Hydrodynamical models of Extrasolar Planets: Formation and Evolution

Planets form in discs of gas and dust that are commonly found around young stars. Within these discs, small dust grains collide with each other to create bigger dust particles. In a few million years, this process can lead to the formation of a full-sized planet like the Earth. If there is enough material available, more massive 'Superearths' can be formed. When they are massive enough, typically a few times the mass of the Earth, planets may start to attract gas from the surrounding disc, which eventually results in the formation of a gas giant planet like Jupiter, which is more than 300 times the mass of the Earth. Because more material is available far away from the star, we would expect gas giant planets to form far from the star, while Earth-like planets form closer in. This is indeed what we see in the Solar system. The first extrasolar planet, however, was a gas giant planet that was orbiting more than 20 times closer to its star than the Earth is to the Sun. These planets are now called 'Hot Jupiters'. They were a real surprise, but fortunately there was a relatively easy explanation: this planet was not formed where it is now; it has migrated inward from its place of birth under influence of the surrounding disc. Eventually the disc evaporates, and migration will stop. However, theoretical migration rates are much to fast: they predict that all planets should quickly plunge into the central star before the disc has time to evaporate. This is a serious problem, and it is important to understand the details of planetary migration in order to understand the formation of planets. Much progress has been made over the past few years in refining numerical models to come up with more realistic migration rates. Very recently, it became clear that modeling the detailed temperature structure of the disc is imminent. Many of the implications of these temperature effects remain to be resolved, and this is part of the proposed research. A few years ago, a new, very fast type of planetary migration was discovered. This runaway migration may take place in very massive discs, for intermediate mass planets (like Neptune), and because it is so fast it can easily come to dominate the overall outcome of migration. Because the disc is very massive, it should feel its own gravity in numerical models. However, this is very difficult to model because it is computationally very expensive. As part of this proposal, I would like to use the newest computational facilities to resolve this issue and study this very interesting type of migration in discs that feel their own gravity. Another interesting surprise regarding extrasolar planets was their occurence in binary stars. The planet orbits one of the stars, with the companion star orbiting around them. This poses a real challenge to planet formation theory, because the gravitational influence of the companion strongly disturbs the dust particles that are supposed to form planets. But not only are the dust particles disturbed, the gas is as well. Recent studies on planet formation in these interesting systems did not take into account the evolution of the gas. As part of this proposal, I would like to study the influence of real gas dynamics on planet formation in binary systems, to get a better picture of the possibility of forming planets in these hostile environments. Extrasolar planets are mostly found on very eccentric orbits, for which the disctance to the central star varies by more than a factor of 2 during one orbit. The Hot Jupiters, however, are all on circular orbits, and we think that this is due to tidal effects. The strong tides that are raised on a planet so close to the star slowly forces the planet into a circular orbit. During this process, a huge amount of energy is dumped into the atmosphere of the planet, and some may not even survive. I would like to study the effect of these tides on the structue and appearance of close-in extrasolar planets.