Discovery Of Small Star Giving Birth To Huge Planet Leaves Scientists Puzzled

NDTV

06-06-2025

Astronomers were left stunned after they found a giant planet, about the same size as Saturn, orbiting a really small red dwarf star, a study published in Nature Astronomy revealed. The star, named TOI-6894, is only about a fifth the mass of the Sun. The observation is quite intriguing, as notable planet formation theories suggest that it shouldn't have happened, as small stars should host small planets similar to Earth and Mars, not big ones. Astronomers believe that small stars are not expected to have suitable conditions for planet formation.
TOI-6894 is roughly 240 light-years from Earth in the constellation Leo. It is the smallest-known star to host a large planet. Meanwhile, the planet, called TOI-6894b, is a gas giant, like the Milky Way has Saturn and Jupiter.
The planet is completing an orbit in approximately three days, as it is about 40 times closer to its star as compared to Earth is to the Sun. Its location suggests that TOI-6894b's surface must be hot. In size, it is bigger than Saturn but slightly smaller than Jupiter. In mass, it is 56% that of Saturn and 17% that of Jupiter.
"I was very excited by this discovery. I originally searched through TESS observations of more than 91,000 low-mass red-dwarf stars looking for giant planets," says astrophysicist Edward Bryant of the University of Warwick in the UK, who led the international research team.
"Then, using observations taken with one of the world's largest telescopes, ESO's VLT, I discovered TOI-6894b, a giant planet transiting the lowest mass star known to date to host such a planet. We did not expect planets like TOI-6894b to be able to form around stars this low-mass. This discovery will be a cornerstone for understanding the extremes of giant planet formation," Bryant said.
Bryant added, "The question of how such a small star can host such a large planet is one that this discovery raises - and we are yet to answer."
The star and planet system was discovered during an investigation of NASA's TESS (Transiting Exoplanet Survey Satellite) and the European Southern Observatory's Chile-based Very Large Telescope (VLT).
"In small clouds of dust and gas, it's hard to build a giant planet," said exoplanet scientist and study co-author Vincent Van Eylen of University College London's Mullard Space Science Laboratory.
"This is because to build a giant planet, you need to quickly build a large planet core and then quickly accrete (accumulate) a lot of gas on top of that core. But there's only so much time to do it before the star starts shining and the disk rapidly disappears. In small stars, we think there's simply not enough mass available to build a giant planet quickly enough before the disk disappears," Van Eylen added.
How are planets formed?
The formation process starts with the Giant Molecular Cloud, in which a giant cloud of gas and dust collapses under its own gravity. After that, the centre of the cloud becomes a protostar, which eventually becomes a star.
Then a disk of gas and dust forms around the protostar. The small particles in the disk stick together, growing into larger bodies called planetesimals.
Then the collision and merger of Planetesimals happen, forming larger planetary embryos. The embryos undergo differentiation, where heavier elements sink to the centre.