An exploration of expected exoplanets for a 6m class direct imaging space observatory

The Astro2020 Decadal Survey recommended as the next strategic astrophysics mission a 6 m class space telescope capable of high-contrast direct imaging of Earth-size exoplanets in about one hundred habitable zones of nearby sun-like stars.(1) The expected number of imageable exoplanets for such a telescope depends on the architecture and the metrics used to evaluate those architectures. In this paper, we assess the yield of notional coronagraph-only, starshade-only, and hybrid starshade/coronagraph architectures for several metrics. We evaluate the exoplanet yield for a 20% bandwidth, SNR=5, R=70 water search metric; a 20% bandwidth, SNR=8.5, R=140 oxygen search metric; and a 4x20% bandwidth metric, SNR=8.5, R=7 for 450-700 nm and R=140 for 700-1000 nm, which is tailored for a coronagraph's sequence of 20% bandwidth sub-spectra. We bound the number of expected exoplanets by considering three cases of a priori knowledge: the case of no prior knowledge that requires a photometric blind search for exoplanets; the theoretical case of perfect prior knowledge that skips the photometric blind search and performs only spectral characterizations using realistic mission scheduling constraints (this approach shows the upper bound and when target exhaustion is reached); and a case of partial prior knowledge via a hypothetical, future extreme precision radial velocity instrument with 3 cm/s sensitivity. This work is an initial study of the potential exoplanet science return for the Decadal-recommended large infrared/optical/UV Great Observatory (IROUV).