JWST Reveals Metal-Poor Atmosphere on Forbidden Exoplanet TOI-5205b, Challenging Giant Planet Formation Models
Webb telescope finds a Jupiter-sized planet orbiting a tiny red dwarf has fewer heavy elements in its atmosphere than its own star, defying all existing formation theories.
Overview
NASA’s James Webb Space Telescope has delivered a puzzling atmospheric portrait of TOI-5205b, a Jupiter-sized gas giant that orbits a red dwarf star roughly four times Jupiter’s own size. Transmission spectroscopy of three transits reveals an atmosphere with fewer heavy elements than the planet’s host star — a result that contradicts every established model of how giant planets acquire their chemical inventory, according to findings published in The Astronomical Journal.
A Planet That Should Not Exist
TOI-5205b has been labeled a “forbidden” planet since its discovery by NASA’s Transiting Exoplanet Survey Satellite (TESS). At 1.08 Jupiter masses and 1.03 Jupiter radii, the planet is unremarkable in size. What makes it anomalous is its host star: an M4-type red dwarf with only about 39 percent of the Sun’s mass. Standard planet formation models — both core accretion and disk instability — struggle to explain how such a small, low-mass star could assemble enough material in its protoplanetary disk to produce a gas giant of this scale, as ScienceDaily reported.
The planet orbits at just 0.02 AU from its star with a period of 1.63 days, and it blocks approximately 6 percent of the star’s light during transit — an extraordinarily large fraction that made it an ideal target for JWST’s Near-Infrared Spectrograph.
What Webb Found
The JWST observations, part of the GEMS survey studying transiting giants around M-dwarf stars, detected methane and hydrogen sulfide in the planet’s atmosphere. More significantly, the data revealed that the atmosphere’s metallicity — its concentration of elements heavier than hydrogen and helium — is lower than that of the host star itself, according to Phys.org.
“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,” said Dr. Anjali Piette, a member of the research team, as quoted by Phys.org.
This finding is significant because giant planets are generally expected to be enriched in heavy elements relative to their stars. During formation, a growing planet sweeps up solid material — rock and ice — from the surrounding disk, concentrating metals in its envelope. Jupiter, for example, has an atmospheric metallicity several times that of the Sun. TOI-5205b reverses this expectation entirely.
An Interior-Atmosphere Disconnect
The research team, led by Dr. Caleb Canas of NASA’s Goddard Space Flight Center and Dr. Shubham Kanodia of the Carnegie Institution for Science, found that while the atmosphere appears metal-poor, models suggest the planet’s bulk composition is roughly 100 times more metal-rich than the atmosphere indicates. The implication is that heavy elements migrated inward during formation, settling into the planet’s deep interior rather than remaining mixed throughout the envelope, according to ScienceDaily.
The resulting atmosphere is carbon-rich and oxygen-poor, a chemical profile that influences everything from cloud formation to thermal structure. The team also had to correct for contamination from starspots on the host red dwarf, which can mimic or mask atmospheric signals in transmission spectra.
What We Don’t Know
The mechanism that drove this apparent stratification remains unclear. Whether the heavy elements were sequestered during initial accretion, migrated downward over time through gravitational settling, or were somehow excluded from the upper atmosphere by an unknown process is an open question.
It is also unknown whether TOI-5205b is a one-of-a-kind outlier or the first identified member of a broader population of metal-depleted giant planets around low-mass stars. The GEMS survey is observing additional M-dwarf giant planet systems with JWST, and future results may determine whether this atmospheric pattern recurs.
The findings add to a growing body of JWST results that have complicated the relatively tidy picture of giant planet formation that prevailed before the telescope’s launch. Earlier Webb observations have revealed unexpected atmospheres on rocky exoplanets and sulfur-dominated super-Earth atmospheres that similarly defied theoretical expectations, suggesting that the diversity of planetary atmospheres extends well beyond what pre-Webb models anticipated.