The identification of TO-6894B, an exoplanet roughly 86% the size of Jupiter orbiting the low-mass Redd star (0.2 solar masses), underscores the importance of enhancing our comprehension of the formation mechanisms of giant planets and their protoplanetary disc environments.
Artist’s illustration of TOI-6894B behind its host star. Image credit: Markgarlic/Warwick University.
The TOI-6894 system is located approximately 73 parsecs (238 light years) away in the Leo constellation.
This planet was discovered through a comprehensive analysis of data from NASA’s Transiting Exoplanet Survey Satellite (TESS), aimed at locating giant planets around low-mass stars.
“I was thrilled by this discovery. My initial focus was on observing a low-mass red star with TESS, in search of a giant planet,” remarked Dr. Edward Bryant, an astronomer from the University of London.
“Then, utilizing observations from ESO’s Very Large Telescope (VLT), one of the most substantial telescopes globally, I identified TO-6894B, a giant planet orbiting the smallest known star with such a companion planet.”
“I never anticipated that a planet like TOI-6894B could exist around such a low-mass star.”
“This finding will serve as a foundational element in our understanding of the boundary conditions for giant planet formation.”
TOI-6894B is a low-density gas giant, with a radius slightly exceeding that of Saturn, which has only 50% of its mass.
The parent star is the lowest mass star yet found to host a massive planet, being just 60% of the mass of the next smallest star observed with such a planet.
“Most stars in our galaxy are actually small, and it was previously believed that they couldn’t support a gas giant,” stated Dr. Daniel Baylis, an astronomer at Warwick University.
“Therefore, the fact that this star has a giant planet significantly impacts our estimates of the total number of giant planets likely to exist in the galaxy.”
“This is a fascinating discovery. We still don’t completely understand why relatively few stars can form such large planets,” commented Dr. Vincent Van Eilen, an astronomer at the University of London.
“This drives one of our objectives to search for more exoplanets.”
“By exploring different planetary systems compared to our own solar system, we can evaluate our models and gain insights into how our solar system was formed.”
The prevailing theory of planetary formation is known as core accretion theory.
According to this theory, the cores of planets are initially formed by accreting material, and as the core grows, it attracts gases that eventually create its atmosphere.
Eventually, the core becomes sufficiently large to initiate the runaway gas accretion process, leading to the formation of a gas giant.
However, forming gas giants around low-mass stars presents challenges, as the gas and dust necessary for planetary formation in their protoplanetary discs is limited, hindering the formation of a sufficiently large core to kickstart this runaway process.
The existence of TOI-6894B indicates that this model may be insufficient and that alternative theories need to be considered.
“Considering TO-6894B’s mass, it might have been formed through an intermediate core-fault mechanism, whereby the protoplanet forms and accumulates gas steadily without orbiting, making it large enough to undergo runaway gas accretion,” Dr. Edward explained.
“Alternatively, it might have formed due to an unstable gravitational disk.”
“In certain cases, the disk surrounding the star can become unstable due to the gravitational forces it exerts on itself.”
“These disks may fragment as gas and dust collapse, leading to planet formation.”
However, the research team found that neither theory fully accounted for the formation of TOI-6894B based on the data available.
“Based on the stellar irradiation affecting TOI-6894B, we anticipate that its atmosphere is primarily influenced by methane chemistry, which is quite rare to identify.”
“The temperatures are low enough that atmospheric observations may even reveal the presence of ammonia.”
TOI-6894B might serve as a benchmark for methane-dominated atmospheric studies and an ideal laboratory for investigating planetary atmospheres containing carbon, nitrogen, and oxygen beyond our solar system.
Survey results will be featured in the journal Nature Astronomy.
____
Bryant et al. A giant exoplanet in orbit around a 0.2 solar mass star. Nature Astronomy, Published online on June 4th, 2025. doi:10.1038/s41550-025-02552-4
Source: www.sci.news
