Protoplanetary Disk
A rotating disk of gas and dust surrounding a young star, where planets are actively forming from colliding particles and growing cores.
A rotating disk of gas, dust, and planetesimals orbiting a young star in the early stages of planetary formation. Material in the disk gradually coalesces into larger bodies through collision and gravitational attraction, eventually forming planets and other solar system objects. Protoplanetary disks typically persist for millions of years before dissipating.
What this means in real life
When you watch dust swirl around a drain in a bathtub, the pattern resembles how particles in a protoplanetary disk spiral inward and clump together—except the disk orbits a star and takes millions of years instead of seconds.
What it isn’t
It is not a fully formed planetary system. A protoplanetary disk is the chaotic, dusty construction zone where planets are still assembling; it contains far more loose material and fewer distinct bodies than a mature system like our own.
Commonly misused online
People sometimes use it interchangeably with 'accretion disk,' but accretion disks can form around black holes and neutron stars too. A protoplanetary disk is specifically the planet-forming variety around young stars.