Earth-Sized Exoplanet Found Orbiting Nearby Ultracool Red Dwarf

Ultracool dwarf stars are widespread in the solar neighborhood. Nevertheless, their extremely low brightness has left their planetary population largely unexplored, and only one of them, TRAPPIST-1, has been found to host a transiting planetary system, until now. Astronomers with the SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project have now detected an Earth-sized planet in a 17-hour orbit around the nearby ultracool dwarf star SPECULOOS-3.

SPECULOOS-3 is an M6.5 spectral type dwarf star located 16.8 parsecs (55 light-years) away in the constellation of Cygnus.

Also known as LSPM J2049+3336 and TIC 230741378, the star is more than twice as cold as our Sun, as well as ten times less massive and a hundred times less luminous.

“We designed SPECULOOS specifically to observe nearby ultracool dwarf stars in search of rocky planets that lend themselves well to detailed studies,” said Dr. Michaël Gillon, an astronomer at the University of Liège.

“In 2017, our SPECULOOS prototype using the TRAPPIST telescope discovered the famous TRAPPIST-1 system made up of seven Earth-sized planets, several of them potentially habitable. This was an excellent start!”

The newfound exoplanet, dubbed SPECULOOS-3b, orbits its parent star once every 17 hours.

The alien world is likely to be tidally locked, so its dayside always faces the star, which has an average temperature of around 2,600 degrees Celsius.

Due to its short orbit, SPECULOOS-3b receives almost times more energy per second than the Earth does from the Sun and is therefore literally bombarded with high-energy radiation.

“SPECULOOS-3b is practically the same size as our home planet,” Dr. Gillon said.

“A year, i.e. an orbit around the star, lasts around 17 hours. Days and nights, on the other hand, should never end. We believe that the planet rotates synchronously, so that the same side, called the dayside, always faces the star, just like the Moon does for the Earth. On the other hand, the nightside would be locked in endless darkness.”

“The discovery of SPECULOOS-3b shows our worldwide network functions well and is ready to detect yet more rocky worlds orbiting very low mass stars,” said University of Birmingham’s Professor Amaury Triaud.

“While ultracool dwarf stars are cooler and smaller than our Sun, their lifespan is over a hundred times longer — around 100 billion years — and they are expected to be the last stars still shining in the Universe.”

“This long life span could offer opportunities for extraterrestrial life on orbiting planets to develop.”

“The small size of ultracool dwarfs makes it easier to detect small planets,” said Dr. University of Birmingham, an astronomer at the University of Birmingham.

“SPECULOOS-3b is special in that its stellar and planetary properties make it an optimal target for Webb, which is capable to get information about the composition of the rocks that make its surface.”

The study was published in the journal Nature Astronomy.

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M. Gillon et al. Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3. Nat Astron, published online April 10, 2024; doi: 10.1038/s41550-024-02271-2