Direct imaging characterization of cool gaseous planets

TL;DR

This work articulates a compelling science case for the Habitable Worlds Observatory to directly image and spectroscopically characterize cool gas giants with $T_{ ext{eff}}<400$ K in reflected light. By combining UV–visible–near-IR spectra with polarimetry and phase-resolved measurements, the approach aims to constrain molecular abundances, cloud microphysics, and upper-atmosphere temperature structure across a broad range of planet sizes, separations, and host types, filling a gap between hot exoplanets and Solar System giants. The paper outlines an actionable science objective, a detailed parameter space (abundances, aerosols, temperature profiles, longitudinal albedo, and survey size), and a plan for observations and follow-up modeling to derive robust atmospheric retrievals, phase-resolved maps, and evolutionary insights. The anticipated results will advance formation theories, cloud physics, and photochemical processes under diverse irradiation environments, enabling meaningful cross-planet comparisons and placing the Solar System in a broader galactic context.

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.

Figures (13)

• Figure 2: Cloud composition at the $\tau=1$ surface as a function of wavelength and separation from the central star. Black regions are where the atmosphere is dominated by molecular opacity.
• Figure 3: The typical scattering properties of molecules, cloud and haze particles (upper panels) and the simulated total intensity and polarized intensity spectra of a typical Jupiter type planet. Model 1 represents a clear atmosphere, model 2 has a tropospheric cloud layer and model 3 has a stratospheric haze layer. Figures taken from Stam2004, used with permission.
• Figure 4: The observed UV-optical-near infrared spectrum of Jupiter, taken from Fletcher2023 “Jupiter Science Enabled by ESA’s Jupiter Icy Moons Explorer”, used with permission. Methane, ammonia, phosphine, acetylene and ethane are all accessible between 0.1 and 2.0 $\mu$m.
• Figure 5: Simulated contrast as a function of wavelength for a Jupiter sized planet around a 6000K host star at different distances. The upper left panel shows the contrast of the planet compared to the host star. The other panels display the difference of the spectrum when given species are removed from the atmosphere. This can be used to estimate the SNR needed to detect a certain molecule at a given wavelength (e.g. a contrast of 0.1 would require a SNR of 10 to be detected).
• Figure 6: As Fig. \ref{['fig:contrast']} but for the molecule H$_2$S.