Principles of Planetary Formation and Migration

How are planets formed?

The most recognized theory in previous centuries was that of the “primordial nebula” – that is a huge gas and dust rotating cloud, from which the Sun and the planets would have formed.

The theory, known as the Solar Nebular Disk Model (SNDM) or the Solar Nebular Model today, basically states that a large, cold cloud of interstellar gas, composed of hydrogen, helium, and a small fraction of heavier elements present as dust, contracts under its own gravitational force. The collapse can be induced by a shock wave (for example, caused by the explosion of a supernova) passing through the cloud, or it can be spontaneous. The contraction lasts several million years, and the cloud starts rotating faster and faster. Its shape becomes that of a disk, whose diameter is about 10 billion kilometers and whose thickness is about 100 million kilometers.

A large amount of gas is stored in the cloud’s center and as the gravitational contraction heats it up, a protostar is born. An accretion disc is formed by the gas rotating around the star, and the gas slowly drops upon it, until the time (a few thousand years later) when the stellar wind develops. This is a gas flow from the protostar to the outer regions, which transfers part of its angular momentum to the disk’s gas.

Image Credit: HubbleSite

In the meantime, the heat and radiation released by the protostar and the gas flow vaporize the dust grains of the cloud. The protostar begins its evolution into a star, accreting the gas.

The disk starts cooling through energy radiation. Depending on the amount and distribution of gas, it can be gravitationally stable or unstable and form one or more new protostars. In such a way, a binary or multiple stellar system is formed.

Far from the star, the gas is cold enough so that part of the gas condenses into dust and ice. The dust grains merge due to collisions until they form small pieces of rock called planetesimals. Planetesimals merge and form protoplanets. The maximum size of protoplanets depend on their distance from the star and on the chemical composition of the primordial nebula. It is much smaller in the inner regions than in the outer regions since the protostar tends to disrupt and vaporize dust.

The difference in size between terrestrial planets and Jovian planets, something we’ll discuss further in a moment, supports this scenario. The protoplanet formation requires from about 100,000 to some 20 million years to occur.

At this time, the star begins emitting a strong wind which sweeps away the residual gas of the disk. If a protoplanet is massive enough, it can retain part of the gas, so a gaseous planet, otherwise known as a Jovian planet, will form. Otherwise, the gas will be stripped and a terrestrial planet will form.

The later evolution of the planetary system is governed by the collisions between its constituent bodies. Impacts of meteorites and planetesimals upon protoplanets and satellites, generate craters on their surfaces. Many of craters from this time are still visible today on planets within our own Solar System. A good example of this can be found on Mercury. When impacts are especially violent, they can even move the bodies out of their original orbit. Our Solar System went through this phase roughly 4-4.5 billion years ago.

After tens of million years, the last planetesimals are disrupted by collisions, and the star plus its planets becomes a dynamically stable system. A planetary system is thus born.

Basically, around 100 million years pass from the initial contraction of the cloud to this event. Some chemical and geological evidence collected has allowed scientists to date the formation of our Solar System to about 4.7 billion years ago.


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