Observatoire de Paris
<-   In the environment of a star   ->
Planetary systems forming in the Orion nebula
The dark regions are dust disks around young stars, where planets are likely to form.
Copyright : NASA / HST / C. R. O'Dell and S. K. Wong

When a star "switchs on", the collapse of the cloud forms a disk around the proto-star. The study of the Solar System has enabled us to reconstruct the steps which led to the formation of the planets.

  • The material forms a very thin dust disk embedded in a thicker gas disk.
  • In the dust disk, the grains stick together to form larger bodies, the planetesimals (which look like current asteroids or comets)
  • The planetesimals collide to form more massive bodies. As soon as a sufficiently large core forms, it attracts the gas and the planetesimals, it grows faster and forms a planet
  • The planets eject the remaining small planetesimals which fall into the Sun, the planets and the satellites (thus forming the numerous craters visible on the Moon or Mercury), or are destroyed in the neighbourhood of the massive planets to form rings. Some of them are sent on the outer edges of the Solar System, forming the Oort cloud, which is a reservoir for current comets.
  • The interactions between the planets still create collisions, which can explain the formation of the Moon or the swing of the orbit of Uranus. They can also repel each other and migrate until they find a stable configuration. The resonance phenomenon plays a large part in the modelling of the orbits of the different bodies of the Solar System.

The dust disk is made up of rocks and metals close from the star, where it is warm. Beyond a certain distance (called the "ice limit"), the lower temperature enables the ice to form. Because hydrogen (H) and oxygen (O) are the most abundant atoms in the Solar System, there are much more planetesimals and the planets form more rapidly. Jupiter, the largest planet, formed at the ice limit, and the other giant planets, made up of gas and ice, are outside this limit.

This scenario of the formation of planets explains quite well all the properties of the planets of the Solar System. But is it still viable for the exoplanets?...