Earth-sized exoplanet found around “ultracool dwarf”

The search for exoplanets which orbit ultracool stars has turned up a second discovery.

The Earth-sized exoplanet, discovered by the SPECULOOS project, is orbiting a star which is about the size of Jupiter and twice as cool as our Sun, at about 2,600°C.

Studying ultracool dwarf stars and their planets could help narrow the search for potentially habitable exoplanets.

The dwarf star, dubbed SPECULOOS-3, is only about 55 light-years from Earth and follows TRAPPIST-1 as the only ultracool dwarf with orbiting planets. The system is described in a new paper published in Nature Astronomy.

SPECULOOS-3 b, the planet orbiting the ultracool dwarf, is roughly the same size as Earth.

But a year on the exoplanet is only 17 hours, says first author Michaël Gillon from the University of Liège in Belgium.

“Days and nights, on the other hand, should never end,” Gillon adds. “We believe that the planet rotates synchronously, so that the same side, called the day side, always faces the star, just like the Moon does for the Earth. The night side hand would be locked in endless darkness.”

Gillon set up SEPCULOOS (Search for Planets EClipsing ULtra-cOOl Stars) precisely to find such exoplanets around ultracool dwarfs.

“These stars are scattered across the sky, so you must observe them one by one, over a period of weeks, to have a good chance of detecting transiting planets,” Gillon explains. “This requires a dedicated network of professional robotic telescopes.”

“We designed SPECULOOS specifically to observe nearby ultra-cool dwarf stars in search of rocky planets, ” says corresponding author Laetitia Delrez, also at the University of Liège. “In 2017, our SPECULOOS prototype discovered the famous TRAPPIST-1 system made up of 7 Earth-sized planets, including several potentially habitable ones. This was an excellent start!”

Exoplanet and cool star
SPECULOOS-3 b orbiting its star. Credit: NASA/JPL-Caltech.

SPECULOOS-3 b is unlikely to be habitable.

Because it orbits its host star so tightly, it receives almost 16 times as much energy per second as Earth and is bombarded with high-energy radiation. As a result, it probably doesn’t have an atmosphere.

“The fact that this planet has no atmosphere could be a plus in several respects,” says corresponding author Julien de Wit, a professor at MIT. “For example, it could enable us to learn a great deal about ultra-cool dwarf stars, which in turn will make possible more in-depth studies of their potentially habitable planets.”

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