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Search for helium in the atmosphere of four small planets around M-dwarfs

Presentation #625.23 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
Search for helium in the atmosphere of four small planets around M-dwarfs

Over 2000 small planets (1 - 4 Earth radii) have been discovered thanks to space telescopes such as Kepler and TESS, becoming common in the Milky Way. However, their formation and evolution remain incompletely understood. NASA’s Kepler space telescope revealed that this population has a bimodal distribution (radius valley; 1.5-2 Earth radii) to be consistent with the transition from rocky to non-rocky planets (Fulton et al. 2017). In addition, the composition and structure of small planets are likely to vary among different formation models (e.g., Zeng et al. 2019). Searching for the atmospheres of small planets helps us identify these differences. The infrared helium triplet in the planetary atmosphere is a good indicator for investigating the existence of a primary atmosphere accreted from the protoplanetary nebula. Recent studies have revealed that the absorption depth of this triplet is influenced by stellar X-ray and Extreme Ultraviolet (EUV) radiation, with K-type stars being particularly favorable hosts (Oklopcic et al., 2020). Moreover, according to the most recent models considering geometrical effects, planets orbiting nearby late M-dwarfs are the most favorable (Biassoni et al., 2023). However, previous studies have detected the infrared helium triplet in the atmosphere of only two planets orbiting M-dwarfs (Palle et al. 2020, Orell-Miquel et al. 2022), despite successful detections in a dozen other planets. To unravel this puzzle, we need to observe helium absorption in more planets around M-dwarfs. We observed four small planets (~1.7-2.4 Earth radii) near radius valley around M-dwarfs to search for helium triplets with Subaru/IRD and determined an upper limit for each planet. In this poster, we will introduce our analysis method and report on these results and discussions.

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