Protostars and Planets VI, Heidelberg, July 15-20, 2013
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)
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.
Click here to view poster PDF