Hot super-Earths with an hydrogen-rich atmosphere. The study: “Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence” of D. Modirrousta-Galian (INAF-OAPA) recently appeared on ApJ
“Super-Earths” are rocky exoplanets with a mass between that of Earth and Uranus. Some of these planets may also have a close orbit around the parent star. For instance, the super-Earth 55 Cancri e has a mass of 8.6 Earth masses and it orbits at a distance of 0.016 AU (Astronomical Unit, which is the average distance between Earth and the Sun, equal to 150 million km; as a comparison, Mercury orbit has a minor semi axis equal to 0.3 AU). At such a close orbit, these planets are very hot, with surface temperature exceeding 1000 degrees. Besides, they are irradiated by intense UV radiation from their stars.
Because of these properties, hot super-Earth are not expected to host any atmosphere. Anyway, observations provide contradictory evidence. In fact, we know both hot super-Earths that host an hydrogen-rich atmosphere, such as 55 Cancri e, that is know to have an atmosphere despite the temperature on the day-side exceeds 2000 degrees, and super-Earths that do not have any atmosphere, such as CoRoT-7b.
The model developed by the astronomer D. Modirrousta-Galian (INAF – Astronomical Observatory of Palermo), and described in the paper: “Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence” recently appeared on The Astrophysical Journal, suggests an explanation why planets such as 55 Cancri e still host an hydrogen-rich atmosphere. The main requirement for this model is that the planet becomes tidally-locked (e.g. with synchronous rotation and revolution, showing always the same face to its star such as the Moon with Earth) before loosing completely its atmosphere. If this happens, two phenomena may slow down its evaporation. First, the atmosphere must be enriched of heavy elements by the magma ocean on the planet. These heavy atoms feel a strong centrifugal force which may exceed the force due to the pressure gradient from the day- to the nigh-side of the planet. Light atoms, such as hydrogen, must thus diffuse across the heavy elements to flow from the night-side to the day-side, and this slows down the evaporation of the planet atmosphere. Besides, from this model it is also possible to evaluate the time before the planet has become tidally locked. The astronomers D. Locci and G. Micela (INAF – Astronomical Observatory of Palermo) have contributed to this work.
The figure (click here to see the entire picture) shows a simplified model of the atmosphere of a super-Earth at the conditions described by the model.
by Mario Giuseppe Guarcello ( follow mguarce)