Protoplanetary disc

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A protoplanetary disc (also protoplanetary disk, proplyd) is a disc of dust and gas which orbits a protostar, within a solar nebula. Such discs eventually condense into either asteroid belts, moonss, or planets.

Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As the cloud shrinks, conservation of angular momentum causes the random motions originally present in the cloud to become one coherent rotation. This rotation causes the cloud to flatten out (much like forming a flat pizza out of dough) and take the form of a disk.

Further gravitational interactions may cause the dust and gas in the disk to condense 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 protostar.