Groundbreaking Discovery of Giant Planet Challenges Existing Theories

In a remarkable breakthrough, an international team of astronomers has discovered a giant planet, designated TOI-6894b, orbiting the small red dwarf star TOI-6894. This star, which is only about 20% the mass of the Sun, defies established theories of planetary formation that suggest such low-mass stars should not host large planets. The findings were published in the prestigious journal *Nature Astronomy* on June 12, 2025, by a research team led by Dr. Edward Bryant, a Warwick Astrophysics Prize Fellow at the University of Warwick and UCL's Mullard Space Science Laboratory.

The discovery was part of a comprehensive analysis of data from the Transiting Exoplanet Survey Satellite (TESS), which involved examining over 91,000 low-mass red dwarf stars for giant planets. Dr. Bryant expressed his excitement, noting that TOI-6894b is the first giant planet identified around the lowest mass star known to date, which raises significant questions about the conditions necessary for planet formation around such stars.

Dr. Daniel Bayliss, an Associate Professor at the University of Warwick, emphasized the implications of the discovery, stating, "Most stars in our Galaxy are small stars exactly like this, with low masses and previously thought to not be able to host gas giant planets. So, the fact that this star hosts a giant planet has big implications for the total number of giant planets we estimate exist in our Galaxy."

The planet TOI-6894b is characterized as a low-density gas giant, slightly larger than Saturn but with only about 50% of Saturn's mass. This finding contradicts the widely accepted core accretion theory of planet formation, which posits that the formation of gas giants is less likely around low-mass stars due to insufficient gas and dust in their protoplanetary discs.

Dr. Vincent Van Eylen from UCL's Mullard Space Science Laboratory remarked, "It's an intriguing discovery. We don't really understand how a star with so little mass can form such a massive planet!" This unexpected finding suggests that either alternative theories of planet formation may be necessary, or that the existing models need significant refinement.

The research team proposes that TOI-6894b could have formed through an intermediate core-accretion process, where a protoplanet gradually accumulates gas without reaching the mass needed for runaway gas accretion. Another theory proposes that the planet may have formed from a gravitationally unstable disc, where the disc surrounding the star collapses under its own gravitational force, leading to planet formation.

To advance their understanding, the researchers plan to conduct detailed atmospheric analyses of TOI-6894b. Such investigations could reveal the planet's core structure and provide insights into its formation. The planet exhibits an unusually low temperature of approximately 420 Kelvin, making it an exceptional candidate for atmospheric characterization.

Professor Amaury Triaud from the University of Birmingham, a co-author of the study, noted that the atmospheric conditions of TOI-6894b are expected to be dominated by methane chemistry, which is rare in exoplanet studies. He stated, "These observations could even show us ammonia, which would be the first time it is found in an exoplanet atmosphere."

The James Webb Space Telescope (JWST) is scheduled to observe TOI-6894b within the next year, which will further aid in understanding the planet's atmosphere and formation. Co-author Dr. Andrés Jordán, a researcher at the Millennium Institute of Astrophysics and a professor at Adolfo Ibáñez University, highlighted the significance of this discovery, stating, "This system provides a new challenge for models of planet formation, and it offers a very interesting target for follow-up observations to characterize its atmosphere."

As astronomers continue to unravel the mysteries of TOI-6894b, this discovery not only challenges existing theories but also opens new avenues for understanding the diversity of planetary systems in our galaxy. It underscores the need for a re-evaluation of our understanding of how planets can form under conditions previously thought unfavorable for giant planet development.