Chasing rainbows and ocean glints: Inner working angle constraints for the Habitable Worlds Observatory
Authors: Sophia R. Vaughan (Astrophysics, Department of Physics, University of Oxford), Timothy D. Gebhard (Max Planck Institute for Intelligent Systems), Kimberly Bott (Department of Earth and Planetary Sciences, University of California, Riverside), Sarah L. Casewell (Centre for Exoplanet Research, School of Physics and Astronomy, University of Leicester), Nicolas B. Cowan (Department of Earth and Planetary Sciences and Department of Physics, McGill University), David S. Doelman (Leiden Observatory, Leiden University), Matthew Kenworthy (Leiden Observatory, Leiden University), Johan Mazoyer (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris), Maxwell A. Millar-Blanchaer (Department of Physics, University of California, Santa Barbara), Victor J. H. Trees (Department of Geoscience and Remote Sensing, Delft University of Technology), Daphne M. Stam (Delft University of Technology), Olivier Absil (STAR Institute, Université de Liège), Lisa Altinier (Aix Marseille Université, CNRS, CNES, LAM), Pierre Baudoz (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris), Ruslan Belikov (NASA Ames Research Center), Alexis Bidot (Université Grenoble Alpes, CNRS, IPAG), Jayne L. Birkby (Astrophysics, Department of Physics, University of Oxford), Markus J. Bonse (ETH Zurich, Institute for Particle Physics and Astrophysics), Bernhard Brandl (Leiden Observatory, Leiden University), Alexis Carlotti (Université Grenoble Alpes, CNRS, IPAG), Elodie Choquet (Aix Marseille Université, CNRS, CNES, LAM), Dirk van Dam (Leiden Observatory, Leiden University), Niyati Desai (Department of Astronomy, California Institute of Technology), Kevin Fogarty (NASA Ames Research Center), J. Fowler (Department of Astronomy and Astrophysics, University of California, Santa Cruz), Kyle van Gorkom (Steward Observatory, University of Arizona), Yann Gutierrez (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris), Olivier Guyon (Steward Observatory, University of Arizona), Sebastiaan Y. Haffert (Steward Observatory, University of Arizona), Olivier Herscovici-Schiller (DTIS, ONERA, Université Paris Saclay), Adrien Hours (Université Grenoble Alpes, CNRS, IPAG), Roser Juanola-Parramon (NASA Goddard Space Flight Center), Evangelia Kleisioti (Leiden Observatory, Leiden University), Lorenzo König (STAR Institute, Université de Liège), Maaike van Kooten (National Research Council Canada, Herzberg Astronomy and Astrophysics Research Center), Mariya Krasteva (European Space Agency, ESTEC), Iva Laginja (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris), Rico Landman (Leiden Observatory, Leiden University), Lucie Leboulleux (Université Grenoble Alpes, CNRS, IPAG), David Mouillet (Université Grenoble Alpes, CNRS, IPAG), Mamadou N'Diaye (Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange), Emiel H. Por (Space Telescope Science Institute), Laurent Pueyo (Space Telescope Science Institute), Frans Snik (Leiden Observatory, Leiden University)
Abstract: NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimised for planets near quadrature, but HabWorlds' coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet's orbital inclination and/or the coronagraph's inner working angle (IWA). We use a recently-created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ${\sim}20-60^\circ$ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ${\sim}{46}$ systems, while the ocean glint signature at phase angles of ${\sim}130-170^\circ$ would be accessible in ${\sim}{16}$ systems, assuming an IWA${=}62$ mas ($3\lambda/D$). Improving the IWA${=}41$ mas ($2\lambda/D$) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.
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