Diana Valencia

Internal structure of massive terrestrial planets

Abstract 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) 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 R ∝ 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 with a 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 R ∝ M ∼ 0.3 .

Detailed Models of Super-Earths: How Well Can We Infer Bulk Properties?

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 this paper we describe our detailed interior models for a rich variety of such massive terrestrial and ocean planets in the 1-10 M⊕ 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).

INEVITABILITY OF PLATE TECTONICS ON SUPER-EARTHS

The recent discovery of super-Earths (masses ≤ ) has initiated a discussion about conditions for habitable 10 M worlds. Among these is the mode of convection, which influences a planet’s thermal evolution and surface conditions. 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 1 regime disappear as mass M increases: super-Earths, even if dry, will exhibit plate tectonic behavior. Subject headings: Earth — planetary systems — planets and satellites: general

Massive terrestrial planets (super-Earths): detailed physics of their interiors

Planets in the mass range of 1–12 Earth masses do not exist in our Solar System, but now there is direct evidence that they orbit other stars. Near-future experiments will allow determination of their masses and radii with 5–10% uncertainty, their host star composition and age and, in some cases, planetary atmosphere properties. Current theoretical models will be capable of producing a limited and well-defined family of solutions for such planets (with no H–He envelopes) at the 2% level. This will allow us to understand the physical properties of super-Earths as a class provided we discover a large enough sample and preferably around small stars.

Habitability from Tidally Induced Tectonics

The stability of Earths climate on geological timescales is enabled by the carbon-silicate cycle that acts as a negative feedback mechanism stabilizing surface temperatures via the intake and outgas of atmospheric carbon. On Earth, this thermostat is enabled by plate tectonics that sequesters outgassed CO2 back into the mantle via weathering and subduction at convergent margins. Here we propose a separate tectonic mechanism -- vertical recycling -- that can serve as the vehicle for CO2 outgassing and sequestration over long timescales. The mechanism requires continuous tidal heating, which makes it particularly relevant to planets in the habitable zone of M stars. Dynamical models of this vertical recycling scenario and stability analysis show that temperate climates stable over Gy timescales are realized for a variety of initial conditions, even as the M star dims over time. The magnitude of equilibrium surface temperatures depends on the interplay of sea weathering and outgassing, which in turn depends on planetary carbon content, so that planets with lower carbon budgets are favoured for temperate conditions. Habitability of planets such as found in the Trappist-1 may be rooted in tidally-driven tectonics.