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

May 2023 • 2023ApJ...948L..11M

Authors • Moran, Sarah E. • Stevenson, Kevin B. • Sing, David K. • MacDonald, Ryan J. • Kirk, James • Lustig-Yaeger, Jacob • Peacock, Sarah • Mayorga, L. C. • Bennett, Katherine A. • López-Morales, Mercedes • May, E. M. • Rustamkulov, Zafar • Valenti, Jeff A. • Adams Redai, Jéa I. • Alam, Munazza K. • Batalha, Natasha E. • Fu, Guangwei • Gonzalez-Quiles, Junellie • Highland, Alicia N. • Kruse, Ethan • Lothringer, Joshua D. • Ortiz Ceballos, Kevin N. • Sotzen, Kristin S. • Wakeford, Hannah R.

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 ( χν2=1.0 ) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

Links

• PREPRINT http://arxiv.org/abs/2305.00868
• ELECTR https://doi.org/10.3847/2041-8213/accb9c
• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2023ApJ...948L..11M
• PDF https://iopscience.iop.org/article/10.3847/2041-8213/accb9c/pdf
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2023ApJ...948L..11M