NASA recently announced the Habitable Worlds Observatory, a coronagraphic mission to detect rocky planets in their habitable zones, assess their habitability, and search for biosignatures. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase variations are one of the main ways we expect to be able to detect oceans, via specular reflections off the surface water. The range of scattering phases accessible for an exoplanet can be limited by its orbital inclination or the coronagraph’s inner working angle. We use the list of target stars for the Habitable Worlds Observatory to estimate the number of exo-Earths that could be searched for non-Lambertian scattering phenomena. Here we will present our methodology and the relationship between inner working angle and accessible phase angles. From these results, we quantify the number of systems for which we expect to be able to detect ocean glint (and other scattering processes), as a function of the accessible inner working angle.
To reduce the amount of stellar light for exoplanet detection, coronagraphs feature amplitude masks in pupils plane(s) and/or focal plane(s), where a large fraction of photons are stopped -- and generally not used. Here, we give an overview of where potentially useful stellar (and circumstellar) photons are lost. We review existing concepts that use these lost photons, and propose generic strategies to make use of them for various applications. We particularly focus on wavefront sensing applications, but also explore how these photons can be used for calibration measurements, or for additional scientific observations.
The detection and characterization of Earth-like exoplanets around Sun-like stars is a primary science motivation for the Habitable Worlds Observatory. However, the current best technology is not yet advanced enough to reach the 10−10 contrasts at close angular separations and at the same time remain insensitive to low-order aberrations, as would be required to achieve high-contrast imaging of exo-Earths. Photonic technologies could fill this gap, potentially doubling exo-Earth yield. We review current work on photonic coronagraphs and investigate the potential of hybridized designs which combine both classical coronagraph designs and photonic technologies into a single optical system. We present two possible systems. First, a hybrid solution which splits the field of view spatially such that the photonics handle light within the inner working angle and a conventional coronagraph that suppresses starlight outside it. Second, a hybrid solution where the conventional coronagraph and photonics operate in series, complementing each other and thereby loosening requirements on each subsystem. As photonic technologies continue to advance, a hybrid or fully photonic coronagraph holds great potential for future exoplanet imaging from space.
Looking to the future of exo-Earth imaging from the ground, core technology developments are required in visible Extreme Adaptive Optics (ExAO) to enable the observation of atmospheric features such as oxygen on rocky planets in visible light. UNDERGROUND (Ultra-fast AO techNology Determination for Exoplanet imageRs from the GROUND), a collaboration built in Feb. 2023 at the Optimal Exoplanet Imagers Lorentz Workshop, aims to (1) motivate oxygen detection in Proxima Centauri b and analogs as an informative science case for high-contrast imaging and direct spectroscopy, (2) overview the state of the field with respect to visible exoplanet imagers, and (3) set the instrumental requirements to achieve this goal and identify what key technologies require further development.
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