Interior structure models of GJ436b

¹ Institut für Physik, Universität Rostock, Universitätsplatz 3, 18051 Rostock, Germany
e-mail: nadine.nettelmann@gmx.de
² Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
³ Astrophysikalisches Institut und Universitäts-Sternwarte Friedrich-Schiller-Universität Jena, Schillergässchen 2-3, 07745 Jena, Germany

Received: 4 March 2009
Accepted: 6 August 2010

Abstract

Context. GJ436b is the first extrasolar planet discovered that resembles Neptune in mass and radius. Two more are known (HAT-P-11b and Kepler-4b), and many more are expected to be found in the upcoming years. The particularly interesting property of Neptune-sized planets is that their mass M_p and radius R_p are close to theoretical M − R relations of water planets. Given M_p, R_p, and equilibrium temperature, however, various internal compositions are possible.

Aims. A broad set of interior structure models is presented here that illustrates the dependence of internal composition and possible phases of water occurring in presumably water-rich planets, such as GJ436b on the uncertainty in atmospheric temperature profile and mean density. We show how the set of solutions can be narrowed down if theoretical constraints from formation and model atmospheres are applied or potentially observational constraints for the atmospheric metallicity Z₁ and the tidal Love number k₂.

Methods. We model the interior by assuming either three layers (hydrogen-helium envelope, water layer, rock core) or two layers (H/He/H₂O envelope, rocky core). For water, we use the equation of state H₂O-REOS based on finite temperature – density functional theory – molecular dynamics (FT-DFT-MD) simulations.

Results. Some admixture of H/He appears mandatory for explaining the measured radius. For the warmest considered models, the H/He mass fraction can reduce to 10^-3, still extending over  ~0.7R_⊕. If water occurs, it will be essentially in the plasma phase or in the superionic phase, but not in an ice phase. Metal-free envelope models have 0.02 < k₂ < 0.2, and the core mass cannot be determined from a measurement of k₂. In contrast, models with 0.3 < k₂ < 0.82 require high metallicities Z₁ < 0.89 in the outer envelope. The uncertainty in core mass decreases to 0.4M_p, if k₂ ≥ 0.3, and further to 0.2M_p, if k₂ ≥ 0.5, and core mass and Z₁ become sensitive functions of k₂.

Conclusions. To further narrow the set of solutions, a proper treatment of the atmosphere and the evolution is necessary. We encourage efforts to observationally determine the atmospheric metallicity and the Love number k₂.