The discovery of TOI-6894b — an exoplanet around 86% the size of Jupiter in orbit around a 0.2-solar-mass red dwarf — highlights the need for a better understanding of giant planet formation mechanisms and the protoplanetary disk environments in which they occur.
An artist’s illustration of TOI-6894b behind its host star. Image credit: Mark Garlick / University of Warwick.
The TOI-6894 system is located approximately 73 parsecs (238 light-years) away in the constellation of Leo.
The planet has been discovered as part of a large-scale investigation of data from NASA’s Transiting Exoplanet Survey Satellite (TESS), looking for giant planets around low-mass stars.
“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 Dr. Edward Bryant, an astronomer at the University of Warwick and University College London.
“Then, using observations taken with one of the world’s largest telescopes, ESO’s Very Large Telescope (VLT), I discovered TOI-6894b, a 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-6894b is a low-density gas giant with a radius a little larger than Saturn’s but with only ~50% of Saturn’s mass.
The parent star is the lowest mass star to have a transiting giant planet discovered to date and is just 60% the size of the next smallest star to host such a planet.
“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, an astronomer 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.”
“It’s an intriguing discovery. We don’t really understand how a star with so little mass can form such a massive planet,” said Dr. Vincent Van Eylen, an astronomer at University College London.
“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.”
The most widely held theory of planet formation is called the core accretion theory.
A planetary core forms first through accretion (gradual accumulation of material) and as the core becomes more massive, it eventually attracts gases that form an atmosphere.
It then gets massive enough to enter a runaway gas accretion process to become a gas giant.
In this theory, the formation of gas giants is harder around low-mass stars because the amount of gas and dust in a protoplanetary disk around the star (the raw material of planet formation) is too limited to allow a massive enough core to form, and the runaway process to occur.
Yet the existence of TOI-6894b suggests this model cannot be completely accurate and alternative theories are needed.
“Given the mass of the planet, TOI-6894b could have formed through an intermediate core-accretion process, in which a protoplanet forms and steadily accretes gas without the core becoming massive enough for runaway gas accretion,” Dr. Edward said.
“Alternatively, it could have formed because of a gravitationally unstable disk.”
“In some cases, the disk surrounding the star will become unstable due to the gravitational force it exerts on itself.”
“These disks can then fragment, with the gas and dust collapsing to form a planet.”
But the team found that neither theory could completely explain the formation of TOI-6894b from the available data, which leaves the origin of this giant planet as an open question for now.
“Based on the stellar irradiation of TOI-6894b, we expect its atmosphere is dominated by methane chemistry, which is exceedingly rare to identify,” said University of Birmingham’s Professor Amaury Triaud.
“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.”
The findings appear in the journal Nature Astronomy.
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E.M. Bryant et al. A transiting giant planet in orbit around a 0.2-solar-mass host star. Nat Astron, published online June 4, 2025; doi: 10.1038/s41550-025-02552-4
