Exo-Earth yield of a 6 m space telescope in the near-infrared

The Astro2020 Decadal Survey recommended a “future large IR/O/UV telescope optimized for observing habitable exoplanets and general astrophysics” that would “search for biosignatures from a robust number of about 25 habitable zone [exo]planets,”¹ now dubbed the Habitable Worlds Observatory (HWO). The search for biosignatures requires high quality spectra over a broad bandwidth and sufficient signal-to-noise. The combination of wavelength, spectral resolution, bandwidth, and signal-to-noise-ratio impacts the number of exo-Earths that can be spectrally characterized. Previous work (Morgan et al. 2022)² evaluated the number of Earth-size, habitable zone exoplanets (denoted here as yield) that could be spectrally characterized over the wavelength range of 500-1000 nm for a 6-m diameter exoplanet direct imaging mission for coronagraph-only and hybrid coronagraph-starshade architectures for three prior knowledge cases: the nominal case of a blind-search survey, the upper-bound case of perfect prior knowledge, which is useful to determine if target depletion occurs, and the partial prior knowledge case of a hypothetical extreme precision radial velocity survey. In this paper, we extend previous exoplanet yields to include wavelengths out to 1.8 microns. Because the IWA for coronagraphs is proportional to wavelength, the achievable spectral coverage will be different for every planet detected. We present the spectral coverage achieved across individual target stars, as well as the ensemble target set, for a coronagraph-only architecture and a hybrid coronagraph + starshade architecture. We use the three prior knowledge cases. The coronagraph spectral characterization is simulated in the near-infrared for each of the 10% sub-bands individually, as if it were the only spectral characterization performed during the mission, and then as a broadband spectral characterization performed in sequence over the sub-bands. The starshade achieves the broadband spectral equivalent simultaneously. We also examine the capabilities of the 60 m starshade point design and investigate the benefits of refueling.