Atmospheric characterization of cold exoplanets with a 1.5-m space coronagraph

Abstract

Several small space coronagraphs have been proposed to characterize cold exoplanets in reflected light. Studies have mainly focused on technical feasibility because of the huge star/planet flux ratio to achieve in the close-in stellar environment (108-1010 at 0.2'). However, the main interest of such instruments, the analysis of planet properties, has remained highly unexplored so far. We performed numerical simulations to assess the ability of a small space coronagraph to retrieve spectra of mature Jupiters, Neptunes and super-Earths under realistic assumptions. We describe our assumptions: exoplanetary atmosphere models, instrument numerical simulation and observing conditions. Then, we define a criterion and use it to determine the required exposure times to measure several planet parameters from their spectra (separation, metallicity, cloud and surface coverages) for particular cases. Finally, we attempt to define a parameter space of the potential targets. In the case of a solar-type star, we show that a small coronagraph can characterize the spectral properties of a 2-AU Jupiter up to 10 pc and the cloud and surface coverage of super-Earths in the habitable zone for a few stars within 4-5 pc. Potentially, SPICES could perform analysis of a hypothetical Earth-size planet around α Cen A and B.