Protoplanetary disk - Wikipedia

Protoplanetary disk - Wikipedia
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Not to be confused with
Protoplanetary nebula
Gas and dust surrounding a newly formed star
Atacama Large Millimeter Array
image of
HL Tauri
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 not be considered an
accretion disk
; while the two are similar, an accretion disk is hotter and spins much faster, accreting matter onto a central body; it is also found on
black holes
, not only 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 called
proplyds
Formation
edit
The evolutionary sequence of protoplanetary disks with substructures
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.
Protostars
are formed 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.
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
, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet discovered is 25 million years old.
Protoplanetary disk. Simulated spiral arm vs observational data.
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
. 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.
Supernova remnant
ejecta producing
planet-forming material
Planetary system
edit
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
15
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
A model of a protoplanetary disk
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
edit
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 "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
edit
Main articles:
Abiogenesis
and
Panspermia
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
edit
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
Components of proplyd 177-341W in the
Orion Nebula
observed with
VLT
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.
See also
edit
Wikimedia Commons has media related to
Protoplanetary disks
Accretion disk
Circumplanetary disk
– Accumulation of matter around a planet
Debris disk
Disk wind
– material ejected from a disk
Disrupted planet
Exoasteroid
Formation and evolution of the Solar System
Herbig–Haro object
Nebular hypothesis
Q-PACE
– a spacecraft mission to study accretion
Planetary system
References
edit
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Archived
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Further reading
edit
Davis, Sanford S. (2006). "A New Model for Water Vapor and Ice Abundance in a Protoplanetary Nebula".
American Astronomical Society, DPS Meeting #38, #66.07
38
: 617.
Bibcode
2006DPS....38.6607D
Barrado y Navascues, D. (1998).
"The Castor moving group: The age of Fomalhaut and Vega"
Astronomy and Astrophysics
339
(3):
831–
839.
arXiv
astro-ph/9905243
Bibcode
1998A&A...339..831B
. Archived from
the original
on 2007-09-29
. Retrieved
2007-06-22
Kalas, Paul
; Graham, J.; Clampin, M. (2005). "A planetary system as the origin of structure in Fomalhaut's dust belt".
Nature
435
(7045):
1067–
70.
arXiv
astro-ph/0506574
Bibcode
2005Natur.435.1067K
doi
10.1038/nature03601
PMID
15973402
S2CID
4406070
Williams, J. P.; Cieza, L. A. (2011). "Protoplanetary Disks and Their Evolution".
Annual Review of Astronomy and Astrophysics
49
(1):
67–
117.
arXiv
1103.0556
Bibcode
2011ARA&A..49...67W
doi
10.1146/annurev-astro-081710-102548
S2CID
58904348
Armitage, P. J. (2011). "Dynamics of Protoplanetary Disks".
Annual Review of Astronomy and Astrophysics
49
(1):
195–
236.
arXiv
1011.1496
Bibcode
2011ARA&A..49..195A
doi
10.1146/annurev-astro-081710-102521
S2CID
55900935
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