Astronomers have discovered a giant Saturn-sized planet orbiting TOI-6894, the tiniest known star to host such a world.
This surprising find challenges established theories that small stars lack the material to form giant planets.
Discovery of a Giant Planet Around a Tiny Star
A group of astronomers from around the world, including scientists from the University of Liège and teams in the UK, Chile, the USA, and across Europe, has identified a massive planet circling an unusually small star. It is the tiniest one ever known to host such a large companion.
The star, named TOI-6894, is a red dwarf that has just 20% of the Sun’s mass. Red dwarfs are the most common type of star in our galaxy. However, it has long been believed that stars of this size don’t have enough material around them to form or hold on to giant planets. Despite this, researchers have now detected clear evidence of a giant planet, TOI-6894b, orbiting this very star. The findings appear in the journal Nature Astronomy.
TESS Spots It First, Earth Confirms
NASA’s Transiting Exoplanet Survey Satellite (TESS) first spotted the unusual signal from this system. The detection was part of a larger project searching for giant planets orbiting small stars, led by Dr. Edward Bryant of UCL’s Mullard Space Science Laboratory.
To confirm the signal, scientists conducted a wide-ranging campaign using telescopes on Earth. These included observatories from the SPECULOOS and TRAPPIST projects, both coordinated by the University of Liège.
Dr. Khalid Barkaoui, a researcher involved with both telescope programs, led the follow-up observations. “The transit signal was unambiguous in our data,” he said. “Our analysis ruled out all alternative explanations — the only viable scenario was that this tiny star hosts a Saturn-sized planet with an orbital period of just over three days. Additional observations confirmed that its mass is about half that of Saturn. This is clearly a giant planet.”
A Record-Breaking Star-Planet Combo
TOI-6894 now holds the record as the smallest star ever found to host a transiting giant planet. Its radius is 40% smaller than any previous known host of such a planet.
Prof. Jamila Chouquar, who was an astronomer at ULiege at the time of the discovery, added: “We previously believed that stars this small couldn’t form or hold on to giant planets. But stars like TOI-6894 are the most common type in the Milky Way — so our discovery suggests there may be far more giant planets out there than we thought.”
Rethinking How Planets Form
According to current planet formation models, giant planets are rare around small stars. This is because their protoplanetary disks — the gas and dust reservoirs from which planets form — are thought to lack the material needed to build massive cores and accrete thick gas envelopes.
Dr. Mathilde Timmermans, member of the SPECULOOS team and ULiege astronomer at the time of the discovery, noted: “The existence of TOI-6894b is hard to reconcile with existing models. None can fully explain how it formed. This shows that our understanding is incomplete, and underscores the need to find more such planets. That’s exactly the goal of MANGO, a SPECULOOS sub-program led by myself and Dr. Georgina Dransfield at the University of Birmingham.”
A New Era of Exoplanet Discovery
Prof. Michaël Gillon, Fund for Scientific Research – FNRS Research Director at ULiege and head of the SPECULOOS and TRAPPIST programs, concluded: “This giant planet orbiting a tiny star reveals that planetary diversity in the galaxy is even greater than we imagined. Most of the targets observed by SPECULOOS and TRAPPIST are similar stars, or even smaller — so we’re well positioned to uncover more cosmic outliers in the years ahead.”
Explore Further: This Giant Planet Shouldn’t Exist – But Astronomers Just Found It Around a Tiny Star
Reference: “A transiting giant planet in orbit around a 0.2-solar-mass host star” by Edward M. Bryant, Andrés Jordán, Joel D. Hartman, Daniel Bayliss, Elyar Sedaghati, Khalid Barkaoui, Jamila Chouqar, Francisco J. Pozuelos, Daniel P. Thorngren, Mathilde Timmermans, Jose Manuel Almenara, Igor V. Chilingarian, Karen A. Collins, Tianjun Gan, Steve B. Howell, Norio Narita, Enric Palle, Benjamin V. Rackham, Amaury H. M. J. Triaud, Gaspar Á. Bakos, Rafael Brahm, Melissa J. Hobson, Vincent Van Eylen, Pedro J. Amado, Luc Arnold, Xavier Bonfils, Artem Burdanov, Charles Cadieux, Douglas A. Caldwell, Victor Casanova, David Charbonneau, Catherine A. Clark, Kevin I. Collins, Tansu Daylan, Georgina Dransfield, Brice-Olivier Demory, Elsa Ducrot, Gareb Fernández-Rodríguez, Izuru Fukuda, Akihiko Fukui, Michaël Gillon, Rebecca Gore, Matthew J. Hooton, Kai Ikuta, Emmanuel Jehin, Jon M. Jenkins, Alan M. Levine, Colin Littlefield, Felipe Murgas, Kendra Nguyen, Hannu Parviainen, Didier Queloz, S. Seager, Daniel Sebastian, Gregor Srdoc, R. Vanderspek, Joshua N. Winn, Julien de Wit and Sebastián Zúñiga-Fernández, 4 June 2025, Nature Astronomy.
DOI: 10.1038/s41550-025-02552-4
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1 Comment
A Saturn-sized planet circling a tiny red dwarf is rewriting what scientists thought was possible in planetary systems.
VERY GOOD.
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