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

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.

💡 Research Summary

The white paper makes a compelling case for incorporating spectropolarimetric capability into NASA’s upcoming Habitable Worlds Observatory (HWO), slated for launch in the 2040s. While JWST and the forthcoming Ariel mission have demonstrated the power of transit spectroscopy for probing atmospheric composition, they cannot directly access the reflected‑light spectra of Earth‑like planets that orbit at larger separations. HWO’s planned instrument suite— a high‑contrast coronagraph for imaging and low‑resolution spectroscopy, a high‑resolution imager (HRI) with a grism, and the candidate French‑led high‑resolution spectropolarimeter “Pollux”—will enable the first systematic study of reflected light from temperate, potentially habitable worlds across the UV, optical, and near‑infrared (0.2–2 µm).

The central scientific argument is that linear polarization of reflected light is exquisitely sensitive to cloud particle size, composition, shape, vertical distribution, and to surface properties such as ocean glint or vegetation red‑edge. Simulations presented in the paper show that models which are indistinguishable in total flux become clearly separable when the degree of linear polarization is measured, even at modest signal‑to‑noise. Polarization thus breaks the severe degeneracies that plague pure albedo spectra, allowing robust retrieval of cloud microphysics and surface signatures. This capability is essential for assessing planetary habitability, because clouds dominate the energy budget and can mask or mimic biosignature gases.

From a technical standpoint the authors outline three likely pathways for polarimetry on HWO. The coronagraph, led by NASA, will include an infrared channel for which the UK Astronomy Technology Centre (UKATC) is already developing a spectrograph design. Two UK‑funded studies aim to lead the HRI hardware, and the Pollux concept (R > 60,000, 0.12–1.8 µm) already involves UK scientists in both instrument design and modeling. The paper sets a performance target of detecting polarized signals at the 0.1–1 ppm level (relative to total flux), an order of magnitude better than the best current ground‑based polarimeters (e.g., HIPPI‑2 at 3.5 ppm). Achieving this will require low‑noise detectors, high‑precision optics, rigorous calibration strategies, and substantial high‑performance computing for forward modeling and retrieval.

The United Kingdom’s strategic contribution is framed in two complementary domains. First, hardware: the UK can provide key components for the coronagraph’s IR arm, the HRI optics, and potentially the polarization module of Pollux. Second, science and data: the UK community will supply high‑temperature molecular line lists, refractive indices for a variety of cloud condensates, and a comprehensive library of polarized reflectance spectra for Earth‑like planets at different evolutionary stages. These assets build on the UK’s leadership in Ariel, PLATO, and JWST MIRI, and will be essential for the mission’s design‑phase trade studies.

Timing is critical. All mission‑critical technologies must reach Technology Readiness Level 5 by the 2029 Mission Concept Review. The authors argue that the next decade is the window to mature polarimetric optics, detectors, and calibration hardware, while simultaneously developing the theoretical and laboratory data needed for interpretation. International collaboration is emphasized: partnerships with NASA, ESA, and European institutions (e.g., the VenSpec‑H polarimeter on ESA’s EnVision Venus mission) can share risk and leverage existing expertise.

In conclusion, spectropolarimetry is not a luxury add‑on but a scientific necessity for HWO to fulfill its goal of characterising the atmospheres, clouds, and surfaces of temperate exoplanets. By taking a leading role in both instrument development and theoretical modeling, the UK can secure a prominent position in the next generation of exoplanet exploration, train a new cohort of space‑science engineers, and drive technological innovation that will benefit the broader aerospace sector.