High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations
Sing, David K.; Lustig-Yaeger, Jacob; Fu, Guangwei; May, E. M.; Moran, Sarah E.; Peacock, Sarah; Stevenson, Kevin B.; Kirk, James; López-Morales, Mercedes; MacDonald, Ryan J.; Mayorga, L. C.; Sotzen, Kristin S.; Valenti, Jeff A.; Alam, Munazza K.; Batalha, Natasha E.; Bennett, Katherine A.; Gonzalez-Quiles, Junellie; Kruse, Ethan; Lothringer, Joshua D.; Rustamkulov, Zafar; Wakeford, Hannah R.; Ortiz Ceballos, Kevin N.; Adams Redai, Jéa I.; Highland, Alicia N.
United States, United Kingdom
Abstract
Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars' highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8-5.2 μm JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3× Earth's insolation) super-Earth GJ 486b (1.3 R ⊕ and 3.0 M ⊕). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2σ - 3.3σ, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2σ) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits ( ${\chi }_{\nu }^{2}=1.0$ ) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.