Direct imaging characterization of cool gaseous planets

TL;DR

This paper advocates for the Habitable Worlds Observatory to directly image and analyze cool gas giant exoplanets below 400 K, aiming to understand their atmospheres and formation in unprecedented detail.

Contribution

It proposes a new observational approach using HWO for direct imaging and spectroscopy of cool gas giants, filling a significant gap in exoplanet characterization.

Findings

Enables characterization of exo-Jupiters in reflected light.

Provides insights into atmospheric composition and cloud properties.

Bridges observational gap between hot exoplanets and Solar System giants.

Abstract

Cool gas giant exoplanets, particularly those with properties similar to those of Jupiter and Saturn, remain poorly characterized due to current observational limitations. This white paper outlines the transformative science case for the Habitable Worlds Observatory (HWO) to directly image and spectroscopically characterize a broad range of gaseous exoplanets with effective temperatures below 400 K. The study focuses on determining key atmospheric properties, including molecular composition, cloud and haze characteristics, and temperature structure, across planets of varying sizes and orbital separations. Leveraging reflected light spectroscopy and polarimetry, HWO will enable comparative planetology of cool gas giants orbiting both solar-type and M-dwarf stars, bridging the observational gap between hot exoplanets and Solar System giants. We present observational requirements and survey strategies necessary to uncover correlations between atmospheric properties and planetary or stellar parameters. This effort will establish critical constraints on planetary formation, cloud microphysics, and the role of photochemistry under diverse irradiation conditions. The unique capabilities of HWO will make it the first facility capable of characterizing true exo-Jupiters in reflected light, thus offering an unprecedented opportunity to place the Solar System in a broader galactic context.