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A protoplanetary disk is a rotating
circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are ...
of dense gas and dust surrounding a
young newly formed
young newly formed
star, a
T Tauri star T Tauri stars (TTS) are a class of variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking t ...
, or
Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young (<10Myr) star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lin ...
. The protoplanetary disk may also be considered an
accretion disk An accretion disk is a structure (often a ) formed by diffuse material in al motion around a massive . The central body is typically a . causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional ...

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 A proplyd, a syllabic abbreviation of an ionized protoplanetary disk, is an externally illuminated Photoevaporation, photoevaporating protoplanetary disk, disk around a Young stellar object, young star. Nearly 180 proplyds have been discovered in th ...
. In July 2018, the first confirmed image of such a disk, containing a nascent
exoplanet An exoplanet or extrasolar planet is a outside the . The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1992. This was followed by the confirmation of a ...

exoplanet
, named
PDS 70b PDS 70 (V1032 Centauri) is a low-mass T Tauri star in the constellation Centaurus. Located approximately 370 light-years from Earth, it has a mass of , and is approximately 10 million years old. The star has a protoplanetary disk ...

PDS 70b
, was reported.


Formation

Protostar A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 5 ...
s form from
molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
s consisting primarily of
molecular hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the sam ...
. When a portion of a molecular cloud reaches a critical size,
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
, or density, it begins to collapse under its own
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
. As this collapsing cloud, called a
solar nebula The formation and evolution of the Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) ...
, 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 In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...
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 In mechanics Mechanics (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...
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 vertical direction. The outcome is the formation of a thin disc supported by gas pressure in the vertical direction. 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, before the disk disappears, perhaps being blown away by the young star's
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet discovered is 25 million years old. 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
AU
, and only their innermost parts reach temperatures above 1000
K
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 The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

Hubble Space Telescope
have shown proplyds and planetary disks to be forming within the
Orion Nebula The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the Orion (constellation), constellation of Orion. It is one of the brightest nebulae, and is visible to t ...

Orion Nebula
. 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. 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 The magnetorotational instability (MRI) is a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter and i ...
(MRI) no longer operates. 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. The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.


Planetary system

The
nebular hypothesis The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
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
planetesimal 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 ab ...
s. This process competes against the
stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown d ...
, which drives the gas out of the system, and gravity (
accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucle ...

accretion
) and internal stresses (
viscosity The viscosity of a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

viscosity
), which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both terrestrial and giant planets. Some of the moons of Jupiter, Saturn, and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks. The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...

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 Beta Pictoris (abbreviated β Pictoris or β Pic) is the second brightest star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest star ...

Beta Pictoris
, 51 Ophiuchi) to billions of years (e.g.
Tau Ceti Tau Ceti, Latinized from τ Ceti, is a single star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, neares ...

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 Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is Ab ...

radiation pressure
(typically hundreds to thousands of years), it is thought that this dust is from the collisions of planetesimals (e.g.
asteroids An asteroid is a minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a ...

asteroids
,
comets A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comets
). Hence the debris disks around these examples (e.g.
Vega Vega is the brightest star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun ...

Vega
, Alphecca,
Fomalhaut Fomalhaut , designation Alpha Piscis Austrini (α Piscis Austrini, abbreviated Alpha PsA, α PsA) is the brightest star in the constellation A constellation is an area on the celestial sphere in which a group of ...

Fomalhaut
, etc.) are not truly "protoplanetary", but represent a later stage of disk evolution where extrasolar analogs of the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
and
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
are home to dust-generating collisions between planetesimals.


Relation to abiogenesis

Based on recent computer model studies, the complex organic molecules necessary for
life Life is a characteristic that distinguishes physical entities A bubble of exhaled gas in water In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a ...

life
may have formed in the protoplanetary disk of dust grains surrounding the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
before the formation of the Earth. According to the computer studies, this same process may also occur around other
stars A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. ...

stars
that acquire
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...

planets
. (Also see Extraterrestrial organic molecules).


Gallery

File:Pitch perfect in DSHARP at ALMA.jpg, 20 protoplanetary discs captured by the High Angular Resolution Project (DSHARP). File:Cosmic shadow of HBC 672.jpg, A shadow is created by the protoplanetary disc surrounding the star HBC 672 within the nebula. File:Young planet creates a scene.jpg, Protoplanetary disc AS 209 nestled in the young
Ophiuchus Ophiuchus () is a large constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inan ...
star-forming region. Image:Feeding a Baby Star with a Dusty Hamburger.jpg, Protoplanetary disk HH 212. File:Spring Cleaning in an Infant Star System.jpg, By observing dusty protoplanetary discs, scientists investigate the first steps of planet formation. Image:Boulevard_of_Broken_Rings.jpg, Concentric rings around young star HD 141569A, located some 370 light-years away. Image:NASA-14114-HubbleSpaceTelescope-DebrisDisks-20140424.jpg, Debris disks detected in images of young stars, ''HD 141943'' and ''HD 191089'' - images at top; geometry at bottom. Image:Protoplanetary disk HH-30.jpg, Protoplanetary disk HH-30 in
Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological characters named Taurus * Taurus (astrology), the astrological sign * ''Bos taurus'' ...
- disk emits the reddish
stellar jet An astrophysical jet is an astronomy, astronomical phenomenon where outflows of Ionization, ionised matter are emitted as an extended beam along the rotation, axis of rotation. When this greatly accelerated matter in the beam approaches the speed of ...
. Image:Artist’s Impression of a Baby Star Still Surrounded by a Protoplanetary Disc.jpg, Artist's impression of a protoplanetary disk. Image:M42proplyds.jpg, A proplyd in the
Orion Nebula The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the Orion (constellation), constellation of Orion. It is one of the brightest nebulae, and is visible to t ...

Orion Nebula
. Image:Artist's impression of the disc around a young star.ogv, Video shows the evolution of the disc around a young star like HL Tauri (artist concept). Image:GW Orionis 2.jpg, Image of the circumtrinary disc around GW Orionis.


See also

*
Accretion disk An accretion disk is a structure (often a ) formed by diffuse material in al motion around a massive . The central body is typically a . causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional ...

Accretion disk
* *
Debris disk A debris disk (), or debris disc (), is a of dust and debris in orbit around a . Sometimes these disks contain prominent rings, as seen in the image of on the right. Debris disks are found around stars with mature planetary systems, including ...
*
Disrupted planet In astronomy, a disrupted planet is a planet or exoplanet or, perhaps on a somewhat smaller scale, a planetesimal, moon, exomoon or asteroid that has been disrupted or destroyed by a nearby or passing astronomical body or object such as a star. Ne ...
*
Formation and evolution of the Solar System The formation and evolution of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all i ...
*
Herbig–Haro object Herbig–Haro (HH) objects are bright patches of nebula, nebulosity associated with newborn stars. They are formed when narrow jets of partially plasma (physics), ionised gas ejected by stars collide with nearby clouds of gas and dust at several ...
*
Nebular hypothesis The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
* Q-PACE, a spacecraft mission to study accretion *
Planetary system A planetary system is a set of gravity, gravitationally bound non-Star, stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems m ...


References


Further reading

* . * * * * {{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science *Protoplanetary disk Articles containing video clips