New observations by the James Webb Space Telescope (JWST) are shedding light on an extremely rare and “forbidden” exoplanet known as TOI-5205 b. Scientists have discovered that the giant planet’s atmosphere contains fewer heavy elements than its host star. This surprising result could change the way researchers understand the early stages of giant planet formation.
The survey results are astronomical journalcomes from an international team led by Caleb Kanyas of NASA’s Goddard Space Flight Center, with assistance from Carnegie Science’s Shubham Kanodia and others.
giant planet orbiting a small star
TOI-5205 b is about the same size as Jupiter, but it is a much smaller star, roughly four times the size of Jupiter and only about 40 percent as massive as the Sun. When a planet passes in front of a star in a phenomenon known as a “transit,” about 6 percent of the star’s light is blocked.
During these passes, astronomers used spectroscopes to separate the star’s light into its component colors. This technique allows us to determine the chemical composition of a planet’s atmosphere and gain insight into how the atmosphere formed and evolved along with its host star.
Planet formation theory puzzle
Planets typically form within rotating disks of gas and dust surrounding young stars. Although this process is widely accepted, systems like TOI-5205 b challenge existing models. Giant planets orbiting small, cold stars at close distances are difficult to explain using current theories.
To investigate these unusual systems, Kanodia, Kanyas, and Jessica Libby Roberts of the University of Tampa are leading JWST’s largest Cycle 2 exoplanet program: Red Dwarfs and Seven Giants. The project focuses on rare worlds like TOI-5205 b, often referred to as GEMS (Giant Exoplanets Around M Dwarfs).
JWST detects unexpected atmospheric chemical reaction
TOI-5205 b was first identified in 2023 when Kanodia led follow-up observations based on data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Now, researchers have used JWST to examine its atmosphere in detail for the first time.
After observing three passes, the team encountered an unexpected result. The planet’s atmosphere contains significantly fewer heavy elements compared to hydrogen than Jupiter’s. Even more surprising, its metallicity is lower than that of its host star, making it unlike any giant planet studied to date.
The data also revealed the presence of methane (CH4) and hydrogen sulfide (H2S) in the atmosphere.
Heavy elements may be hidden deep inside
To better understand these findings, researchers Simon Müller and Rabbit Held from the University of Zurich used an advanced model of the planet’s interior. Their results suggest that the entire planet is about 100 times more metal-rich than its atmosphere.
“We observed much lower metallicities than models predicting the planet’s bulk composition, calculated from measurements of the planet’s mass and radius. This suggests that heavy elements moved into the interior during formation and that the interior and atmosphere are not currently mixing,” Kanodia explained. “In summary, these results suggest a planetary atmosphere that is very carbon-rich and oxygen-poor.”
GEMS survey and future research
This study is part of the broader GEMS survey, which aims to study giant planets passing around M dwarf stars to better understand their formation, internal structure, and atmosphere. The research team includes Carnegie astronomers Peter Gao, Johanna Teske and Nicole Wallach, as well as former Carnegie postdoctoral fellow Anjali Piet, now at the University of Birmingham.
Additional contributors include researchers from institutions such as Johns Hopkins University Applied Physics Laboratory, Academia Sinica Institute of Astronomy and Astrophysics, Catholic University, University of Maryland, California Institute of Technology, NASA Goddard, University of St. Andrews, Pennsylvania State University, University of California, Irvine, Tata Institute for Fundamental Research, and the University of Amsterdam.
Correcting star spots improves accuracy
The research team also took into account interference caused by the host star’s star point. These dark, active regions can distort observations by brightening certain wavelengths and obscuring some of the atmospheric signal.
By correcting for these effects, the researchers improved the accuracy of their measurements. Wallack and Kandia are currently refining this approach with a new JWST project focused on the same system. Their work could help future studies of planets orbiting active stars produce more reliable results.
