Biosignatures & Paleobiology

A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ~486b from JWST Observations

By Keith Cowing
Status Report
May 1, 2023
Filed under , , , , , ,
A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ~486b from JWST Observations
Eureka! spectroscopic and white light curves from two transits of GJ 486b. The top two rows contain the spectroscopic light curves (left), our best-fit models (center), and subsequent residuals (right) for each transit. Most evident in the data are wavelength-dependent ramps near 3.2 µm that we readily remove. The bottom row depicts the white light curves from each detector (NRS1 and NRS2) before removing their systematic trends. Correlated noise is evident in the residuals and is likely due to thermal cycling (Rigby et al. 2022). The standard deviation of the normalized residuals is 140 ppm for NRS1 and 165 ppm for NRS2. The complete figure set (3 images, one for each reduction) is available in the online journal. — astro-ph.EP

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 micron JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3x 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 2.2 – 3.3 σ, 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 (χ2ν = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

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

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

Comments: 18 pages, 7 figures, 5 tables. Accepted in ApJ Letters. Co-First Authors
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2305.00868 [astro-ph.EP] (or arXiv:2305.00868v1 [astro-ph.EP] for this version)
Submission history
From: Sarah Moran
[v1] Mon, 1 May 2023 15:10:34 UTC (8,092 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻