THE NATURE OF THE ATMOSPHERE OF THE TRANSITING SUPER-EARTH GJ 1214b

The newly discovered planet GJ 1214b is the first known transiting super-Earth requiring a significant atmosphere to explain its observed mass and radius. Models for the structure of this planet predict that it likely possesses an H-He envelope of at least 0.05% of the total mass of the planet. However, models without a significant H-He atmosphere are not entirely ruled out by the available data. Here, we explore a range of possible atmospheres for the planet, ranging from solar composition gas, to pure CO(2) or water (steam). We present transmission and emission spectra for each of these cases. We find that, if GJ 1214b possesses a hydrogen-rich atmosphere as expected, then the primary transit depth for such an atmosphere would vary at a level of up to 0.3% as a function of wavelength, relative to the background light of its M-dwarf host star. Observations at this level of precision are potentially obtainable with current space-based instrumentation. Successful detection of the transmission signature of this planet at the similar to 0.1% level would therefore provide confirmation of the hydrogen-rich nature of the planetary atmosphere. It follows that transmission spectroscopy at this level of precision could provide a first glimpse into answering the question of whether planets in the super-Earth mass regime (1-10 M(circle plus)) more closely resemble large terrestrial planets or small gas giant planets.