protoplanetary disk

A protoplanetary disk is the flat, spinning pancake of gas and dust that surrounds a newborn star. When a slowly rotating cloud collapses, it cannot all fall straight to the centre, and the leftover swirling material settles into a disk in the star's equatorial plane, like the rings of a spinning record. It is at once the conveyor belt that feeds the growing star and the raw material from which planets are later built.

The disk exists because of a strict rule of physics: angular momentum, the quantity of spin, is conserved. As the cloud shrinks it must spin faster, and gas that is moving sideways too fast simply cannot reach the centre; instead it orbits, piling up into a thin, rotating disk perhaps a few hundred times the Earth-Sun distance across. Within the disk, gas slowly spirals inward and falls onto the star (accretion), while friction makes the inner disk hot and the outer disk stays cold enough for ices. Over a few million years the dust grains in the disk collide and stick, growing from specks to pebbles to boulders to the seeds of planets, even as the star siphons away and the disk's gas evaporates.

Protoplanetary disks matter because they tie together two great stories: how stars finish growing and how planets begin. The same disk that delivers the last gas onto the star is the birthplace of planets, moons, asteroids, and comets, so the existence of these disks around essentially every young star is a major reason we expect planets to be common. (The planets themselves are covered elsewhere; here the focus is the disk as part of the star's birth.) Modern telescopes can now photograph these disks directly, revealing gaps and rings that are likely carved by planets forming inside them.