THE DIRECT DETECTABILITY of GIANT EXOPLANETS in the OPTICAL

Motivated by the possibility that a coronagraph will be put on the Wide-field Infrared Survey Telescope (WFIRST)/Astrophysics Focused Telescope Assets (AFTA), we explore the direct detectability of extrasolar giant planets (EGPs) in the optical. We quantify a planet's detectability by the fraction of its orbit for which it is in an observable configuration (f_obs). Using a suite of Monte Carlo experiments, we study the dependence of f_obs upon the inner working angle (IWA) and minimum achievable contrast (C_min) of the direct-imaging observatory; the planet's phase function, geometric albedo, single-scattering albedo, radius, and distance from Earth; and the semimajor axis distribution of EGPs. We calculate phase functions for a given geometric or single-scattering albedo, assuming various scattering mechanisms. We find that the Lambertian phase function can predict significantly larger f_obss with respect to the more realistic Rayleigh phase function. For observations made with WFIRST/AFTA's baseline capabilities (C_min, ∼ 10^-9, IWA ∼ 0.2), Jupiter-like planets orbiting stars within 10, 30, and 50 pc of Earth have volume-averaged observability fractions of ∼12%, 3%, and 0.5%, respectively. At 10 pc, such observations yield f_obs > 1% for low- to modest-eccentricity planets with semimajor axes in the range ∼2-10 AU. If C_min = 10^-10, this range extends to ∼35 AU. We find that, in all but the most optimistic configurations, the probability for detection in a blind search is low (< 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.