Extrasolar Terrestrial Planets

• Extrasolar planetary host stars are known to be chemically unusual. They are enriched in key terrestrial planet forming elements such as Iron, Silicon, Magnesium and Oxygen. Presumably, if the host star is enriched then the planetary building blocks that formed out of the same molecular cloud would also be enriched in the same way. As these systems already host at least one planet, it is likely that terrestrial planet formation also occured within the system. This leads us to ask the question: If the stars are enriched in these elements, what would any terrestrial planets be like? Would they also be enriched? Would they be chemically similar to Earth or be completely different?

• To answer this questions, we have combined dynamical models of the formation of terrestrial planets by accretion with chemical models of the composition of solids present within the disc. This way we can not only simulate terrestrial planet formation but also make first order preditions about the bulk elemental composition of the planet itself. This approach resulted in us determining that there are two likely classes of planet present within these systems: thse that resemble Earth in terms of their bulk composition and those that are enriched in Carbon. The Carbon enrichment ranged from relatively minor enrichments (less that 5 wt% of the simulated planet) to being one of the dominant species (comprising up to 54 wt% of the simulated planet).

• These Carbon-rich planets are unlikely to behanve in a similar manner as Earth due to the drastically different properties of carbon phases. Gloabl plate tectonics, geothermal activity and atmospheric composition are some o the planetary processes and properties likely to be affected. Similarly, the spectral signal for a Carbon-rich terrestrial planet is expected to be drastically different to that of a Mg-silicate planet (like Earth).

Giant Planet Migration: Watering the Planets

• It is believed that the giant planets did not form in their current positions. Instead they formed elsewhere in the disc and migrated to their present location. As a result of this migration, solid material within the disc will have been moved around. Refractory-rich material from the hotter, inner disc will have been pushed out to the cooler outer disc while volatile rich material (rich in water ice, clathrates and other hydrates) will have been dragged in to the inner disc. Thus terrestrial planet compositions are likely to change drastically once we incorporate migration.

• Simulations are currently running to examine the effects of migration on the bulk elemental composition of extrasolar terrestrial planets. These simulations include both the species previously studied as well as addtional ices, clathrates and hydrates. It is hoped that we will not only gain a better understanding of how the Earth obtained its water but also help us to place constraints on which types of systems we should examine for other Earth-like planets (composed of Mg-silicate with liquid water also present).