Protoplanetary disk

Last updated February 19, 2024

A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.

Formation

Protostars form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, 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.^[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'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.^[7]^[8]

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 AU, and only their innermost parts reach temperatures above 1000 K. They are very often accompanied by 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.^[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 the magnetorotational 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.

15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg

15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg

Supernova remnant ejecta producing planet-forming material.

Planetary system

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 star V883 Orionis. Soot-line1.jpg — 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 star V883 Orionis.

The nebular 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 into planetesimals. This process competes against the stellar 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 of Jupiter, Saturn, and Uranus 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 the planets are arranged in an ecliptic 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 obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System.

Debris disks

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 to Poynting Robertson drag, collisions, and radiation pressure (typically hundreds to thousands of years), it is thought that this dust is from the collisions of planetesimals (e.g. asteroids, comets). Hence the debris disks around these examples (e.g. Vega, Alphecca, Fomalhaut, etc.) are not truly "protoplanetary", but represent a later stage of disk evolution where extrasolar analogs of the asteroid belt and Kuiper belt are home to dust-generating collisions between planetesimals.

Relation to abiogenesis

Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.^[20] According to the computer studies, this same process may also occur around other stars that acquire planets.^[20] (Also see Extraterrestrial organic molecules.)

Gallery

Illustration of the dynamics of a proplyd
20 protoplanetary discs captured by the High Angular Resolution Project (DSHARP).^[21]
A shadow is created by the protoplanetary disc surrounding the star HBC 672 within the nebula.^[22]
Protoplanetary disc AS 209 nestled in the young Ophiuchus star-forming region.^[23]
Protoplanetary disk HH 212.^[24]
By observing dusty protoplanetary discs, scientists investigate the first steps of planet formation.^[25]
Concentric rings around young star HD 141569A, located some 370 light-years away.^[26]
Debris disks detected in HST images of young stars, HD 141943 and HD 191089 - images at top; geometry at bottom.^[27]
Protoplanetary disk HH-30 in Taurus - disk emits the reddish stellar jet.
Artist's impression of a protoplanetary disk.
A proplyd in the Orion Nebula.
Video shows the evolution of the disc around a young star like HL Tauri (artist concept).
Image of the circumtrinary disc around GW Orionis.^[28]
An artist's concept of a protoplanetary disk

Related Research Articles

<span class="mw-page-title-main">Planetesimal</span> Solid objects in protoplanetary disks and debris disks

Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its 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. 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. 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.

A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth.

A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets.

Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.

Planetary migration occurs when a planet or other body in orbit around a star interacts with a disk of gas or planetesimals, resulting in the alteration of its orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for hot Jupiters. The generally accepted theory of planet formation from a protoplanetary disk predicts that such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals.

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, into an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

TW Hydrae is a T Tauri star approximately 196 light-years away in the constellation of Hydra. TW Hydrae is about 80% of the mass of the Sun, but is only about 5-10 million years old. The star appears to be accreting from a face-on protoplanetary disk of dust and gas, which has been resolved in images from the ALMA observatory. TW Hydrae is accompanied by about twenty other low-mass stars with similar ages and spatial motions, comprising the "TW Hydrae association" or TWA, one of the closest regions of recent "fossil" star-formation to the Sun.

There is evidence that the formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

A bipolar outflow comprises two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars, or with evolved post-AGB stars.

A debris disk, or debris disc, is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks are found around stars with mature planetary systems, including at least one debris disk in orbit around an evolved neutron star. Debris disks can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets.

HD 100546, also known as KR Muscae, is a pre-main sequence star of spectral type B8 to A0 located 353 light-years from Earth in the southern constellation of Musca. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.

In planetary astronomy, the grand tack hypothesis proposes that Jupiter formed at a distance of 3.5 AU from the Sun, then migrated inward to 1.5 AU, before reversing course due to capturing Saturn in an orbital resonance, eventually halting near its current orbit at 5.2 AU. The reversal of Jupiter's planetary migration is likened to the path of a sailboat changing directions (tacking) as it travels against the wind.

A circumstellar disc is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.

Pebble accretion is the accumulation of particles, ranging from centimeters up to meters in diameter, into planetesimals in a protoplanetary disk that is enhanced by aerodynamic drag from the gas present in the disk. This drag reduces the relative velocity of pebbles as they pass by larger bodies, preventing some from escaping the body's gravity. These pebbles are then accreted by the body after spiraling or settling toward its surface. This process increases the cross section over which the large bodies can accrete material, accelerating their growth. The rapid growth of the planetesimals via pebble accretion allows for the formation of giant planet cores in the outer Solar System before the dispersal of the gas disk. A reduction in the size of pebbles as they lose water ice after crossing the ice line and a declining density of gas with distance from the sun slow the rates of pebble accretion in the inner Solar System resulting in smaller terrestrial planets, a small mass of Mars and a low mass asteroid belt.

In planetary science a streaming instability is a hypothetical mechanism for the formation of planetesimals in which the drag felt by solid particles orbiting in a gas disk leads to their spontaneous concentration into clumps which can gravitationally collapse. Small initial clumps increase the orbital velocity of the gas, slowing radial drift locally, leading to their growth as they are joined by faster drifting isolated particles. Massive filaments form that reach densities sufficient for the gravitational collapse into planetesimals the size of large asteroids, bypassing a number of barriers to the traditional formation mechanisms. The formation of streaming instabilities requires solids that are moderately coupled to the gas and a local solid to gas ratio of one or greater. The growth of solids large enough to become moderately coupled to the gas is more likely outside the ice line and in regions with limited turbulence. An initial concentration of solids with respect to the gas is necessary to suppress turbulence sufficiently to allow the solid to gas ratio to reach greater than one at the mid-plane. A wide variety of mechanisms to selectively remove gas or to concentrate solids have been proposed. In the inner Solar System the formation of streaming instabilities requires a greater initial concentration of solids or the growth of solid beyond the size of chondrules.

PDS 70 is a very young T Tauri star in the constellation Centaurus. Located 370 light-years from Earth, it has a mass of 0.76 M_☉ and is approximately 5.4 million years old. The star has a protoplanetary disk containing two nascent exoplanets, named PDS 70b and PDS 70c, which have been directly imaged by the European Southern Observatory's Very Large Telescope. PDS 70b was the first confirmed protoplanet to be directly imaged.

<span class="mw-page-title-main">Circumplanetary disk</span> Accumulation of matter around a planet

A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. Around the planets, they are the reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.

<span class="mw-page-title-main">RW Aurigae</span> Young binary star system in the constellation Auriga

RW Aurigae is a young binary system in the constellation of Auriga about 530 light years away, belonging to the Taurus-Auriga association of the Taurus Molecular Cloud. RW Aurigae B was discovered in 1944.

<span class="mw-page-title-main">CQ Tauri</span> Star in the constellation Taurus

CQ Tauri is a young variable star in the equatorial constellation of Taurus. It is too faint to be visible to the naked eye with an apparent visual magnitude that ranges from 8.7 to 12.25. The distance to this star is approximately 487 light years based on parallax measurements, and it is drifting further away with a radial velocity of ~23 km/s. It appears to be part of the T-association Tau 4. CQ Tauri lies close enough to the ecliptic to undergo lunar occultations.

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