Surface Imaging of Proxima b and Other Exoplanets: Albedo Maps, Biosignatures, and Technosignatures

Seeing oceans, continents, quasi-static weather, and other surface features on exoplanets may allow for detecting and characterizing life outside the solar system. The Proxima b exoplanet resides within the stellar habitable zone, possibly allowing for liquid water on its surface, as on Earth. However, even the largest planned telescopes will not be able to resolve its surface features directly. Here we demonstrate an inversion technique to indirectly image exoplanet surfaces using observed unresolved reflected light variations over the course of the exoplanet’s orbital and axial rotation: ExoPlanet Surface Imaging (EPSI). We show that the reflected light curve contains enough information to detect both longitudinal and latitudinal structures and to map exoplanet surface features. We demonstrate this using examples of solar system planets and moons, as well as simulated planets with Earth-like life and artificial structures. We also describe how it is possible to infer the planet and orbit geometry from light curves. Then, we show how albedo maps of Proxima b can be successfully reconstructed for tidally locked, resonance, and unlocked axial and orbital rotation. Such albedo maps obtained in different wavelength passbands can provide “photographic” views of distant exoplanets. We estimate the signal-to-noise ratio necessary for successful inversions and analyze telescope and detector requirements necessary for the first surface image reconstructions of Proxima b and other nearby exoplanets using EPSI. This is a significant challenge, but the success of such measurements depends heavily on large-aperture diffraction-limited telescope performance—a feat that may be achieved on the ground before it is in space.