Using the Near-Infrared Spectrograph (NIRSpec) onboard the NASA/ESA/CSA James Webb Space Telescope, astronomers have characterized the atmosphere of TOI-5205b, an extrasolar gas giant orbiting a small, dim red dwarf star. These observations have revealed an atmosphere unexpectedly poor in heavy elements, raising fresh questions about how such ‘forbidden’ alien worlds take shape and evolve.
The Jupiter-sized planet TOI-5205b has a surface temperature of 737 K and orbits its parent star, TOI-5205, at a distance of 0.02 AU. Image credit: Sci.News.
TOI-5205b is a short-period (1.63 days) gas giant only 1.03 times larger than Jupiter and 1.08 times as massive.
First discovered in 2022, the planet orbits TOI-5205, an M4-type star that is itself approximately 39% the size and the mass of the Sun.
Also known as TIC 419411415, the system is located some 283 light-years away in the constellation of Vulpecula.
“Short-period (less than 10 days), Jupiter-mass planets were the first type of exoplanet discovered around main-sequence Sun-like stars, but their formation process remains uncertain,” said Dr. Caleb Cañas from NASA’s Goddard Space Flight Center and his colleagues.
“The growing number of short-period giant exoplanets around M dwarf stars presents additional complications to the theories of gas giant formation.”
“These worlds are difficult to form through core accretion because the low disk masses and long orbital timescales for M dwarfs impede the efficient formation of massive planetary cores capable of initiating runaway gas accretion.”
“These planets represent an extreme regime of planet formation for the mid- to late-M dwarfs because the high planet-to-star mass ratios require core masses that exceed the estimated dust mass in the protoplanetary disk.”
Using Webb’s Near Infrared Spectrograph, the astronomers observed three transits of TOI-5205b.
They were surprised to see that the planet’s atmosphere has a lower concentration of heavy elements — relative to hydrogen — than a gas giant planet in our own Solar System like Jupiter. It even has a lower metallicity than its own host star.
This makes it stand out among all the giant planets that have been studied to date.
Additionally, although less shocking, the transits revealed methane and hydrogen sulfide in TOI-5205-b’s atmosphere.
To contextualize their findings, the researchers deployed sophisticated models of planetary interiors to predict that the entirety of TOI5205-b’s composition is about 100 times more metal rich than its atmosphere, as measured by the transits.
“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,” said Carnegie Science’s Dr. Shubham Kanodia.
“This suggests that its heavy elements migrated inward during formation and now its interior and atmosphere are not mixing.”
“In summary, these results suggest a very carbon-rich, oxygen-poor planetary atmosphere.”
The findings were published in the Astronomical Journal.
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Caleb I. Cañas et al. 2026. GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere. AJ 171, 260; doi: 10.3847/1538-3881/ae4976
