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Exoplanet Hyper Illumination: Analytical and Numerical Approaches

Presentation #223.05 in the session Exoplanet Atmospheres (Poster)

Published onOct 23, 2023
Exoplanet Hyper Illumination: Analytical and Numerical Approaches

Exoplanet researchers can model the emissions of exoplanet systems due to reflected and thermal light in detail. In addition, planetary scientists continually develop precise global climate models. These models depend on the illumination of the planet by its host star to estimate exoplanet parameters, such as temperature and albedo. For most exoplanets, the host star illuminating the planet is far away and can be treated as a point source. In such cases, half of the exoplanet is illuminated. Exoplanets that orbit very close to their host star experience hyper illumination, in which more than half of the exoplanet surface is illuminated. For example, the hot Jupiter Kepler-91 b orbits its host star at about 2.5 stellar radii and is approximately 70% illuminated. The potentially habitable planet TRAPPIST-1 d’s orbital radius is about 40 stellar radii, and it is about 51% illuminated. Neglecting the excess illumination of hyper illuminated exoplanets risks their reflected light being mistaken for thermal light during a secondary eclipse and underestimating the instellation at the poles for climate models. Accounting for hyper illumination requires treating the exoplanet’s host star as an extended source. Previous work shows that doing so results in an exoplanet with multiple illumination zones. The fully illuminated zone receives light from the full apparent disk of the host star, and the two penumbral zones from less. In addition, an analytical expression for the illumination within an exoplanet’s fully illuminated zone was determined, and work within the penumbral zones is ongoing. STARRY, first used for thermal maps, was extended in 2022 to include reflected light. To do so, STARRY treats the host star as either a point source, or a spherical source. In the latter case, it approximates the host star as a set of point sources located on the star’s surface. In this work, we present the analytical results obtained thus far, and a comparison between the analytical results and those of STARRY.

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