Search for exospheres of temperate exoplanets

Observations of transiting giant planets at ultraviolet (UV) wavelengths revealed that some of them display extended exospheres composed of neutral hydrogen atoms escaping from the planet. The Earth is known to possess its own exosphere, also called geocorona, but such a feature has not yet been detected for a small rocky exoplanet. The existence of a neutral hydrogen exosphere around small planets can be used as an evidence for the presence of water in their lower atmosphere, an information that is not easily accessible at longer wavelengths. Thus, we address in this work the possibility of detecting the exosphere of an Earth-like exoplanet around nearby stars using the current and future UV instrumentation. Based an empirical model of the geocorona from recent observations, our results show that such detection can only be performed using the proposed LUVOIR telescope, owing to its higher spectral resolution, detector sensitivity, and mirror size. Moreover, we found that a putative Earth-like exosphere could be reliably detected around nearby M dwarfs, such as TRAPPIST-1, Ross 128, and Proxima Cen, in few visits using LUVOIR. Owing to the large size of the Earth’s exosphere (> 40 Earth radii), we also determined that larger M dwarfs could be better targets for such a search, since the exosphere will have more stellar surface to block and thus display a deeper transit signal – provided the star is nearby and has a large radial velocity to avoid Lyman-alpha absorption by the interstellar medium.

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