11:00 〜 13:00
[PAE17-P06] The Lowest Mass Sub-Neptunes with Hydrogen-rich Atmospheres Identified from the Kepler's Multiplanetary Systems
キーワード：トランジット、トランジット時刻変動法、ケプラー宇宙望遠鏡、サブネプチューン、惑星大気、光蒸発
Radius and mass measurements of transiting planets have revealed the presence of short-period rocky planets with a mass less than ~3 Earth masses. Due to the high irradiation by their host stars, they could be the remnants of gas-rich planets that have experienced a complete atmospheric loss. However, it has been unclear if such low-mass planets could indeed acquire hydrogen-rich atmospheres from the protoplanetary disks, because their progenitors, which would be less massive than ~3 Earth masses and larger than ~1.8 Earth radii, have yet been firmly discovered. To search for such planets, we analized the Kepler's TTV systems consisting of two planets by the photodynamical modeling technique, identifying three planets (Kepler-29b, Kepler-29c, and Kepler-200c) that have masses lower than 4 Earth masses and compositions consistent with rocky cores and hydrogen-rich atmospheres. Among them, Kepler-29b and c have the lowest masses (2.3 and 1.9 Earth masses, respectively) among other known planets with masses well determined and with hydrogen-rich atmospheres. We estimate the lifetime of their atmospheres, which are subject to photoevaporation, to be ~10^8-9 yr (depending on the assumed XUV flux); this is comparable to or shorter than the typical age of field stars. We find that the lifetime can be an order of magnitude longer if the atmospheres contain substantial amount of water.