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

TL;DR

This paper argues that Earth and Venus offer a natural laboratory for climate evolution, and that a statistical census of Venus Zone and Habitable Zone terrestrial exoplanets will illuminate the occurrence of post‑runaway greenhouse states. It proposes an observing program for the Habitable Worlds Observatory that combines direct imaging, UV/optical/NIR spectroscopy, and spectropolarimetry to diagnose sulfur chemistry, haze/cloud microphysics, and redox states, aided by precursor exoplanet data and Solar System Venus mission context, to detect signatures such as SO$_2$ and H$_2$SO$_4$ clouds. End‑to‑end retrievals with radiative transfer modeling (e.g., PSG) are used to define spectral requirements and demonstrate detectable signatures even in hazy atmospheres, including Venus‑like atmospheric pressures ($P_s\approx 10$ bar). The work outlines a path to demographic inferences, discovery yield, and robust biosignature interpretation, leveraging synergy with Venus missions to anchor exoplanet inferences in Solar System scale measurements.

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 $μ$m, optical/NIR coverage to $\gtrsim$1.5 $μ$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.

Figures (5)

• Figure 1: The distribution of currently known exoplanets from the NASA Exoplanet Archive that are in the VZ and have radii $R_p \lesssim 2$$R_\oplus$. The inner VZ boundary is indicated by the red line on the left, and the outer VZ boundary is shown on the right. As of November 2025 there are 370 potential exoVenuses, and this number will continue to grow as there are over 7000 TESS planet candidates yet to be confirmed. The VZ will be calculated and translated against the IWA for all proposed HWO priority targets.
• Figure 2: Properties of VZ candidates and their host stars. Left: Host star V magnitude, angular separation of the planet, and incident flux received by the planet. The two vertical dashed lines indicate an IWA of 50 and 100 mas. Right: Distance to the planetary systems and the semi-major axis of the planet. Systems where the planet is known to transit are shown in blue, otherwise they are shown in red.
• Figure 3: Modeled JWST observations of an exoplanet with a 10-bar, Venus-like atmosphere orbiting an M-type star. Left: A modeled transit spectrum of the exoplanet, with and without haze, if observed using JWST NIRSpec PRISM. The opaque haze layer truncates much of the spectrum, making CO$_2$ the only potentially detectable absorption feature. Right: A modeled eclipse spectrum of the exoplanet using JWST MIRI. The presence of an atmosphere may be able to be deduced from this spectrum, although the haze would inhibit estimates of the atmosphere's composition.
• Figure 4: Simulated HWO spectra of an exoplanet with a 10-bar, Venus-like atmosphere. The upper panels assume the planet is orbiting an M-type star, whereas the lower panels assume a solar-type star. Top-left: UV and optical transmission spectrum, showing SO$_2$ absorption. Top-right: UV and optical reflectance spectrum during secondary eclipse. Bottom-left: Reflectance spectrum at optical and NIR wavelength from direct imaging observations with HWO. Bottom-right: Reflectance spectrum at UV and optical wavelengths from direct imaging observations with HWO.
• Figure 5: Simulated unpolarized reflectance (left) and signed degree of linear polarization (right) spectra for a Venus-like exoplanet around a solar-type star. The optically thick H$_2$SO$_4$ clouds flatten both spectra and suppress the NIR spectral features in the polarization, while the CO$_2$ Rayleigh scattering strongly polarizes the light at the shorter wavelengths.