Discovery of an Earth-sized planet orbiting the supercold dwarf star SPECULOOS-3

The SPECULOOS project revealed the existence of an Earth-sized planet around SPECULOOS-3, a nearby star similar in size to Jupiter and twice as cold as our Sun

The SPECULOOS project, led by astronomer Michel Guyon of the University of Liège, discovered a new Earth-sized planet around SPECULOOS-3, an ultracold dwarf star, as small as Jupiter, twice as cold as our Sun and located 55 light-years from Earth. After the famous TRAPPIST-1 planetary system, SPECULOOS-3 is the second planetary system found around a star of this type.

Cold dwarf stars are the least massive stars in our universe, similar in size to Jupiter, twice as cold as the Sun, ten times less massive and a hundred times less luminous than our Sun. Their lifespan is a hundred times longer than our sun, and they will be the last stars to shine when the universe cools and darkens. Although much more common than Sun-like stars, supercold dwarfs are still poorly understood due to their low luminosity. In particular, very little is known about their planets, even though they make up a significant part of the stellar population of our Milky Way Galaxy.

Against this backdrop, the SPECULOOS consortium, led by the University of Liège, has announced the discovery of a new Earth-sized planet orbiting a nearby cold dwarf star. The planet SPECULOOS-3 b is located about 55 light-years from Earth (very close in cosmic terms! Our own galaxy, the Milky Way, spans more than 100,000 light-years). SPECULOOS-3 is only the second planetary system discovered around a star of this type: "SPECULOOS-3 b is almost the same size as our planet," explains astronomer Michel Guyon, author of the first paper published in Nature Astronomy. A year, meaning an orbit around the star, lasts about 17 hours. The days and nights, however, will never end. We believe that the star rotates synchronously, so that the same side, called the day side, always faces the star, just as the moon does to the earth. On the other hand, the night side will be locked in eternal darkness."

The SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project, launched and led by astronomer Michel Guyon, was specifically designed to search for planets around the nearest supercold dwarf stars. "These stars are scattered throughout the sky, so you have to observe them one by one, over several weeks, to have a good chance of finding transiting planets," the researcher continues. "This requires a dedicated network of professional robotic telescopes." This is the idea behind SPECULOOS, which is shared by the universities of Liège, Cambridge, Birmingham, Bern, MIT and ETH Zurich.

"We designed SPECULOOS specifically to observe nearby supercold dwarfs in search of rocky planets that are suitable for detailed studies," notes Laetitia Dellera, an astronomer at the University of Liège. In 2017, the SPECULOOS prototype using the TRAPPIST telescope discovered the famous TRAPPIST-1 system consisting of seven Earth-sized planets, including several potentially habitable ones. It was a great start!”

The star SPECULOOS-3 is more than twice as cold as our Sun, with an average temperature of about 2,600°C. Due to its extremely short orbit, the star receives almost sixteen times more energy per second than the Earth from the Sun and is therefore bombarded with high-energy radiation. "In such an environment, the existence of an atmosphere around the star is highly improbable," says Julian de Wit, a professor at MIT and co-director of the SPECULOOS Northern Observatory and the Artemis telescope, jointly developed by the University of Liège and MIT, and is a pillar the medium of this discovery. "The fact that this planet has no atmosphere may be an advantage in several ways. For example, it may allow us to learn a lot about super-dwarf stars, which in turn will enable more in-depth studies of their potentially habitable planets."

SPECULOOS-3 b is proving to be an excellent target for the JWST space telescope, whose data is revolutionizing our vision of the universe. "With the JWST, we can even study the mineralogy of the star's surface!" Enthusiastic Elsa Ducrou, former researcher at the University of Liège and now at the Paris Observatory.

"This discovery demonstrates the SPECULOOS Northern Observatory's ability to identify Earth-sized planets suitable for detailed studies. And that's just the beginning! Thanks to the financial support of the Valonia Region and the University of Liège, two new telescopes, Orion and Apollo, will soon join Artemis on the plateau of the Teide volcano in Tenerife, to speed up the hunt for these fascinating stars," concludes Michel Guyon.

for the scientific article