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

By S. Moran, Kevin B. Stevenson, D. Sing, R. MacDonald, J. Kirk, J. Lustig-Yaeger, S. Peacock, L. Mayorga, Katherine A. Bennett, Mercedes López-Morales and 14 more. Astrophysical Journal Letters, 2023

135 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b01ce64342171fe8c60bac77db1c6a959441aa80

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. R. MacDonald
3. D. Deming
4. M. Line
5. Jon Fortney
6. H. Knutson
7. A. Showman
8. Laura Kreidberg

In Instrumentation

1. N. Madhusudhan
2. D. Deming
3. R. MacDonald
4. M. Line
5. H. Knutson
6. Jon Fortney
7. A. Showman
8. Arazi Pinhas

Citations in the corpus

• Carbon-bearing Molecules in a Possible Hycean Atmosphere (2023, methodology): “The nondetection of H2O is further evidence against the effects of unocculted stellar heterogeneities on the present transmission spectrum, given that H2O in cool unocculted starspots may contaminate the spectrum, as recently reported (Barclay et al. 2021; Moran et al. 2023).”