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TOI-5205 b

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TOI-5205 b

Jupiter-sized exoplanet orbiting a low-mass M dwarf (~0.4 M☉, roughly four times Jupiter’s size). Member of the “forbidden” GEMS class — giant planets around small stars whose existence challenges current core-accretion / disk-instability formation theory.

Discovery and basic properties

  • Initially flagged by NASA TESS and confirmed in 2023 by Kanodia via ground-based follow-up.
  • Blocks ~6% of the host star’s light at transit.

2026 JWST atmosphere result

Reported in src-sciencedaily-forbidden-exoplanet-atmosphere-2026-04 (Cañas, Kanodia, Libby-Roberts et al., The Astronomical Journal 2026):

  • Three JWST transits combined into a transmission spectrum.
  • Atmosphere is metal-poor — below both Jupiter’s metallicity and the host star’s metallicity. First known case for a transiting giant.
  • Detected molecules: methane (CH₄) and hydrogen sulfide (H₂S).
  • Interpreted as carbon-rich, oxygen-poor.
  • university-of-zurich interior models (Muller and Helled) imply the bulk planet is ~100× more metal-rich than the observed atmosphere — heavy elements migrated inward during formation, and interior–atmosphere mixing has ceased.
  • The paper develops a starspot-correction methodology to compensate for stellar-activity contamination of M-dwarf spectra.

Why it matters

  • First giant for which atmospheric metallicity is sub-stellar — a stress-test of existing exoplanet-atmosphere-models that typically assume bulk composition tracks the atmosphere.
  • Provides a direct probe of planetary interior–atmosphere decoupling as a migration signature.

See also

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