Exoplanet characterization with NASA's Habitable Worlds Observatory

TL;DR

The paper advocates leveraging NASA's Habitable Worlds Observatory to achieve direct, high-contrast spectroscopic characterization of temperate exoplanets, enabling detection of biosignatures and habitability indicators across a UV–visible–IR range. It outlines a multi-faceted observational strategy (coronagraphic reflected-light spectroscopy, transit-imaging spectroscopy, phase curves, polarimetry, and high-resolution UV spectroscopy) and identifies the critical technical and scientific developments required for readiness. It emphasizes the UK's strategic role, including hardware contributions, laboratory astrophysics, and modelling, supported by sustained funding and international collaboration to maximize return from the mission. The work underscores the transformative potential of HWO for exoplanet atmospheres, climate, and biosignature research, with broad implications for science leadership and economic benefits through UK industry involvement and long-term research investments.

Abstract

Exoplanet atmosphere characterization has seen revolutionary advances over the last few years, providing us with unique insights into atmospheric chemistry, dynamics and planet formation mechanisms. However, true solar system analog planets remain inaccessible. A major goal for exoplanet science over the coming decades is to observe, and characterize, temperate rocky planets and cool gas giants in orbit around solar-type stars, with the prospect of detecting signs of habitability or even life. Characterization and categorization of these planets relies on direct spectroscopic observations capable of identifying molecular species in their atmospheres; however, these observations represent a substantial engineering challenge due to the extreme contrast between a temperate, Earth-sized exoplanet and its parent star. NASA's next flagship mission, the Habitable Worlds Observatory (HWO) - planned for launch in the mid-2040s - will boast a coronagraphic instrument capable of reaching the needed 10$^{-10}$ contrast, on an ultrastable platform enabling long integration times to achieve the required signal to noise. HWO will cover near-ultraviolet to the near-infrared wavelengths, enabling detections of key biosignature molecules and habitability indicators such as ocean glint and a vegetation `red edge'. Via early involvement in this groundbreaking observatory, including a potential UK instrument contribution, the UK exoplanet community now has an important opportunity to influence the telescope's design. To maintain our international competitiveness, we must be at the forefront of observational campaigns with HWO when it eventually launches, and this comes with the need for parallel development in laboratory astrophysics and computational modelling. Maximising our exploitation of this transformative NASA mission requires consistent financial support in these areas across the next two decades.

Figures (2)

• Figure 1: Figure 1 is taken from schwieterman "Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life", Figure 4, published in Astrobiology and reproduced with permission. A synthetic UVO-optical Earth radiance spectrum at quadrature phase (half illumination) in terms of geometric albedo. This spectrum was generated by the VPL 3D spectral Earth model robinson2011schwieterman2015. Strong absorption features from O$_2$, O$_3$, H$_2$O, CO$_2$, N$_2$O, and CH$_4$ are labeled, in addition to Rayleigh scattering and the location of the vegetation red edge (VRE).
• Figure 2: Figure 2 is taken from the Science Case Development Document https://docs.google.com/document/d/18rb6H7SNd15Is_NzoQkmoaqEocDkcVt5/edit#heading=h.rxylahxbnjx8. This is a simulated reflected light spectrum in the optical and ultraviolet showing the observable effects of SO$_2$ and H$_2$SO$_4$ clouds. The figure is reproduced here with the permission of S. R. Kane.