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ScienceDaily: 'Forbidden' exoplanet TOI-5205 b has unexplained metal-poor atmosphere (April 2026)

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ScienceDaily — This “forbidden” exoplanet has an atmosphere scientists can’t explain

JWST transmission-spectroscopy of toi-5205-b — a Jupiter-sized giant orbiting a 0.4 M☉ M dwarf — finds a metal-poor atmosphere whose metallicity falls below that of its host star, an unprecedented result for a transiting giant. The team detects methane (CH₄) and hydrogen sulfide (H₂S) and interprets the atmosphere as carbon-rich and oxygen-poor. Interior models from the university-of-zurich suggest the bulk planet is ~100× more metal-rich than its visible atmosphere, implying heavy elements migrated inward during formation and the interior and atmosphere are no longer mixing. The work is part of the GEMS Survey (Red Dwarfs and the Seven Giants), JWST’s largest Cycle 2 exoplanet program, targeting the “forbidden” class of giant planets around small stars — systems poorly explained by current migration theory. The paper also pioneers a starspot-correction technique that improves spectra of planets around active hosts.

Key Claims

  • toi-5205-b is a Jupiter-sized planet around a ~0.4 M☉ M dwarf, blocking ~6% of stellar light at transit; first confirmed by shubham-kanodia in 2023 from nasa TESS data.
  • Three JWST transits yield an atmospheric metallicity lower than both Jupiter’s and the host star’s — a first for any giant studied.
  • CH₄ and H₂S detected in the atmosphere; framing is “very carbon-rich, oxygen-poor.”
  • Interior models (Muller and Helled, university-of-zurich) indicate the planet’s bulk is ~100× more metal-rich than the observed atmosphere, suggesting heavy elements migrated inward and interior–atmosphere mixing has ceased.
  • Systems like toi-5205-b — labelled GEMS (giant exoplanets around M-dwarf stars) — challenge core-accretion/disk-instability formation models for low-mass stars.
  • GEMS Survey (Kanodia, Cañas, Libby-Roberts) is JWST’s largest Cycle 2 exoplanet program.
  • The team’s starspot-correction methodology (extended by Wallack and Kanodia in a follow-on JWST project) is relevant to all M-dwarf exoplanet spectra.

Notable Quotes

“We observed much lower metallicity than our models predicted for the planet’s bulk composition… This suggests that its heavy elements migrated inward during formation and now its interior and atmosphere are not mixing… these results suggest a very carbon-rich, oxygen-poor planetary atmosphere.” — shubham-kanodia

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