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A proplyd in the Orion Nebula.

The protoplanetary disk HH-30 in Taurus, about 450 light years away. The disk emits the reddish stellar jet, a common structure of these formations.

An artist's impression of protoplanetary disk.

See also: Planetary formation and Formation and evolution of the Solar System

A protoplanetary disk (or proplyd) is a rotating disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig star. The protoplanetary disk may be considered an accretion disk because gaseous material may be falling from the inner edge of the disk onto the surface of the star, but this process should not be confused with the accretion process thought to build up the planets themselves.

Protoplanetary disks around T Tauri stars differ from the discs surrounding the primary components of close binary systems in their size and temperature. Protoplanetary discs have radii up to 1000 astronomical units and are rather cool. Only their innermost parts reach temperatures above 1000 kelvins. They are very often accompanied by jets.

Protostars typically 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 more dense, 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 becomes smaller. This rotation causes the cloud to flatten out - much like forming a flat pizza out of dough - and take the form of a disk. 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 solar wind, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk ever discovered is 25 million years old.

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 accretion, which pulls material into the central T Tauri star.

Protoplanetary disks have been observed around several young stars in our galaxy, the first being found around the star Beta Pictoris in 1984. Recent observations by the Hubble Space Telescope have shown proplyds and planetary discs to be forming within the Orion Nebula.

Astronomers have discovered large discs of material, which may themselves be protoplanetary discs, around the stars Vega, Alphecca and Fomalhaut, all of which are very close to the Sun.

The Castor co-moving group of stars containing Vega and Fomalhaut has recently been isolated. Using data from the Hipparcos satellite telescope the Castor group was found to have an estimated age of 200 ± 100 million years. This indicates that the infrared excesses seen around Vega and Fomalhaut are likely due to a disk of debris from colliding planetesimals rather than a protoplanetary disk. Successful imaging of Fomalhaut's disk by the Hubble Space Telescope confirms this.

The name protoplanetary nebula is sometimes employed (for example in Davis 2006) when discussing protoplanetary disks. However, this can lead to confusion with the same term subsequently being also employed when discussing the unrelated concept of planetary nebulas. (See protoplanetary nebula for more information on naming.)

Water is the only known substance found in planetary disks in both the solid (ice) and gaseous phases together in large quantities. Consequently, determining their relative proportions is useful in characterizing the physical state of a nebula and the planet formation process. Typical protoplanetary disks are composed of a wide range of densities and temperatures which results in an array of gas/ice ratios. Ice predominates far and mid nebula while gaseous water tends to dominate the centerplane area of the near nebula and above the disk photosphere.(Davis 2006)