A 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 also be considered an 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.
Protostars 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. 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 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, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk yet 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 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.
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 are arranged in an ecliptic plane.
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).
In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported.