org/web/20211117095828/https://articles.adsabs.harvard.edu/pdf/1996ApJ...457..355G|archivedate=2021-11-17}}</ref>

It is believed that these disks consist of a turbulent envelope of plasma, also called the active zone, that encases an extensive region of quiescent gas called the dead zone.<ref name="Layered Accretion In T Tauri Disks"/> The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.
{{Multiple image|direction=horizontal|align=center|width=300|image1=15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg|image2=15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg| footer_align = center |footer=[[Supernova remnant]] ejecta producing [[Nebular hypothesis|planet-forming material]].}}