Radius and structure models of the first super-earth planet

With improving methods and surveys, the young field of extrasolar planet studies has recently expanded into a qualitatively new domain - terrestrial ( mostly rocky) planets. The first such planets were discovered during the past year and a half, judging by their measured masses of less than 10 M+ (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 compositions can be completely terrestrial, or they may harbor an extensive ocean ( water and ices) above a rocky core. We model the structure and properties of the first super-Earth ( mass similar to 7.5 M+), discovered in 2005, illustrating the possible compositions and providing radius evaluations in view of future detection of similar planets by transits. We find that there exists a threshold in radius above which a super-Earth most certainly has an extensive water content. In the case of GJ 876d, this threshold is at about 12,000 km. Our results show that unique characterization of the bulk composition of super-Earths will be possible with future transit studies.