An international team of astronomers has discovered a Saturn-sized planet orbiting a star that’s only 20% the mass of the Sun – a pairing that should not exist.
The extreme example of a planetary mismatch consists of a red dwarf, codenamed TOI-6894, and its giant companion, TOI-6894b. It is the smallest known star ever found to host such a large planet.
While red dwarfs like TOI-6894 are the most common type of star in the Milky Way, scientists previously believed that stars this small were incapable of forming or retaining such giant planets.
A research team led by Dr. Edward Bryant from UCL Mullard Space Science Laboratory in the UK first identified the exceptional system in data from NASA’s Transiting Exoplanet Survey Satellite (TESS).
The planetary nature of the discovery was then confirmed by an extensive ground-based observation campaign that involved several telescopes, including those of the SPECULOOS and TRAPPIST, both led by the University of Liège in Belgium.
Researchers from Chile and the US, as well as from elsewhere in Europe, were also involved in the study that was published in the science journal Nature Astronomy.
“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,” said Dr. Khalid Barkaoui, researcher on the SPECULOOS and TRAPPIST teams.
Further observations showed that the planet’s mass is about half that of Saturn, the second-largest planet in the Solar System after Jupiter. TOI-6894 is now the smallest star known to host a transiting giant planet.
“We previously believed that stars this small couldn’t form or hold on to giant planets,” said Prof. Jamila Chouquar, who was an astronomer at Liège at the time of the discovery. The fact that they’re so common now challenges that notion, she added.
Discovery challenges planetary formation theories
Scientists believe that giant planets don’t normally form around small stars because they lack the gas and dust needed to build them. This new discovery now challenges that idea as well.
“The existence of TOI-6894b is hard to reconcile with existing models. None can fully explain how it formed,” said Dr. Mathilde Timmermans, member of the SPECULOOS team and ULiege astronomer at the time of the discovery.
“This shows that our understanding is incomplete, and underscores the need to find more such planets,” she added.
According to Prof. Michaël Gillon, head of the SPECULOOS and TRAPPIST programs, the discovery of the giant planet orbiting a tiny star “reveals that planetary diversity in the galaxy is even greater than we imagined” and that more “cosmic outliers” are likely to be uncovered.
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