A strange giant planet orbiting a small star is forcing astronomers to rethink how planetary systems form.
Astronomers using the James Webb Space Telescope (JWST) have taken a closer look at TOI-5205 b, a strange exoplanet often labeled “forbidden,” and found something unexpected. Its atmosphere appears to be unusually poor in heavy elements compared to its own star. That runs counter to long-standing ideas about how giant planets form and evolve.
The study, published in The Astronomical Journal, was led by Caleb Cañas at NASA’s Goddard Space Flight Center, with key contributions from Shubham Kanodia of Carnegie Science and an international team of researchers.
TOI-5205 b is similar in size to Jupiter but orbits a much smaller star, which is about four times the size of Jupiter and roughly 40 percent the mass of the Sun. When the planet crosses in front of its star in an event known as a “transit,” it blocks about six percent of the star’s light. Scientists analyze these transits using spectrographs, which separate light into different colors, to identify the chemical makeup of the planet’s atmosphere and better understand its origins.
Challenging Planet Formation Theories
Planets form within rotating disks of gas and dust that surround young stars. While astronomers generally agree that giant planets develop in these disks, systems like TOI-5205 b are difficult to explain. A large planet orbiting so close to a small, cool star does not fit neatly into existing models.
To investigate these unusual systems, 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. The project focuses on rare worlds like TOI-5205 b, often grouped under the label GEMS, or giant exoplanets around M dwarf stars.
Kanodia first confirmed TOI-5205 b in 2023 using follow-up observations of data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which had identified it as a candidate. He now co-leads the team that has carried out the first detailed study of its atmosphere.
Data from three observed transits revealed an unexpected pattern. The planet’s atmosphere contains fewer heavy elements relative to hydrogen than Jupiter and even less than its own star. This is unusual because giant planets are typically enriched in heavier elements compared to their stars.
The observations also detected methane (CH₄) and hydrogen sulfide (H₂S) in the atmosphere, adding further detail to its chemical profile.
A Mismatch Between Atmosphere and Interior
To interpret these results, researchers Simon Muller and Ravit Helled at the University of Zurich used models of planetary structure. Their analysis suggests that the planet as a whole is about 100 times more metal-rich than its atmosphere, based on transit measurements.
“We observed much lower metallicity than our models predicted for the planet’s bulk composition, which is calculated from measurements of a planet’s mass and radius. This suggests that its heavy elements migrated inward during formation and now its interior and atmosphere are not mixing,” Kanodia explained. “In summary, these results suggest a very carbon-rich, oxygen-poor planetary atmosphere.”
The team also accounted for the effects of starspots on the host star, which can distort observational data. These dark regions altered the measured light by enhancing certain wavelengths and obscuring possible atmospheric signals.
Wallack and Kanodia are now testing this correction method in a newer JWST study of the same system. This approach could improve future observations of planets orbiting active stars.
Reference: “GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere” by Caleb I. Cañas, Jacob Lustig-Yaeger, Shang-Min Tsai, Simon Müller, Ravit Helled, Shubham Kanodia, Dana R. Louie, Giannina Guzmán Caloca, Peter Gao, Jessica Libby-Roberts, Kevin K. Hardegree-Ullman, Knicole D. Colón, Ian Czekala, Megan Delamer, Te Han, Andrea S.J. Lin, Suvrath Mahadevan, Erin M. May, Joe P. Ninan, Anjali A. A. Piette, Guðmundur Stefánsson, Kevin B. Stevenson, Johanna Teske and Nicole L. Wallack, 31 March 2026, The Astronomical Journal.
DOI: 10.3847/1538-3881/ae4976
The research is part of the GEMS Survey, a program dedicated to studying transiting giant planets around M-dwarf stars to understand their formation, structure, and atmospheres. The research group also includes Carnegie astronomers Peter Gao, Johanna Teske, and Nicole Wallack, as well as recently departed Carnegie postdoctoral fellow Anjali Piette, now on faculty at University of Birmingham.
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