A Giant Planet and a Small Star Are Shaking Up Conventional Cosmological Theory

WIRED

09-07-2025

Jul 9, 2025 7:00 AM A gas giant comparable in size to Saturn has been found orbiting a small red dwarf star. The discovery is making experts reconsider conventional notions of how planets form. An illustration of the giant gas planet TOI-6894b orbiting the small, dim red dwarf star TOI-6894. Illustration: University of Warwick/Mark Garlick
Many of the stars in the Milky Way galaxy are small, dim red dwarfs—stars much smaller than the sun in both size and mass. TOI-6894, located far away from Earth, is one of them.
Astronomers previously thought a star like this could not have large planets circulating it, because its mass is only about 20 percent of the sun, meaning its planetary system—generated from materials surrounding the star—would not have contained enough mass to form a giant body like Saturn or Jupiter.
But when observing TOI-6894, an international research team detected a clear transit signal—a temporary decrease in a star's brightness caused by a planet passing across it. This newly discovered planet, named TOI-6894b, blocks 17 percent of the star's light, indicating the planet is fairly large. The signal was picked up by the Transiting Exoplanet Survey Satellite (TESS), an observation instrument launched by NASA to hunt for planets orbiting stars outside of our solar system.
This makes TOI-6894 'the lowest mass star known to date to host such a planet,' said Edward Bryant, Astrophysics Prize Fellow at the University of Warwick, in a press statement. The finding appears to upend conventional theory on how planets are formed. 'This discovery will be a cornerstone for understanding the extremes of giant planet formation,' Bryant said.
Astronomers at University College London and the University of Warwick, as part of a global collaboration with partners in Chile, the US, and Europe, trawled through the data of about 91,000 red dwarf stars observed by TESS before discovering the planet TOI-6894b. After that, the nature of TOI-6894b was clarified by additional observations made with other telescopes. According to these, TOI-6894b's radius is slightly larger than Saturn's, but its mass is only about half that of the ringed giant. Its density is extremely light at only 0.33 g/cm³, indicating that it is an expanding gas planet.
TOI-6894 is nearly 40 percent smaller than the previous record for the smallest star with a planet of this size. This fact poses a serious contradiction to conventional theories of planet formation.
The widely accepted planetary formation model, the 'core-accumulation theory,' proposes that a ring of dust and rocks—known as protoplanetary disk—forms around a star, and that materials in this disk then gather together to form the cores of planets. After starting out this way, larger gas planets then accrete gases around their cores to become gigantic. But if the mass of the star is small, the mass of its protoplanetary disk tends to be small as well. In such a scenario, the nucleus necessary for the formation of a giant gas planet will not grow.
Based on this theory, it is estimated that more than 120 times more solid matter than that of the Earth would be required to form TOI-6894b. However, the observed disk surrounding the star TOI-6894 contains only 58 times the mass of the Earth at most. This raises the possibility of an alternative planet-formation mechanism existing.
One suggestion by Bryant and the team is that planets could form by gradual gas accumulation, without the initial formation of a massive core. 'Alternatively, it could have formed because of a gravitationally unstable disc,' Bryant says. '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.'
However, the press statement goes on to explain that neither of these theories can could completely explain the formation of TOI-6894b, based on the data gathered. For now the planet's origin remains a mystery.
Analyzing the atmosphere of the newly discovered exoplanet might yield answers; it is possible that chemical traces of its formation process remain. Observations of TOI-6894b by the James Webb Space Telescope over the next 12 months are expected to reveal details about its interior structure and atmospheric composition, which will have a major say on whether current theories about planet formation are supported or new ones are needed.
The discovery may force us to rethink our unified model of planet formation, says Andrés Jordán of the Millennium Astrophysics Institute at Adolfo Ibáñez University in Chile. Years of steady observations are pushing the limits of theory, and it may even be time to fundamentally rethink our estimates of the total number of giant planets in the galaxy.
This story originally appeared on WIRED Japan and has been translated from Japanese.