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
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2023
- DOI:
- 10.3847/2041-8213/accb9c
- arXiv:
- arXiv:2305.00868
- Bibcode:
- 2023ApJ...948L..11M
- Keywords:
-
- Exoplanet atmospheres;
- Exoplanet atmospheric composition;
- Transmission spectroscopy;
- Astronomy data reduction;
- Planetary atmospheres;
- Water vapor;
- Stellar faculae;
- Stellar atmospheres;
- Infrared spectroscopy;
- 487;
- 2021;
- 2133;
- 1861;
- 1244;
- 1791;
- 1601;
- 1584;
- 2285;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 18 pages, 7 figures, 5 tables. Accepted in ApJ Letters. Co-First Authors