Astronomers using the James Webb Space Telescope have detected an unusually metal-poor atmosphere on the Jupiter-sized exoplanet TOI-5205 b, which orbits a small, cool star. The planet's atmospheric metallicity is lower than that of its host star, challenging theories of giant planet formation. The findings come from a study led by researchers at NASA's Goddard Space Flight Center and Carnegie Science.
TOI-5205 b, roughly the size of Jupiter, circles a star about four times Jupiter's size and 40 percent as massive as the Sun. During transits, when the planet blocks about six percent of the star's light, scientists analyzed the starlight passing through its atmosphere with JWST spectrographs. This revealed methane and hydrogen sulfide, but far fewer heavy elements relative to hydrogen than expected— even less than in the host star itself, according to the team led by Caleb Cañas of NASA's Goddard Space Flight Center and Shubham Kanodia of Carnegie Science. The planet was first confirmed in 2023 through follow-up observations of NASA's Transiting Exoplanet Survey Satellite data led by Kanodia. Kanodia noted, '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.' Models by Simon Muller and Ravit Helled of the University of Zurich indicate the planet's interior may be 100 times richer in metals than its atmosphere suggests, pointing to poor mixing. The research, published in The Astronomical Journal, is part of the GEMS survey studying giant exoplanets around M-dwarf stars. The team corrected observations for starspot interference to improve accuracy, with further refinements ongoing.