The Descent of Planets

Over 350 planets have been discovered outside our solar system. Most of these 'extrasolar planets' are similar in mass to Jupiter but have orbits that are much closer to their central star. Planet formation is common, with at least 5% of all Sun-like stars having a Jupiter mass planet in close orbit. Many more stars may have planets that are too small or too far from the host star to be detected with current techniques. The systems that are observed are surprisingly diverse and very different from our solar system. Observations cannot trace the full history of planet formation, but do provide snapshots of either early stages of a dusty gas disk orbiting a young star or the late stages after planetary systems or debris disks have formed. As a result, we cannot observationally connect the early and late stages of planet formation, and it is not known how such a diversity of extrasolar systems arises, or what determines the type of solar system that develops. The solution is to build a testable model that can evolve a broad range of protoplanetary disks (including, but not limited to, the observed protoplanetary disks) through to final planetary systems or debris disks. While the model should reproduce all of the observed end states, many of the final systems produced will not have been observed. Such a complete model of planet formation has eluded the astrophysics community because of numerical limitations and incomplete/unknown physics. In order to make the problem of planet formation more tractable the planet formation process is often divided into separate stages, which are then tackled in isolation. This method has had some success, for example, large (~100 km) planetesimals, the planet building blocks, grow into protoplanets (planetary precursers) of about a lunar mass, and simplified simulations go on to create planetary systems from an assumed initial distribution of protoplanets. However, important properties of our own Solar System are not explained, such as the near circular orbits of the rocky planets, nor the diversity of the extrasolar planets and debris disks. During the next five years I will develop sophisticated and realistic computer models of the planet formation process. In particular, I will show how planetesimals grow from km-sized rocks into lunar-sized protoplanets and ultimately entire solar systems or disks of debris. I will produce a catalogue of solar systems that can be compared with observations. This research will also explain details of our own solar system that have yet to be fully understood (rate of rocky-planet formation and water delivery). Ultimately I will be able to determine whether our solar system is unique, or has many counterparts throughout the Galaxy. The challenge to understand how solar systems and terrestrial planets form is very exciting, for these are the places where we know life can develop. Studying planet formation has a wide benefit to society - for thousands of years people have sought to understand the origin of the Earth and its place in the solar system, and whether the Earth is unique or merely one of a vast number of similar planets scattered throughout the Galaxy and Universe. On a more practical note, sophisticated numerical simulations of complex systems are widely applicable to industry, and these transferable skills will be learned by students in a University environment. Much longer term benefits of understanding the collisional evolution of our solar system include defence against catastrophic asteroid impacts.