Imaging Venus-like Worlds: Spectral, Polarimetric, and UV Diagnostics for the Habitable Worlds Observatory

Abstract

Understanding planetary habitability requires a comparative approach that explores the divergent evolutionary outcomes of Earth and Venus. The Habitable Worlds Observatory (HWO) will be uniquely positioned to conduct a statistical and physical census of terrestrial exoplanets spanning the Venus Zone (VZ) and the Habitable Zone (HZ), enabling the detection and atmospheric characterization of post-runaway greenhouse worlds (``exoVenuses''). We present an updated list of VZ exoplanets, which raises the number of known candidates to 370. We describe a science case and an observing strategy for VZ exoplanets that integrates precursor exoplanet detection data and stellar characterization with HWO direct imaging, spectroscopy across the UV/optical/IR, and spectropolarimetry. Our proposed framework emphasizes a pathway toward the diagnosis of sulfur chemistry (SO${}_2$) and aerosol physics (H${}_2$SO${}_4$ clouds/hazes), planetary redox states (O${}_2$/O${}_3$ false positives from hydrogen loss), and cloud microphysics detection (rainbow polarization). We quantify implications for HWO requirements, including UV access to 0.2--0.4 $\mu$m, optical/NIR coverage to $\gtrsim$1.5 $\mu$m, inner working angle (IWA) reaching 0.3--1.5 AU around nearby Sun-like stars, and the SNR/resolution needed for key features. Finally, we outline a community-driven path to producing robust demographic inferences and target selection for optimizing HWO observations.

15 pages, 5 figures, 2 tables, accepted for publication in PASP