Optimally planning and interpreting reflected-starlight observations of exoplanets through a multi-technique approach

Abstract

Directly imaging exoplanets in reflected starlight will enable the atmospheric characterization of long-period exoplanets, which are not accessible to current techniques. The launch of the Roman Space Telescope (planned in the mid-2020s) will pave the way for a future generation of telescopes both in space and on the ground (LUVOIR, HabEx, ELTs...) whose ultimate goal is to directly image Earth-like planets in their habitable zones. However, in order to both plan and interpret direct-imaging observations, a multi-technique approach will be needed. For instance, estimating the inclination of the orbit will affect the prospects of detecting the planet. In this talk, we show with particular examples such as Proxima Centauri c how current astrometric measurements of the orbital inclination determine the detectability of exoplanets. We also find that the characterization of atmospheric properties from directly-imaged spectra of exoplanets will be affected by nonatmospheric parameters such as the planet radius. Through atmospheric retrieval exercises we show that, if the planet radius is constrained, it would be possible to constrain the abundance of gaseous species and detect the presence of clouds. On the other hand, if the planet radius remains unknown it will be challenging to distinguish between cloudy and cloud-free atmospheres. This will be the most frequent scenario, given that most long-period planets will not be observed in transit. Hence, we show that the combination of radial velocity and astrometry campaigns will help plan a target list for future direct-imaging telescopes. In addition, to make the most of direct imaging for atmospheric characterization, our retrieval exercises also motivate long-baseline transit missions that enable to measure the planets' radius.