Observations from NASA's James Webb Space Telescope have uncovered surprising details about TOI-5205 b, a Jupiter-sized "forbidden" exoplanet orbiting a diminutive red dwarf star, revealing an atmosphere significantly poorer in heavy elements than its host star and even our solar system's gas giants.

Caption An artist’s conception of the gas giant planet TOI-5205
An artist’s conception of the gas giant planet TOI-5205 b orbiting a small, cool red dwarf star. Credit Katherine Cain, Carnegie Science.

The findings, published this week in The Astronomical Journal, suggest the planet's interior and atmosphere may not be well mixed, with heavier elements possibly having migrated inward during formation. The results force astronomers to rethink how gas giant planets assemble in the protoplanetary disks around small, cool stars early in their lifetimes.

An international team led by NASA Goddard Space Flight Center's Caleb I. Cañas, with key contributions from Carnegie Science's Shubham Kanodia and collaborators including Anjali A. Piette of the University of Birmingham, analyzed transmission spectroscopy data from three transits of TOI-5205 b using JWST's Near Infrared Spectrograph in PRISM mode. The observations, part of the GEMS (Giant Exoplanets around M-dwarfs with JWST) survey, spanned wavelengths from 0.6 to 5.3 microns and provided an exceptionally clear view thanks to the planet's large transit depth of approximately 6-7%.

TOI-5205 b, discovered in 2023 by the Transiting Exoplanet Survey Satellite and confirmed with ground-based instruments, is a roughly Jupiter-mass planet (about 1.08 Jupiter masses and 0.94-1.03 Jupiter radii) circling an M4 dwarf star roughly 40% the mass of the Sun and only about four times Jupiter's size. Located some 282 light-years away, the system challenges core accretion models because the star's protoplanetary disk likely contained too little solid material to build the massive core needed for runaway gas accretion in the planet's short orbital period of just 1.63 days.

Astronomers dubbed it a "forbidden" planet because standard formation theories struggle to explain how such a massive world could form so close to a low-mass star. The new JWST data adds another layer of mystery: the atmosphere shows sub-solar metallicity, with heavy element abundance relative to hydrogen far lower than expected — potentially as low as 1/100th of the planet's bulk metallicity inferred from mass and radius models.

"We observed much lower metallicity than our models predicted for the planet's bulk composition," Kanodia said. "This suggests that its heavy elements migrated inward during formation, and now its interior and atmosphere are not mixing." The discrepancy implies the planet's rocky and icy core may be enriched in metals while the gaseous envelope remains relatively metal-poor.

The transmission spectrum revealed clear signatures of methane (CH₄) and hydrogen sulfide (H₂S), with indications of a carbon-rich, oxygen-poor composition. Retrieval analyses favored a super-solar carbon-to-oxygen ratio alongside the low overall metallicity. Stellar contamination from unocculted star spots and faculae complicated the shorter-wavelength data, producing a slope in the spectrum and spot-crossing events, but the longer wavelengths allowed robust detection of the molecular features.

Piette emphasized the broader implications: "These findings have implications for our understanding of the giant planet formation process that occurs early in a star's lifespan. The planet having a lower metallicity than its own host star makes it stand out among all the giant planets that have been studied to date."

In our solar system, Jupiter and Saturn exhibit metallicities several times higher than the Sun, consistent with the idea that they accreted heavy elements from the solar nebula. For TOI-5205 b, the opposite appears true relative to its star, raising questions about disk chemistry, migration mechanisms and mixing processes in young planetary systems.

The GEMS program targets transiting giant planets around M dwarfs precisely because their large transit depths and the prevalence of such stars in the galaxy make them ideal laboratories for testing formation theories. M dwarfs, the most common stellar type, host a surprising number of close-in gas giants that push the boundaries of current models.

Despite significant stellar contamination — exceeding 30-sigma in some analyses — the team achieved a greater than 15-sigma detection of the planetary atmosphere. Atmospheric retrievals struggled with haze or cloud constraints at shorter wavelengths due to the contamination but converged on the metal-poor interpretation across both gridded and Bayesian methods.

The results add to a growing body of JWST exoplanet science that continues to reveal diversity in planetary atmospheres and challenge preconceptions. Previous studies of hot Jupiters and other giants have shown varied metallicities, but few have displayed such a stark mismatch with their host stars or interior models.

Kanodia noted that the findings point to a decoupled interior and atmosphere, a scenario not commonly observed or predicted. "In summary, these results suggest a very carbon-rich, oxygen-poor planetary atmosphere," the team concluded.

Astronomers say more observations are needed, including emission spectroscopy or higher-resolution data, to refine constraints on temperature structure, clouds and potential disequilibrium chemistry. Future JWST programs or ground-based instruments may help disentangle remaining stellar effects and probe deeper atmospheric layers.

The discovery of TOI-5205 b in 2023 already stretched disk scaling relations and core accretion timelines. Its proximity to the star — completing an orbit in under two days — means it experiences intense irradiation, yet the atmosphere retains detectable molecules without signs of extreme escape or inflation beyond expectations.

Researchers caution that while the planet's existence was already puzzling, the atmospheric composition introduces new variables: perhaps pebble accretion, disk instabilities or rapid migration played outsized roles. Some models suggest giant planets around M dwarfs could form via gravitational instability rather than core accretion, though the metal-poor envelope complicates that picture as well.

The study highlights JWST's power to characterize even challenging targets amid stellar activity. By observing multiple transits, the team mitigated some variability and built confidence in the decontaminated spectrum.

For the broader field of exoplanetary science, TOI-5205 b serves as a cautionary tale against overly rigid formation paradigms. As JWST and upcoming facilities like the Extremely Large Telescope probe more systems, astronomers expect to find greater diversity, particularly around the abundant red dwarf population.

"Planets are born from the rotating disk of gas and dust that surrounds a star in its youth," Piette explained. "While it is commonly accepted that giant planets form in these cloudy disks, the existence of massive planets like TOI-5205 b orbiting cool stars at close distances raises many questions about this process."

The paper, titled "GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere," appears in the April 2026 issue of The Astronomical Journal. Co-authors include Jacob Lustig-Yaeger, Shang-Min Tsai, Simon Müller, Ravit Helled and many others from institutions across the United States, Europe and beyond.

As data from this and other GEMS targets continue to arrive, scientists anticipate further insights into whether "forbidden" planets represent rare outliers or hint at missing pieces in our understanding of planet formation across the Milky Way.

For now, TOI-5205 b stands as a compelling reminder that the universe still holds surprises, even for worlds that, on paper, should not exist.