Abstract
Abstract The M3V dwarf star L 98-59 hosts three small (R < 1.6 R ⊕) planets. The host star is bright (K = 7.1) and nearby (10.6 pc), making the system a prime target for follow-up characterization with the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST). Herein, we use simulated transmission spectroscopy to evaluate the detectability of spectral features with HST and JWST assuming diverse atmospheric scenarios (e.g., atmospheres dominated by H2, H2O, CO2, or O2). We find that H2O and CH4 present in a low mean molecular weight atmosphere could be detected with HST in one transit for the two outermost planets, while H2O in a clear steam atmosphere could be detected in six transits or fewer with HST for all three planets. We predict that observations using JWST/NIRISS would be capable of detecting a clear steam atmosphere in one transit for each planet and H2O absorption in a hazy steam atmosphere in two transits or less. In a clear, desiccated atmosphere, O2 absorption may be detectable for all three planets with NIRISS. If the L 98-59 planets possess a clear, Venus-like atmosphere, NIRSpec could detect CO2 within 26 transits for each planet, but the presence of H2SO4 clouds would significantly suppress CO2 absorption. The L 98-59 system is an excellent laboratory for comparative planetary studies of transiting multiplanet systems, and observations of the system via HST and JWST would present a unique opportunity to test the accuracy of the models presented in this study.
Highlights
Thousands of planets orbiting stars outside of our solar system have been confirmed, the vast majority of which were detected with the transit method
With current ongoing efforts through Hubble Space Telescope (HST)’s general observer (GO) program, we soon expect to place these initial constraints on L 98-59 b–d (HST-GO-15856 and HST-GO-16448, PI Barclay) and constrain abundances if they are dominated by abundances of H2 or H2O
Our investigation of the potential to detect and characterize the atmospheres of the L 98-59 planets through transmission spectroscopy indicates that 1 transit with HST/Wide-Field Camera 3 (WFC3) could detect a low mean-molecular weight atmosphere on L 98-59 c and d, while 1 transit with James Webb Space Telescope (JWST)/Near-Infrared Imager and Slitless Spectrograph (NIRISS) Single Object Slitless Spectroscopy (SOSS) would allow us to begin distinguishing between a variety of possible atmospheres for each planet in the L 98-59 system
Summary
Thousands of planets orbiting stars outside of our solar system have been confirmed, the vast majority of which were detected with the transit method. While Kepler provided an unprecedented bounty of planets that enabled exoplanet population statistics, relatively few planets discovered by Kepler make ideal follow-up targets due to their large average distances. The Transiting Exoplanet Survey Satellite (TESS; Ricker et al 2015), launched in 2018, is currently performing a near-all-sky survey to search for planets orbiting bright stars in our solar neighborhood. Planets that orbit bright stars are favorable targets to probe their bulk compositions and atmospheres, those orbiting M dwarfs because of the large planet-tostar radius ratio, and favorable transit probabilities for close-in planets. As TESS reveals new exoplanets, the prospect of discovering new worlds and probing their atmospheres provides an exciting opportunity to broaden our understanding of the nature of planets that may resemble our own
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