JWST finds metal-poor atmosphere on TOI-5205 b

Caleb Cañas and collaborators used the James Webb Space Telescope to take the first transmission spectrum of TOI-5205 b and found two things at once: the planet’s atmosphere appears unusually poor in heavy elements, and the signal is heavily muddied by its host star. The result came from three transits observed with JWST’s Near Infrared Spectrograph, according to the team’s paper in *The Astronomical Journal*. TOI-5205 b is already an odd system — a Jupiter-like planet orbiting a much smaller M-dwarf star every 1.63 days. The new data add a second puzzle: the atmosphere does not look like what many formation models would predict. ### Why is TOI-5205 b called “forbidden”? TOI-5205 b was confirmed in 2023 after follow-up work on a TESS candidate, and the label comes from the mismatch between the planet and its star. The planet is about 1.08 times Jupiter’s mass and 0.94 times Jupiter’s radius, while the host is an M4 dwarf with about 39% of the Sun’s mass and 39% of the Sun’s radius, the paper said. Systems like that are considered difficult to make in standard pictures of planet formation because a small star’s disk is not expected to build such a large close-in giant planet so easily. (arxiv.org) ### What exactly did JWST observe? JWST watched three transits of TOI-5205 b with NIRSpec PRISM over wavelengths from 0.6 to 5.3 microns. In transmission spectroscopy, astronomers measure how starlight filters through the planet’s atmosphere during transit, looking for wavelength-dependent changes that reveal molecules and clouds. The team reported a larger transit depth at bluer wavelengths and spot-crossing events in the light curves, both signs that the star itself was contaminating the planetary signal. (arxiv.org) ### What does “stellar contamination” mean here? The paper said unocculted star spots dominate much of the spectrum below about 3.0 microns. That matters because many of the strongest water features sit in that part of the spectrum, and the contamination reduced the team’s ability to say whether water vapor was present. In other words, the telescope was not failing; the star was imprinting its own structure onto the measurement, making the atmosphere harder to read cleanly. (arxiv.org) ### So what did the team detect in the atmosphere? The strongest atmospheric detections were methane and hydrogen sulfide, both seen in the 3.0 to 5.0 micron range where the stellar contamination was less dominant. The authors said both gridded and Bayesian retrievals favored a sub-solar atmospheric metallicity and a super-solar carbon-to-oxygen ratio. In plain terms, the atmosphere appears poorer in elements heavier than hydrogen and helium than expected — even compared with Jupiter, according to the Carnegie Science summary carried by ScienceDaily and other outlets. (arxiv.org) ### Why is the metal-poor result a problem for formation models? The paper said interior models predict a bulk metallicity of about 10% to 20%, roughly 100 times the atmospheric metallicity inferred from the spectrum. That gap suggests the planet’s atmosphere and interior may not be well mixed, the authors wrote. Carnegie Science’s summary said that is surprising because giant planets are usually enriched in heavy elements relative to their stars, not depleted. (arxiv.org) ### What’s the broader takeaway for JWST exoplanet work? Jessica Libby-Roberts, Shubham Kanodia and Cañas are part of JWST’s “Red Dwarfs and the Seven Giants” Cycle 2 program, which targets giant planets around M dwarfs. TOI-5205 b shows why those targets are attractive and difficult at the same time: they can test formation theories, but active small stars can distort transmission spectra enough to change the chemistry astronomers think they are seeing. The next step is more comparative work on similar systems and more modeling of stellar contamination in the same JWST data sets described in the April 2026 paper. (arxiv.org) (sciencedaily.com)