Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2K079

THEORETICAL MASS-RADIUS RELATIONSHIPS OF WATER-RICH LOW-MASS EXOPLANETS: EFFECTS OF THERMAL EVOLUTION AND MASS LOSS

Kurosaki, Kenji (The University of Tokyo)
Ikoma, Masahiro (The University of Tokyo)
Hori, Yasunori (National Astronomical Observatory of Japan)

Abstract:
Recent progress in observational techniques has enabled us to detect exoplanets down to the Earth in size. The mass-radius relationships for short-period super-Earths suggested the existence of water-rich planets, for example, GJ 1214b. Volatile inventories such as water are important clues to understanding the origin and evolution of low-mass planets. Close-in low-mass planets also experience mass loss due to stellar X-ray and EUV (XUV) irradiation. Previously, several studies have examined atmospheric escape of close-in planets with hydrogen-rich envelopes. However, it is not obvious how efficiently close-in water-rich planets lose their water mantle via the energy-limited escape. In this study, we aim to investigate mass-radius relationships for water-rich planets under intense irradiation conditions. We consider a three-layered planet at 0.01-0.1AU around solar-type stars: a rocky core surrounded by a water mantle with a water-vapor atmosphere. We have simulated thermal evolution of water-rich planets for 10Gyr, including the effect of mass loss driven by XUV flux. We have found that water-rich planets above a minimum mass can maintain water mantles, regardless of initial water content from 0% to 100%. The threshold value decreases with semi-major axis: several Earth-mass at 0.03AU and sub-Earths at 0.1AU. Our results predict a habitat area for water-rich planets orbiting solar- type stars in a planetary mass/radius-period diagram. This helps us know the origin and planetary compositions of close-in low-mass planets from the Kepler and other planet surveys.

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