The recent discovery of super-Earths (masses less or equal to 10 earth-masses) has initiated a discussion about conditions for habitable worlds. Among these is the mode of convection, which influences a planets thermal evolution and surface condition
s. On Earth, plate tectonics has been proposed as a necessary condition for life. Here we show, that super-Earths will also have plate tectonics. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. These effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-Earths, even if dry, will exhibit plate tectonic behaviour.
The field of extrasolar planets has rapidly expanded to include the detection of planets with masses smaller than that of Uranus. Many of these are expected to have little or no hydrogen and helium gas and we might find Earth analogs among them. In t
his paper we describe our detailed interior models for a rich variety of such massive terrestrial and ocean planets in the 1-to-10 earth-mass range (super-Earths). The grid presented here allows the characterization of the bulk composition of super-Earths detected in transit and with a measured mass. We show that, on average, planet radius measurements to better than 5%, combined with mass measurements to better than 10% would permit us to distinguish between an icy or rocky composition. This is due to the fact that there is a maximum radius a rocky terrestrial planet may achieve for a given mass. Any value of the radius above this maximum terrestrial radius implies that the planet contains a large (> 10%) amount of water (ocean 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 measur
ed 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.
Planetary formation models predict the existence of massive terrestrial planets and experiments are now being designed that should succeed in discovering them and measuring their masses and radii. We calculate internal structures of planets with one
to ten times the mass of the Earth (Super-Earths) in order to obtain scaling laws for total radius, mantle thickness, core size and average density as a function of mass. We explore different compositions and obtain a scaling law of $Rpropto M^{0.267-0.272}$ for Super-Earths. We also study a second family of planets, Super-Mercuries with masses ranging from one mercury-mass to ten mercury-masses with similar composition to the Earths but larger core mass fraction. We explore the effect of surface temperature and core mass fraction on the scaling laws for these planets. The scaling law obtained for the Super-Mercuries is $Rpropto M^{sim0.3}$.
