From the time of the first discoveries, the properties of exoplanets contradicted the scheme that had been developed of the formation of the Solar System. For example, several planets were discovered 0.05 AU from their star, where the temperature of the protoplanetary disk would not allow ices to condense, nor even refractory materials such as silicates. So the formation of a solid core was not possible. Yet a number of extrasolar planets were found at this distance from their stars (see Chap. 4).
Some authors have suggested other mechanisms for forming a planet very close to a star. Even if these models are dynamically possible, they show that a planet would not be able to cool down, and would lose its atmosphere by evaporation. By contrast, studies show that a planet that formed and had time to cool, and thus contract, far from the star, could then migrate down to 0.05 AU and remain stable. This stability only relates to the core of a planet. The orbital stability of these planets, in contrast, is far less understood (see below).
It is, therefore, generally assumed that most of the giant planets observed close to their parent stars formed like the planets in the Solar System, i.e., outside the ice line, and then migrated to their current positions. To confirm this theory, we still need to find the one or more mechanisms responsible for this migration, and find indications proving that this hypothesis is relevant.
It is important to note that the observations are explained by migrations towards the interior of planetary systems, but several of the mechanisms envisaged by theorists lead to migrations that are either towards the interior of a system or toward the exterior, depending on the initial parameters.
It should also be noted that the discovery of numerous exoplanets with short periods arises from an observational bias: planets with periods greater than the age of the earliest observations have yet to be discovered. In 2007, such observations are 13 years old, in other words the period of a planet close to the ice line, in a system similar to the Solar System. So observations are only just starting to probe the natural domain of giant planets.
So it is possible that the mechanism of migration towards the interior of a planetary system is not a universal phenomenon, and that the exoplanets discovered in the near future will correspond to systems without migration, or even migration towards the outside of planetary systems.
The mechanism most frequently advanced to explain the migration of exoplanets proposes an interaction between the planet and a disk of gas. This mechanism, which was devised to account for planetary rings, occurs within the protoplanetary disk before the disk of gas disperses. The interaction takes the form of a couple between the disk and the planet. This transfer may result, under certain conditions, in a decrease in the semi-major axis of the planet's orbit.
For a planet of low mass with respect to the mass of the disk, the migration is known as Type I migration. The timescale of the migration is:
Was this article helpful?