JWST finds 'forbidden' exoplanet with atmosphere poorer in heavy elements than its star

A Jupiter-sized planet circling a diminutive, cool star has delivered an unexpected jolt to planet-formation theory. New observations from the James Webb Space Telescope (JWST) show that TOI-5205 b’s atmosphere contains fewer heavy elements than its own host star—a result that researchers say is unlike any giant planet studied so far.

The findings, published in The Astronomical Journal, come from an international team led by Caleb Cañas of NASA’s Goddard Space Flight Center with contributions from Carnegie Science’s Shubham Kanodia and others. TOI-5205 b is roughly the size of Jupiter but orbits a star only about four times Jupiter’s size and approximately 40% the mass of the Sun.

When the planet transits, it blocks about 6% of the star’s light. By breaking that starlight into its component colors during three JWST-observed transits, the team identified the chemical fingerprints of the planet’s atmosphere—including methane (CH4) and hydrogen sulfide (H2S).

Giant planets are thought to form in rotating disks of gas and dust around young stars, gradually accumulating heavy elements as they grow. Yet systems like TOI-5205 b—massive planets hugging small, cool M-dwarf stars—are difficult to explain with standard models.

To systematically probe these rare cases, Kanodia, Cañas, and Jessica Libby-Roberts of the University of Tampa are leading JWST’s largest Cycle 2 exoplanet program, Red Dwarfs and the Seven Giants, focusing on giant exoplanets around M-dwarf stars often dubbed GEMS.

TOI-5205 b itself was confirmed in 2023 after Kanodia led follow-up observations of a signal first flagged by NASA’s Transiting Exoplanet Survey Satellite (TESS). JWST’s first detailed look at the planet’s atmosphere revealed that its heavy-element content relative to hydrogen is significantly lower than Jupiter’s—and, more surprisingly, lower than its own star’s.

Interior models by Simon Muller and Ravit Helled at the University of Zurich indicate a stark mismatch: the planet’s bulk composition may be about 100 times more metal rich than its atmosphere appears to be. Kanodia said the data point to heavy elements migrating inward during formation, leaving an interior and atmosphere that are not mixing efficiently.

The measurements, he added, suggest a very carbon-rich, oxygen-poor atmosphere. The work forms part of the broader GEMS Survey to examine transiting giant planets around M dwarfs and clarify how they form, what they are made of, and how their atmospheres evolve.

The collaboration includes researchers from Carnegie Science and a range of U.S. and international institutions. Further observations of TOI-5205 b and similar systems are expected to help test whether poor atmospheric mixing is common among giant planets around small stars—and to refine models of how such “forbidden” worlds come to be.