JWST Observations of Forbidden Exoplanet TOI 5205 b Reveal Unexpectedly Metal Poor Atmosphere Around Red Dwarf Star

New James Webb Space Telescope data reveals the giant planet TOI 5205 b has a metal-poor atmosphere, challenging theories on how planets form around small stars.

By: AXL Media

Published: Apr 3, 2026, 11:21 AM EDT

Source: Information for this report was sourced from Carnegie Institution for Science

The Discovery of a Forbidden World in a Red Dwarf System

The existence of TOI 5205 b has puzzled the astronomical community since its initial confirmation in 2023. This gas giant, roughly the size of Jupiter, orbits a red dwarf star that is only four times its own size and possesses about 40 percent of the mass of the Sun. In the specialized field of exoplanetary science, such a configuration is often referred to as forbidden because the traditional understanding of planet formation suggests that small stars should not have enough material in their protoplanetary disks to create such massive companions. This unlikely world, categorized under the GEMS survey for giant exoplanets around M dwarf stars, represents a significant outlier in our current catalog of the cosmos.

Deciphering Atmospheric Secrets via Transmission Spectroscopy

To understand the history of this unusual planet, researchers utilized the advanced spectrographs of the James Webb Space Telescope to observe the planet as it transited in front of its star. During these events, the planet blocks approximately six percent of the light from the star, allowing astronomers to analyze the filtered starlight for chemical signatures. By splitting this light into its constituent colors, the team can identify the specific gases present in the atmosphere of the planet. These observations are critical for determining the relationship between a planet and its host, providing a chemical roadmap of the environment in which the planet was born.

Surprising Deficit of Heavy Elements Compared to Solar Giants

The most startling revelation from the JWST data was the low concentration of heavy elements, or metallicity, within the atmosphere of TOI 5205 b. While gas giants in our own Solar System like Jupiter are relatively metal rich, this exoplanet shows a much lower concentration of elements heavier than hydrogen and helium. Remarkably, the atmosphere of the planet is even less metal rich than the star it orbits, a finding that contradicts standard planetary formation theories. The detection of methane and hydrogen sulfide further defines a unique chemical profile that stands out among all giant planets studied to date.