A massive planet has been discovered orbiting an unusually small red dwarf star, in a discovery that has left scientists scratching their heads. The gas giant, dubbed TOI-6894b, was found circling a star just 20 percent the mass of our Sun — the smallest star known to host such a large planet.
The findings, published Wednesday in Nature Astronomy, challenge long-standing theories about planet formation, especially around low-mass stars like TOI-6894. The star’s relatively puny size and faint energy output were previously thought to be inadequate for producing or sustaining gas giants.
Astronomers made the discovery while sifting through data from more than 91,000 red dwarf stars captured by NASA’s TESS space telescope. The planet’s presence was confirmed using several ground-based observatories, including Chile’s Very Large Telescope.
TOI-6894b is slightly larger in radius than Saturn but has only about half its mass. It completes a full orbit around its host star in just over three days. Despite its proximity to the star, the planet remains unusually cool — with estimated temperatures under 150°C, far colder than the searing temperatures typically seen in other “hot Jupiter”-type exoplanets.
“This is the smallest star we know of that hosts a giant planet,” said Daniel Bayliss of the University of Warwick. “It could significantly change our estimates of how many giant planets might exist in our galaxy.”
Vincent Van Eylen of University College London, another co-author of the study, added, “We don’t really understand how a star with so little mass can form such a massive planet. Discoveries like this are exactly why we search for exoplanets — to challenge our models and improve our understanding of how planetary systems like our own form.”
Traditional theories of planetary formation, particularly the core accretion model, suggest that stars need to have enough gas and dust in their surrounding protoplanetary disc to build up a core and gather a massive atmosphere. Red dwarfs, being smaller and less material-rich, were believed to be poor candidates for forming gas giants.
An alternative theory proposes that planets form when the disc becomes unstable and fragments, but even this idea struggles to explain the existence of TOI-6894b.
Adding to the intrigue, the planet’s low temperature means it may be possible to detect atmospheric features not yet observed in any exoplanet, including ammonia. “If ammonia is present, it would be a groundbreaking first,” said Amaury Triaud of the University of Birmingham.
The James Webb Space Telescope is expected to conduct further observations of TOI-6894b in the coming year, potentially offering new insights into the chemistry and origins of this enigmatic planet.