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A JWST NIRSpec Transmission Spectrum of the Keystone Super-Earth L 98-59 c

Presentation #625.21 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
A JWST NIRSpec Transmission Spectrum of the Keystone Super-Earth L 98-59 c

Most planets in our galaxy have radii between 1 and 3 times that of Earth at periods less than 100 days, as revealed by the Kepler mission. Because there are no planets of this size range in our solar system, their nature remains mysterious. As we enter the era of JWST, it is possible to study the atmospheres of these small planets for the first time. Here, we present a transmission spectrum of L 98-59 c, one of the 11 planets observed by the JWST COMPASS (Compositions of Mini-Planet Atmospheres for Statistical Study) program focused on the demographics of small exoplanets. In particular, COMPASS investigates the transition from primary to secondary atmospheres, and identifies which (if any) small exoplanets have observable atmospheres. All of our targets are being observed with JWST NIRSpec G395H, which gives visibility of key dominant background gas molecular candidates (CO2, CH4, H2O, and CO). Our study of L 98-59 c yields a flat spectrum of exquisite precision that is relatively uncontaminated by stellar activity. With this, we are able to rule out primordial H2-rich atmospheres even for high-altitude (~1mbar) cloud decks. We are able to rule out atmospheric mean molecular weights (mmw) below 10 g/mol (including pure CH4), while higher mmw atmospheres, or no atmosphere at all, are compatible with our spectrum. This spectrum of a keystone super-Earth will provide important context for community’s efforts to study the atmospheres of small planets.

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