MIGRATING NEPTUNE-CLASS BODIES AS A SOURCE OF LARGE TERRESTRIAL PLANETS

摘要

After the discovery of more than 100 exosolar Jupiter-class planets the detection of Neptune-size and large terrestrial-like bodies will be the next major step in the search for exoplanets. Space-bome telescopes like COROT and Eddington using high precision photometry and the transit technique, will have the capability to detect exoplanets with sizes of 1.5-4 Earth radii at distances between 0.3-1 au. Current theoretical models indicate that the discovered large exoplanets orbiting close to their central star may have either migrated inward from greater distances, or may have formed at their present orbit. In recent studies of close-in giant exoplanets the radiative effective temperature, which is not physically relevant for atmospheric loss processes was used to estimate atmospheric evaporation rates. Therefore, these studies lead to significant underestimations of thermal atmospheric escape rates with values ≤ 10~3 g s~(-1) and to conclusions of long-term atmospheric stability. However, the exosphere temperature, which controls the thermal escape in an upper atmosphere, is usually much higher than the effective temperature, since upper planetary atmospheres are mainly controlled by absorption of X-rays and extreme ultraviolet (XUV) radiation. In this study, a scaling relation from solar system planets is used to estimate the exospheric temperature for exoplanets. This relation is based on the assumption of equilibrium between the XUV heat input and downward heat transport by conduction. We found that large exospheric temperatures, which are typical for hydrogen-dominated thermospheres, develop at close orbital distances to their host stars. These exosphere temperatures lead to hydrodynamic energy limited escape. Further, we estimate the protection effect of upper atmospheres due to an assumed intrinsic planetary magnetic field and simulate atmospheric ion pick up fluxes by a test particle model, which was successfully applied on Venus and Mars. We found that "Hot Neptune's" may lose their entire hydrogen atmospheres by thermal and non-thermal atmospheric escape processes and can evolve into a new type of terrestrial planet, after the development of secondary atmospheres by out-gassing their remaining ice-rocky cores. Moreover, our mass loss estimations applied to Jupiter-class exoplanets agree well with the recent H Lyman a detection of an extended exosphere at HD 209458b and its observation based estimated loss rate of about 10~(10) g s~(-1).