系外行星上的熱潮汐作用

Abstract

Close-in exoplanets receive more stellar irradiation, implying that the thermal tides in these planets may not be negligible compared to the gravitational tides. To investigate the properties of the atmospheric thermal tides excited by the irradiation from their parent stars, we perform a linear analysis to study the atmospheric perturbations of exoplanets. The torque due to the thermal tides acting on the spin of the planet is estimated and compared to that due to the gravitational tides. In particular, we study the thermal tides in a hypothetical super-Earth with a hydrogen-rich atmosphere at about 0:47 AU from a Sunlike star and those in the radiative layer of the hot Jupiter HD 209458b. In the super-Earth case, the torques arising from both tides can be cancelled out in some ranges of forcing periods, inferring that the super-Earth may have been escaped from the stage of being tidally locked by its parent star. In the case of gaseous planets, the thermal bulges have been argued not to exist in the literature. However, our study suggests that the thermal bulges could form when the thermal variations of the atmosphere are not hydrostatically balanced by the redistribution of gas in the layer near the turning point of internal waves. In some ranges of forcing periods and Q-values, both torques can be cancelled out in the case of the hot Jupiter HD 209458b. The results imply that some of the close-in exoplanets, including both super-Earths and gaseous planets, may not lie in synchronously rotating states at their ages of serveral Gyrs.