Webb finds metal-poor atmosphere on giant world around red dwarf

by Simon Mansfield
Sydney, Australia (SPX) Apr 04, 2026
Observations of the unusual exoplanet TOI-5205 b with the James Webb Space Telescope suggest that the giant planet's atmosphere contains fewer heavy elements than its host star, challenging conventional ideas about how such worlds form around low mass stars. The results, published in The Astronomical Journal, come from an international team led by NASA Goddard Space Flight Center astronomer Caleb Canas and including Carnegie Science researcher Shubham Kanodia.

TOI-5205 b is similar in size to Jupiter but orbits a cool red dwarf star that is only about four times Jupiter's size and roughly 40 percent the mass of the Sun. During each transit, when the planet passes in front of its star, it blocks about six percent of the star's light, giving astronomers a strong signal to probe. By dispersing this starlight with spectrographs as the planet moves across the star, the team can infer the atmospheric composition of the planet and build a picture of its formation and evolution.

Giant planets are thought to form in the rotating disk of gas and dust that surrounds a young star, with heavy elements incorporated as the planet assembles. Massive planets in close orbits around small, cool stars, sometimes called forbidden planets, are difficult to reconcile with standard models of disk mass and structure. TOI-5205 b is one of these so-called GEMS, or giant exoplanets around M dwarf stars, that appear to defy expectations based on current planet formation theories.

To better understand these objects, Kanodia, Canas and University of Tampa researcher Jessica Libby-Roberts are leading a large JWST Cycle 2 exoplanet program called Red Dwarfs and the Seven Giants. This program targets transiting giant planets around red dwarf stars to investigate their atmospheres, internal structures and origins. TOI-5205 b is one of the first worlds in this survey to be characterized in detail with Webb's infrared instruments.

Kanodia previously led work confirming the existence of TOI-5205 b in 2023, following up on NASA's Transiting Exoplanet Survey Satellite observations that initially flagged the object as a planetary candidate. The new JWST data provide the first detailed look at the planet's atmospheric composition across multiple wavelengths. The team observed three separate transits to build up sufficient signal for their analysis.

The transit spectra reveal that the atmosphere of TOI-5205 b has a lower concentration of heavy elements, relative to hydrogen, than Jupiter and other gas giants in the Solar System. In addition, the atmospheric metallicity appears to be lower than that of the host star, placing TOI-5205 b among the most metal-poor giant planets yet characterized. This combination of a relatively small host star and a metal-poor giant planet makes the system stand out from previously studied exoplanets.

The Webb observations also show signatures of methane and hydrogen sulfide in the planet's atmosphere. These molecules are expected in the cool atmospheres of gas giants, but their detection around an M dwarf planet provides important constraints on temperature and chemistry. The presence and strength of these spectral features help refine models of the planet's atmospheric structure and its carbon and oxygen inventories.

To interpret the observations, team members Simon Muller and Ravit Helled at the University of Zurich modeled the planet's internal structure to estimate its overall composition. Their calculations indicate that TOI-5205 b as a whole is roughly 100 times more metal-rich than its atmosphere appears to be from the transit spectra. This mismatch implies that most of the heavy elements are locked deep in the interior rather than mixed throughout the atmosphere.

Kanodia said that the measured atmospheric metallicity is much lower than predicted by these interior models, which use the planet's mass and radius as inputs. He and colleagues suggest that heavy elements likely migrated inward during the planet's formation and are no longer efficiently mixed with the outer envelope. In their view, the data point to a carbon-rich, oxygen-poor atmosphere whose composition differs markedly from that of the bulk planet.

The work forms part of the broader GEMS Survey, which focuses on transiting giant planets around M dwarf stars as a way to test formation scenarios across a wide range of stellar masses. Alongside Kanodia, Carnegie astronomers Peter Gao, Johanna Teske and Nicole Wallack contributed to the analysis, as did former Carnegie postdoctoral researcher Anjali Piette, now at the University of Birmingham. The collaboration spans multiple institutions and brings together expertise in observations, atmospheric modeling and interior structure calculations.

Additional co-authors include Jacob Lustig-Yaeger, Erin May and Kevin Stevenson of the Johns Hopkins Applied Physics Laboratory; Shang-Min Tsai of Academia Sinica Institute of Astronomy and Astrophysics; Dana Louie of Catholic University; Giannina Guzman Caloca of the University of Maryland; Kevin Hardegree-Ullman of Caltech; Knicole Colon of NASA Goddard Space Flight Center; Ian Czekala of the University of St. Andrews; Megan Delamer and Suvrath Mahadevan of Penn State University; Andrea Lin and Te Han of the University of California Irvine; Joe Ninan of the Tata Institute of Fundamental Research; and Gudmundur Stefansson of the University of Amsterdam. Their combined efforts have produced one of the most detailed portraits yet of a giant planet around a small, active star.

Because TOI-5205's host star is heavily spotted, the team had to correct their data for the effects of starspots that imprint wavelength-dependent signatures on the transit spectra. Bright or dark regions on the stellar surface can mimic or hide features in a planet's atmosphere if not properly accounted for. Wallack and Kanodia are now applying and validating this correction method in a more recent JWST project targeting the same planetary system, work that will be important for future studies of planets around similarly active stars.

Research Report:GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere