The power of polarimetry for characterising exoplanet atmospheres, clouds, and surfaces with NASA's Habitable Worlds Observatory

TL;DR

The paper argues that incorporating polarimetric capability into NASA's Habitable Worlds Observatory would substantially enhance exoplanet atmosphere and surface characterization by providing stronger constraints on clouds and surface types beyond total-reflectance spectra. It outlines a plan combining coronagraphy, high-resolution imaging, and potential spectropolarimetry to exploit phase curves and polarized reflected light, with the goal of breaking degeneracies in atmospheric and surface models. Key contributions include a detailed justification for polarimetry, an observational and instrument-development roadmap, and a strong case for UK leadership in both theory and hardware development (e.g., Pollux, HRI). The work underscores the practical impact on habitability assessment and international collaboration, positioning the UK to play a central role in delivering polarimetric capabilities for HWO and related missions.

Abstract

The Habitable Worlds Observatory (HWO), planned for launch in the 2040s, represents the next major step in exoplanet characterisation. HWO will, for the first time, enable detailed studies of the atmospheres and surfaces of Earth-like exoplanets through high-contrast reflection spectroscopy across the UV, optical, and near-infrared. These wavelength ranges provide access to key molecular absorption features, including O2, O3, H2O, CO2, and CH4, as well as potential surface biosignatures such as the vegetation red edge or ocean glint, making HWO a cornerstone mission for assessing planetary habitability. Clouds are a dominant factor in determining planetary climate and observability, yet their properties remain highly degenerate when constrained using reflected flux alone. Spectropolarimetry, a measure of the polarisation state of reflected light as a function of wavelength and orbital phase, provides a powerful complementary diagnostic. Polarisation is highly sensitive to cloud particle size, composition, shape, vertical distribution, and surface type, enabling degeneracies between atmospheric and surface models to be broken. Numerous studies have demonstrated the value of polarimetry for characterising a wide range of exoplanets, from hot Jupiters to cooler potentially habitable worlds. HWO's proposed instrument suite includes a coronagraph, a high-resolution imager, and a candidate high-resolution spectropolarimeter, offering multiple pathways to exploit polarimetry across diverse planetary regimes. This white paper argues that incorporating polarimetric capability into HWO instruments would significantly enhance the mission's scientific return. We highlight the unique opportunity for UK leadership in both instrument development and theoretical modelling, and advocate for a strong UK role in shaping HWO's polarimetric capabilities to maximise its impact on exoplanet science.

Figures (3)

• Figure 1: The reflectance spectrum detailing features from O$_2$, O$_3$, H$_2$O, CO$_2$, and CH$_4$ of a modern Earth-like planet from 0.2 - 2 $\mu$m, similar in range expected from HWO (adapted from Westall2024).
• Figure 2: The reflected flux (left) and degree of polarisation (right) (i.e. proportion of the flux that is linearly polarised at a given wavelength) of a Jupiter-like exoplanet for three model atmospheres at an orbital phase of 90$^{\circ}$. Model 1: gas only (no clouds), model 2: the same as model 1 plus a tropospheric cloud layer, model 3: the same as model 2 plus a stratospheric haze layer (from 04Stam).
• Figure 3: The reflected flux (upper panels) and degree of polarisation (lower panels) as a function of orbital phase of a Hot-Jupiter exoplanet for model atmospheres containing clouds made up of different materials: Al$_2$O$_3$ (right), FeO (middle), and Mg$_2$SiO$_4$ (right). Different wavelengths are shown for each model. This figure is adapted from 23ChStHe.