Many small red dwarf stars inhabit the Milky Way galaxy, with the lower-mass ones unlikely to support planet formation. Yet somehow, the tiny red dwarf TOI-6894 defies expectations after astronomers recently discovered a gas giant in its orbit.
At just 20% of the mass of Earth’s Sun, researchers didn’t expect to find an exoplanet gas giant, TOI-6894b, as they scoured data from the ESO’s Very Large Telescope (VLT). This discovery represents the lowest-mass star ever recorded to host a gas giant, as found in a study led by Dr. Edward Bryant, Warwick Astrophysics Prize Fellow and lead author of the paper.
“I was very excited by this discovery. I originally searched through TESS observations of more than 91,000 low-mass red-dwarf stars looking for giant planets,” said Bryant.
VLT Spies the Unexpected Planet Around a Red Dwarf
Armed with observations from one of the largest telescopes, the ESO’s VLT, Bryant discovered the curious giant planet “transiting the lowest mass star known to date to host such a planet.”
“We did not expect planets like TOI-6894b to be able to form around stars this low-mass. This discovery will be a cornerstone for understanding the extremes of giant planet formation.”
TOI-6894 defies the odds by being a surprising 60% smaller than the next lowest mass star. In turn, the gas giant TOI-6894b has a very low density, coming in at about half the mass of Saturn, despite being marginally larger.
“Most stars in our Galaxy are actually small stars exactly like this, with low masses and previously thought to not be able to host gas giant planets,” said Dr. Daniel Bayliss, Associate Professor at The University of Warwick. “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.”
Gas Giant Defies Expectations
One intriguing element of TOI-6894b is its cool atmosphere for a gas giant. By comparison, most exoplanet gas giants are 1000-2000 Kelvin “hot Jupiters”, while TOI-6894b is around just 420 Kelvin.
“Based on the stellar irradiation of TOI-6894b, we expect the atmosphere is dominated by methane chemistry, which is exceedingly rare to identify,” said co-author Professor Amaury Triaud, University of Birmingham. “Temperatures are low enough that atmospheric observations could even show us ammonia, which would be the first time it is found in an exoplanet atmosphere.”
“TOI-6894b likely presents a benchmark exoplanet for the study of methane-dominated atmospheres and the best ‘laboratory’ to study a planetary atmosphere containing carbon, nitrogen, and oxygen outside the Solar System,” Triaud said.
Red Dwarf Star Challenges Models
Dr Vincent Van Eylen from UCL’s Mullard Space Science Laboratory called it “an intriguing discovery,” adding, “We don’t really understand how a star with so little mass can form such a massive planet! This is one of the goals of the search for more exoplanets.”
“By finding planetary systems different from our solar system, we can test our models and better understand how our own solar system formed,’ Eylen added.
Core accretion theory is the most commonly accepted explanation for planet formation. In this theory, planetary cores slowly accrete from bits of solid material in the protoplanetary disc surrounding a star until enough mass is collected to attract and capture a gaseous atmosphere.
A gas giant is formed when the mass becomes so great that it leads to runaway gas collection. Since low-mass stars have much smaller protoplanetary discs, astronomers expect them to be incapable of producing massive cores due to limited raw materials. TOI-6894b shatters this hypothesis, indicating that astronomers must revise their basic assumptions about planet formation.
“Alternatively, it could have formed because of a gravitationally unstable disc. In some cases, the disc surrounding the star will become unstable due to the gravitational force it exerts on itself. These discs can then fragment, with the gas and dust collapsing to form a planet,” Edward said.
Continuing Observations
Further analysis determined that the existing data was a poor match for both theories. In the future, the team believes atmospheric analysis may provide important answers about the strange planet; material distribution measurements would allow astronomers to calculate the core’s size and structure and answer whether it originated through accretion or an unstable disc.
The team has already scheduled observations using the James Webb Space Telescope to examine TOI-6894b’s atmosphere further sometime in the next year.
“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,” said co-author Dr. Andrés Jordán, researcher at the Millennium Institute of Astrophysics and professor at Adolfo Ibáñez University.
“This discovery is the result of a systematic program we have been carrying out for several years from Chile and the UK. Our efforts have allowed us to contribute significantly to a better understanding of how often small stars can form giant planets, and we are providing prime targets for follow-up with space-based platforms.”
The paper “A Transiting Giant Planet in Orbit Around a 0.2-Solar-mass Host Star” appeared on June 04, 2025, in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.