Accretion

Accretion is the process by which small particles and bodies stick together and grow into larger ones — the basic way planets are built.

Accretion is the growth of an object by gathering material onto it. In planet formation, it describes the whole chain of growth from microscopic dust grains to full-sized planets.

It happens in stages. First, grains stick together through surface forces. Then pebbles gather into kilometre-sized planetesimals. Once bodies are big enough, gravity takes over and they sweep up their neighbours, eventually becoming protoplanets and planets. Gas giants also accrete gas from the protoplanetary disk.

Accretion releases heat: every impact turns the energy of motion into warmth, which is one reason young planets are hot enough to melt.