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Protoplanetary disk

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From Wikipedia, the free encyclopedia

Not to be confused withProtoplanetary nebula.

Gas and dust surrounding a newly formed star

Atacama Large Millimeter Array image ofHL Tauri^[1]^[2]

Aprotoplanetary disk is a rotatingcircumstellar disc of dense gas and dust surrounding ayoung newly formed star, aT Tauri star, orHerbig Ae/Be star. The protoplanetary disk may not be considered anaccretion disk; while the two are similar, an accretion disk is hotter and spins much faster; it is also found onblack holes, not stars. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are calledproplyds.

Formation

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The evolutionary sequence of protoplanetary disks with substructures^[3]

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.^[4]

Protostars form frommolecular clouds consisting primarily ofmolecular hydrogen. When a portion of a molecular cloud reaches a critical size,mass, or density, it begins to collapse under its owngravity. As this collapsing cloud, called asolar 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 becausecentripetal 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.^[5] 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,^[6] before the disk disappears, perhaps being blown away by the young star'sstellar wind, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet discovered is 25 million years old.^[7]^[8]

Protoplanetary disk. Simulated spiral arm vs observational data.^[9]

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 1000AU, and only their innermost parts reach temperatures above 1000K. They are very often accompanied byjets.

Protoplanetary disks have been observed around several young stars in our galaxy. Observations by theHubble Space Telescope have shown proplyds and planetary disks to be forming within theOrion Nebula.^[10]^[11]

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.^[12]

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 themagnetorotational instability (MRI) no longer operates.^[13]^[14]

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.^[14] The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.

Supernova remnant ejecta producingplanet-forming material.

Planetary system

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An artist's illustration giving a simple overview of the main regions of a protoplanetary disk, delineated by the soot and frost line, which for example has been observed around the starV883 Orionis.^[15]

Thenebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete intoplanetesimals. This process competes against thestellar wind, which drives the gas out of the system, and gravity (accretion) and internal stresses (viscosity), which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both terrestrial and giant planets.^[16]^[17]

Some of the moons ofJupiter,Saturn, andUranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks.^[18]^[19] The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why theplanets are arranged in anecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System likely contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see. The Earth's moon likely formed after a Mars-sized protoplanet obliquelyimpacted the proto-Earth ~30 million years after the formation of the Solar System.

Debris disks

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Gas-poor disks of circumstellar dust have been found around many nearby stars—most of which have ages in the range of ~10 million years (e.g.Beta Pictoris,51 Ophiuchi) to billions of years (e.g.Tau Ceti). These systems are usually referred to as "debris disks". Given the older ages of these stars, and the short lifetimes of micrometer-sized dust grains around stars due toPoynting Robertson drag, collisions, andradiation pressure (typically hundreds to thousands of years), it is thought that this dust is from the collisions of planetesimals (e.g.asteroids,comets). Hence thedebris disks around these examples (e.g.Vega,Alphecca,Fomalhaut, etc.) are not "protoplanetary", but represent a later stage of disk evolution where extrasolar analogs of theasteroid belt andKuiper belt are home to dust-generating collisions between planetesimals.

Relation to abiogenesis

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Main articles:Abiogenesis andPanspermia

Based on recentcomputer model studies, thecomplex organic molecules necessary forlife may have formed in the protoplanetary disk ofdust grains surrounding theSun before the formation of the Earth.^[20] According to the computer studies, this same process may also occur around otherstars that acquireplanets.^[20] (Also seeExtraterrestrial organic molecules.)

Gallery

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Illustration of the dynamics of aproplyd
20 protoplanetary discs captured by theHigh Angular Resolution Project (DSHARP).^[21]
A shadow is created by the protoplanetary disc surrounding the starHBC 672 within the nebula.^[22]
Protoplanetary discAS 209 nestled in the youngOphiuchus star-forming region.^[23]
Protoplanetary diskHH 212.^[24]
By observing dusty protoplanetary discs, scientists investigate the first steps of planet formation.^[25]
Concentric rings around young starHD 141569A, located some 370 light-years away.^[26]
Debris disks detected inHST images of young stars,HD 141943 andHD 191089 - images at top; geometry at bottom.^[27]
Protoplanetary diskHH-30 inTaurus - disk emits the reddishstellar jet.
Artist's impression of a protoplanetary disk.
A proplyd in theOrion Nebula.
Video shows the evolution of the disc around a young star likeHL Tauri (artist concept).
Image of the circumtrinary disc aroundGW Orionis.^[28]
An artist's concept of a protoplanetary disk
Components of proplyd 177-341W in theOrion Nebula observed withVLT MUSE, showing an ionization front, protoplanetary disk, and tail^[29]
HOPS-315, a still-forming planetary system showing evidence for the earliest stages of planet formation.