Radius and Structure models for the First Super-Earth Planet

With improving methods and surveys, the young field of extrasolar planets has recently expanded into a qualitatively new domain - terrestrial (mostly rocky) planets. The first such planets were discovered during the past year, judging by their measured masses of less than 10 Earth-masses ($M_{\oplus}$) or Super-Earths. They are introducing a novel physical regime that has not been explored before as such planets do not exist in our Solar System. Their composition can be either completely terrestrial or harbour an extensive ocean (water and ices) above a rocky core. We model the structure and properties of the first Super-Earth (mass $\sim$ 7.5 $M_{\oplus}$) discovered in 2005, illustrating the possibilities in composition and providing radius evaluations in view of future detection of similar planets by transits. We find that a threshold in radius exists for which larger values indicate that a Super-Earth most certainly has an extensive water content. In the case of GJ876d this threshold is at about 12000 km. Our results show that unique characterization of the bulk composition of Super-Earths will be possible in future transit studies.