Planet Formation in Realistic Protostellar Disc Models

One of the most important and recent discoveries in astronomy has been of planets orbiting stars other that the Sun, and this has led scientists to ask the question: how did these planetary systems and our own Solar System form ? This proposal is to address exactly this question. There are a number of competing ideas about how planetary systems come into being, but one thing they have in common is the belief that planets form out of the discs of gas and dust that are observed around young stars. To understand how this happens, we first need to know about the environment in which planet formation occurs, and the first part of the proposed research is to use parallel super computers to simulate sophisticated models of so-called protoplanetary discs. These simulations will take into account a number of new physical processes that have not been included in models of this type so far: heating by radiation from the central star, chemical effects, and the cooling of the disc by radiation from its surface. Once the new disc models have been computed, we will use further super computer simulations to study a number of outstanding questions that scientists have about the way in which planets form. The first question to be examined is the effect that turbulence in the disc has on the movement of small bodies called planetesimals that form in the early stages of planetary growth. The situation here is analogous to an aeroplane travelling through turbulence in the Earth's atmosphere, and the planetesimals are similarly jostled about. If this jostling becomes too severe, then the time taken for planetesimals to grow into planets becomes very long, and may make it difficult to form a completed planetary system in the required time of a few million years, which is the time for which the discs around young stars are observed to exist. It is obviously very important to check that our theory of how planet formation occurs is able to form planets in the required time. The next question we will look at is how the orbits of low mass planets evolve when they form in these discs. These planets will have a mass between that of the Earth and 50 times the mass of the Earth, and current theory tells us that these planets spiral into the central star very rapidly when they form and interact with the disc. This is obviously a big problem, and we will examine whether this very fast inward migration is halted or slowed down in the new disc models. The last question we will examine is what happens when a gas giant planet like Jupiter or Saturn forms in a protoplanetary disc. A massive planet like this is able to gravitationally push the nearby gas in the disc away, thus forming a gap in the planet's vicinity. We wish to understand how this gap formation happens, and the structure of the disc near the planet. A gas giant planet becomes massive by taking gas from the disc which then forms an envelope around the planet. We will examine how this 'gas accretion' happens so that we can understand why planets become as massive as they are observed to be. A giant planet is also expected to slowly spiral in toward the central star by interacting with the surrounding disc. We will examine how quickly this inward migration happens so that we can understand the observations of planets orbiting other stars.