Authors

Thérèse Encrenaz

Affiliations

LESIA, Observatoire de Paris

Abstract

Using our knowledge of solar-system planetary atmospheres, it is possible to derive a simple classification of exoplanets’ atmospheres, on the basis of a few parameters: their mass, their distance to their host star and the stellar type of this star. The key parameters are (1) the exoplanet’s mass versus the critical mass beyond which the planet’s gravity can accrete the surrounding protostellar gas (i.e. about 10 terrestrial masses), and (2) their effective temperature, which allows to locate them with respect to the snow line of the protostellar disk (i.e. the distance beyond which molecules condense in ices). This classification, based on thermochemical equilibrium, is however no more than a first-order approach as it does not take into account migration effects.

Infrared spectra of (exo)planets are characterized by two components : the reflected stellar component which typically peaks in the visible range, and the thermal emission which corresponds to the absorbed part of the stellar flux re-readiated by the planet. The peak of this emission ranges from about 1 μm (for an exoplanet close to its star) to the far-IR (in the case of Neptune-analogs). For interpreting the thermal emission of exoplanets, it is essential to know their thermal structure in order to identify emission and absorption signatures. Separating the bands requires, for most of the major expected species, a resolving power of at least 30. A higher resolving power (about 200) is required for resolving the 5-μm and 10-μm spectral regions.