JWST Finds Sub-Solar Metallicity Atmosphere on Exoplanet

JWST observations of the giant exoplanet TOI-5293Ab, orbiting an early M-dwarf, reveal a sub-Solar metallicity atmosphere. The study used two NIRSpec/PRISM transits to characterize the atmosphere. The first transit was affected by heterogeneity crossings across the entire transit chord, rendering inferences unreliable. However, the second transit allowed for modeling of the heterogeneity crossings, resulting in a transmission spectrum free from stellar contamination. Bayesian free chemistry retrievals indicate a low atmospheric metallicity (log [M/H] = -1.03 (+0.53/-0.44) x Solar) and a slightly super-solar C/O ratio (1.23 (+2.94/-0.75)).

The retrievals provide strong evidence for CH4 and low significance detections of CO2, H2O, and NH3. Thermal evolution models suggest that the radius of TOI-5293Ab is inflated above theoretical expectations (~ 1.07 R_J), despite having a temperature of ~ 700 K. This makes it difficult to constrain its bulk composition. The study highlights the challenges of characterizing exoplanet atmospheres around rapidly changing M-dwarfs. The findings contribute to the growing sample of Giant Exoplanets around M-dwarf Stars (GEMS), which helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass.

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