JWST finds "forbidden" exoplanet with unexpectedly metal-poor atmosphere

Friday 03 April 2026 - 07:50

By: Dakir Madiha

JWST finds "forbidden" exoplanet with unexpectedly metal-poor atmosphere

The James Webb Space Telescope has delivered new findings about TOI-5205 b, a Jupiter-sized exoplanet already considered anomalous for orbiting a star far too small to have formed it. Observations published this week in The Astronomical Journal show that the planet's atmosphere contains fewer heavy elements than expected — even fewer than its host star — setting it apart from every other giant planet studied to date and challenging established models of gas giant formation.

TOI-5205 b was confirmed in 2023 after detection by NASA's Transiting Exoplanet Survey Satellite. It matches Jupiter in mass and size but orbits an M-dwarf star with only about 40 percent of the Sun's mass — a pairing so improbable under standard planetary formation theory that scientists labeled it "forbidden." When the planet crosses in front of its star, it blocks roughly six percent of the star's light, producing one of the deepest transits ever recorded for a confirmed exoplanet around a main-sequence star.

An international team led by Caleb Cañas of NASA's Goddard Space Flight Center and Shubham Kanodia of Carnegie Science used JWST's NIRSpec instrument to capture transmission spectra across three transits, covering wavelengths from 0.6 to 5.3 micrometers. The observations were conducted as part of the GEMS program — Giant Exoplanets around M-dwarf Stars — the largest exoplanet study in JWST's Cycle 2, designed specifically to investigate worlds like TOI-5205 b.

The spectra confirmed the presence of methane and hydrogen sulfide in the planet's atmosphere. The most significant finding, however, was a sub-solar metallicity: a concentration of heavy elements well below what models predict for a planet of this mass. Interior modeling indicates that TOI-5205 b's overall composition is roughly 100 times richer in metals than what its observable atmosphere reflects, suggesting that the planet's interior and outer layers are not well mixed.

"We observed a 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," Kanodia said. "This suggests that its heavy elements migrated inward during formation and that now its interior and atmosphere are not mixing."

The analysis was complicated by significant stellar contamination from starspots on the host star, which obscured potential atmospheric signatures at shorter wavelengths and prevented a definitive detection of water vapor. The research team is now conducting follow-up observations with JWST in the same system to validate their correction methods — an approach that could prove useful for studying other planets around similarly active stars.