A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star.
The protoplanetary disk may also be considered an accretion disk for the star itself, because gasses 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. Protostars mainly 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.
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 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, and only their innermost parts reach temperatures above 1000 Kelvin. They are very often accompanied by jets. Protoplanetary disks have been observed around several young stars in our galaxy.
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.
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), which pulls material into the central T Tauri star. Some of the moons of Jupiter, Saturn, and other planets 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 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.