Astronomers at the University of London and the University of Warwick have made a groundbreaking discovery, finding the smallest known star to host a transiting giant planet. The star, TOI-6894, is just 20% of the mass of our Sun and was expected to be unable to support the formation of large planets. However, observations from the Transiting Exoplanet Survey Satellite (TESS) revealed the unmistakable signature of a giant planet, TOI-6894b.
TOI-6894b is a low-density gas giant with a radius slightly larger than Saturn’s but with only 50% of Saturn’s mass. The discovery challenges the leading theory of planet formation, known as the core accretion theory, which suggests that the formation of large planets is unlikely around low-mass stars.
The team found that neither the accretion nor an unstable disc could completely explain the formation of TOI-6894b from the available data. To shed light on this mystery, astronomers plan to conduct a detailed atmospheric analysis of the planet, which may reveal whether it formed through accretion or via an unstable disc.
The atmosphere of TOI-6894b is unusually cold for a gas giant, with temperatures as low as 420 Kelvin. This makes it one of the most promising targets for studying cool atmospheres in exoplanets. The James Webb Space Telescope is set to observe the planet’s atmosphere within the next 12 months, which may help unravel the secrets behind TOI-6894b’s formation.
The discovery has significant implications for our understanding of how often small stars can form giant planets and provides a new challenge for models of planet formation.
Source: https://www.ucl.ac.uk/news/2025/jun/giant-planet-discovered-orbiting-tiny-star