
A protoplanetary disk is a rotating disk of gas and dust surrounding a young star, from which planets and planetary systems form.A protoplanetary disk consists of gas (mostly hydrogen) and dust, heated by the star's radiation. The inner part is hotter, there ices evaporate; the outer part is colder, where icy planets and comets form. Disks show gaps — traces of forming planets. Disk lifetime is several million years.
The concept of the protoplanetary disk goes back to the nebular hypothesis of I. Kant (1755) and P. Laplace (1796). In the 20th century, the theory was developed by V. S. Safronov, who mathematically described planet accumulation. Observational confirmations came with space telescopes: IRAS (1980s) detected an excess of infrared radiation from young stars, and Hubble and ALMA directly imaged disks.
Disk formation is a consequence of angular momentum conservation during the collapse of a rotating cloud. Gas and dust falling to the center increase their rotation speed, and centrifugal force in the equatorial plane balances gravity, forming a disk. Further evolution: dust settles to the midplane, grows through sticking. When planetesimals reach ~km size, gravitational focusing kicks in. In the gas phase, planetary migration is possible. Chemical transformations occur in the disk (beyond the ice line). The disk phase ends via photoevaporation or accretion, leaving a planetary system.
Observations of protoplanetary disks help understand how our Solar System formed and predict the architecture of exoplanetary systems.
Astronomers discovered a disk around the star HL Tauri with a clear ring structure, revealing planetary embryos.

























