How do planets and stars form?

Let’s start with stars. Stars form by the slow contraction under gravity of a very large cloud of gas and dust particles in space. The gas and dust clouds are very common and we know of many regions of star formation in our Milky Way Galaxy. As the gas and dust clouds contract, small centres of condensation form which eventually become new stars. The whole process takes millions of years, but that is a relatively short time for changes to take place in space.

When stars form there is always some matter around the star left over and this forms into a disk around the equator of the rotating star. This matter is also able to condense into gaseous or solid bodies in a disk. These are called planetesimals and their formation takes place on a longer timescale than the central star, about which they orbit. Eventually these planetesimals attract more of the debris remaining in the disk and they contract further to a spherical shape under their own gravity. At this point they form a system of planets, like those in our solar system.

There are at least 200 thousand million stars in our Milky Way (of which the Sun is just one) and probably most of them (if not all) have planetary systems with planets orbiting around their parent stars.

We see that gravitational contraction of matter in the space between stars plays a central role in the formation of both stars and planets.

Our expert

Professor John Hearnshaw answered this question. He is a Fellow of Royal Society Te Apārangi, which means he’s one of the top experts in his field of physics and astronomy. John works at the University of Canterbury and has a particular interest in planetary systems and astrophysics.

For more information on our expert, visit his profile:  Professor John Hearnshaw

PHOTO CREDIT: NASA Cosmic fireworks