Around the star

When the star “ignites”, the collapse of the cloud forms a disc around the proto-star. Our understanding of the planets in our Solar System enabled us to recreate the following steps in the formation of planets and other objects orbiting around the Sun:

• The disc stratifies into a very thin disc of dust within a thicker disc of gas.

• In the dust disc, the grains stick together to create larger bodies, planetesimals ^?  (which resemble asteroids ^? or comets ^? ).

• During a series of collisions, the planetesimals ^? will form even more massive bodies. As soon as a large enough core is formed, it attracts more and more planetesimals ^? and the gas. It then grows even faster and forms a planet.

• The planets eject the remaining small planetesimals ^? , which fall into the Sun, onto planets and satellites ^? (creating the countless craters visible on the Moon or on Mercury), or break up in the vicinity of the massive planets, creating rings. Some are dispatched to the far reaches of the Solar System, forming the Oort Cloud ^? , a reservoir of comets ^? .

• Interactions between the planets will lead to more collisions, which explains the formation of the Moon and the axial tilt of Uranus. They can also repel each other and migrate into stable positions. The resonance phenomenon also plays a major role in modelling the orbits of the bodies in the Solar System that we can see today.

• Once formed, the planets can change places by being dragged into the gas disc and by repelling each other.

Circumstellar discs:

In addition to gas, the dust disc contains rocks and metals near the star, where it is hotter. Further from the star, the lower temperature enables gas to condense. As hydrogen (H) and oxygen (O) are highly abundant atoms, there are many planetesimals ^? at this location and the planets form more rapidly. The largest planet, Jupiter, was formed at this point, called the frost line, while the other giant planets, made of gas and ice, are beyond this limit.

Several circumstellar discs have been observed and will make it possible to verify whether the above scenario – written for our Solar System – is the same elsewhere in the Galaxy ^? ?. Observation of the disc around HL Tau, an extremely young star, shows that planet formation begins very early on in the life of the star.

The life of the planet will then be primarily governed by the life and death of its star.