Can WFIRST With an Optical Coronagraph Directly Image Jupiter Analogs?

The Direct Detectability of Giant Exoplanets in the Optical

Authors:

Greco et al

Abstract:

Motivated by the possibility that an optical coronagraph will be put on WFIRST/AFTA, we present an exploration of the general character of the direct detectability of extrasolar giant planets (EGPs) in the optical. We quantify a planet's direct detectability by the fraction of its orbit for which it is in an observable configuration--defined to be its observability fraction (fobs). Using a suite of Monte Carlo experiments, we study the dependence of fobs upon various technological and astrophysical parameters, including the inner working angle (IWA) and minimum achievable contrast (Cmin) of the direct-imaging observatory; the planet's scattering phase function, geometric albedo, single-scattering albedo, radius, and distance from Earth; and the semi-major axis distribution of EGPs. We assume cloud-free, homogeneous atmospheres and calculate phase functions for a given geometric or single-scattering albedo, assuming various scattering mechanisms. We find that the often-assumed Lambertian phase function can predict significantly larger fobs values with respect to the more physically motivated Rayleigh phase function. For observations made with WFIRST/AFTA's baseline coronagraphic capabilities (Cmin∼10−9, IWA∼0.2″), Jupiter-like planets orbiting stars within 10, 30, and 50 pc from Earth have volume-averaged (assuming a uniform distribution of stars) observability fractions of ∼12%, 3%, and 0.5%, respectively. Using a plausible estimate for the occurrence rate of EGPs, we find that, in all but the most optimistic configurations, the probability a blind search will lead to a detection is low (less than 5%). However, with orbital parameter constraints from long-term radial-velocity campaigns and Gaia astrometry, the tools we develop in this work can be used to determine both the most promising systems to target and when to observe them.