Astronomers on Wednesday announced they have discovered a massive planet orbiting a tiny star, a bizarre pairing that has stumped scientists.
Most of the stars across the Milky Way are small red dwarfs such as TOI-6894, which has only 20 percent the mass of our sun.
It had not been thought possible that such puny, weak stars could provide the conditions needed to form and host huge planets, but an international team of astronomers have detected the unmistakable signature of a gas giant planet orbiting the undersized TOI-6894, according to a study in the journal Nature Astronomy.
Photo: NASA via AP
That makes the star the smallest star yet known to host a gas giant. The planet has a slightly larger radius than Saturn, but only half its mass. It orbits its star in a little more than three days.
The astronomers discovered the planet when searching through more than 91,000 low-mass red dwarfs observed by NASA’s Transiting Exoplanet Survey Satellite telescope.
Its existence was confirmed by ground-based telescopes, including Chile’s Very Large Telescope.
“The fact that this star hosts a giant planet has big implications for the total number of giant planets we estimate exist in our galaxy,” study coauthor Daniel Bayliss of England’s University of Warwick said in a statement.
Another coauthor, Vincent Van Eylen, of University College London, said it was an “intriguing discovery.”
“We don’t really understand how a star with so little mass can form such a massive planet,” he said. “This is one of the goals of the search for more exoplanets. By finding planetary systems different from our solar system, we can test our models and better understand how our own solar system formed.”
The most prominent theory for how planets form is called core accretion.
The process begins when a ring of gas and dust — called a protoplanetary disc — which surrounds a newly formed star builds up into a planetary core. That core attracts more gas that forms an atmosphere, eventually snowballing into a gas giant.
Under that theory, it is difficult for low-mass stars to host giant planets, because there is not enough gas and dust to begin building a core in the first place.
A rival theory proposes that such planets instead form when their protoplanetary disc becomes gravitationally unstable and breaks up, with the collapsing gas and dust forming a planet.
Neither theory seems to explain the existence of the newly discovered planet, TOI-6894b, the researchers said.
The planet also interests scientists because it is strangely cold.
Most of the gas giants discovered outside our solar system so far have been what are known as “hot Jupiters,” where temperatures soar well over 1,000°C.
The newly discovered planet appears to be less than 150°C, the researchers said.
“Temperatures are low enough that atmospheric observations could even show us ammonia, which would be the first time it is found in an exoplanet atmosphere,” study coauthor Amaury Triaud of England’s University of Birmingham said.
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