Astronomers using the James Webb Space Telescope have discovered a Jupiter-sized exoplanet, TOI-5205 b, orbiting a small, cool red dwarf star.
The discovery is significant because the planet's existence and composition contradict long-standing models of how giant planets form around tiny stars. Current theories suggest that small stars should not have enough material in their disks to produce such massive planets.
Observations of the planet's atmosphere revealed an unexpectedly low metallicity, meaning it is poor in heavy elements [1]. This atmospheric composition is atypical for a planet of its size and location, leading researchers to describe it as a "forbidden" planet [2].
According to a report published April 6, 2026 [1], the data suggests that TOI-5205 b does not fit the expected chemical profile of giant planets orbiting M-dwarf stars. The presence of a Jupiter-sized body around such a small star suggests that the process of planetary accretion may be more complex than previously understood [2].
The James Webb Space Telescope allowed scientists to analyze the light passing through the planet's atmosphere to determine its makeup [3]. By identifying the lack of heavy metals, the team confirmed that the planet's formation likely deviated from the standard core-accretion model [1].
Scientists are now re-evaluating the relationship between star size and planetary mass. The discovery of TOI-5205 b indicates that giant planets can emerge in environments previously thought to be incapable of supporting them [2]. This finding pushes the boundaries of astrophysics and suggests that the diversity of planetary systems in the galaxy is greater than current simulations predict [3].
“TOI-5205 b is a Jupiter-sized planet with an unexpectedly low-metallicity atmosphere.”
The existence of TOI-5205 b suggests that the 'core-accretion' model, which posits that a solid core must form before gas is gathered, may not be the only way giant planets are created. If giant planets can form around low-mass M-dwarf stars with low-metallicity atmospheres, astronomers may need to rewrite the rules regarding which stars can host gas giants, potentially increasing the estimated number of such planets across the Milky Way.
