California Institute of Technology Jet Propulsion Laboratory

Fact Sheet

Intro

Note To Editors

Press Release

Visuals

Bios

Fact Sheet

How Planets Form

An elegant tale of how planets are born is unfolding in the universe. It begins in the spaces between stars. They may look empty but in fact are full of dust and gas from stars that died long ago. This is the stuff from which planets are made.

Cosmic dust and gas collect into vast cloudy systems. When these gas clouds become dense enough, they collapse under the force of gravity and the intense heat and pressure that follows. The process forms a new star that is surrounded by a dense cocoon of gas and dust called a dust envelope. Ninety-nine percent of the mass in the envelope is gas.

As the star ages, material rains down from the envelope onto a pancake-like structure called a disc. Material from the disc pours onto the star. Ultimately, the envelope disappears, leaving a stable disc. It is out of these "protoplanetary" discs that astronomers believe planets are born.

In time, dust particles in the discs collide and clump together, forming larger bodies, called embryonic planets or "planetesimals." These planetesimals are considered the building blocks of planets. They also collide to form even larger bodies, or planets.

This is the first phase in the life of a new planetary system. As the system matures, the gas disappears. This leaves the planets orbiting through a disc consisting mostly of small dust grains replenished by colliding planetesimals and other rocky bodies. This "debris" disc represents a later stage of evolution.