Earth-sized exoplanet discovered around ultracool dwarf star

• Astronomers with the SPECULOOS project discovered an Earth-sized exoplanet orbiting an ultracool dwarf star. It’s only the second planetary system astronomers have found so far around such a star.
• The planet, SPECULOOS-3 b, is about the same size as Earth but orbits very close to its star, completing an orbit in only 17 hours. The planet is also likely tidally locked to its star.
• Radiation from the star has likely decimated any atmosphere the planet had.

Exoplanet around an ultracool dwarf star

For only the second time, astronomers have discovered an Earth-sized exoplanet, SPECULOOS-3 b, orbiting an ultracool dwarf star. Researchers from the SPECULOOS project, led by the University of Liège in Belgium, said they found the new planet on May 15, 2024. SPECULOOS-3 b is about 55 light-years away. The planet is almost the same size as Earth and orbits its star in only 17 hours. It likely has little to no atmosphere, however. That’s because – even though its star is much dimmer than our sun – the planet orbits so close to it that radiation from the star blasts its surface.

Partner universities included the Universities of Cambridge, Birmingham and Bern, Massachusetts Institute of Technology and ETH Zürich.

The international research team published the peer-reviewed details of the discovery in Nature Astronomy on May 15, 2024.

Meet Earth-sized exoplanet SPECULOOS-3 b

The Search for Planets EClipsing ULtra-cOOl Stars (SPECULOOS) project, led by astronomer Michaël Gillon at the University of Liège in Belgium, helped an international team of astronomers discover the new Earth-sized world. SPECULOOS-3 b is very close in size to Earth. And it’s just 55 light-years away from us. Its year is much shorter than ours, however, at only 17 hours long. The researchers think it is tidally locked to its star. This means that one side is always facing the star, while the other faces outward, in permanent night. Gillon said:

SPECULOOS-3 b is practically the same size as our planet. 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 day side, always faces the star, just like the moon does for the Earth. On the other hand, the night side would be locked in endless darkness.

Astronomer Michaël Gillon and the #SPECULOOS team discovered a new Earth-sized exoplanet around SPECULOOS-3, an "ultracool dwarf" star as small as Jupiter, twice as cold as our Sun, and located 55 light-years from Earth. #ULiège @NatureAstronomy @NASA https://t.co/2bQGJNhBmz pic.twitter.com/K0l9RBpbD6

— Université de Liège (@UniversiteLiege) May 15, 2024

No atmosphere on SPECULOOS-3 b

But while the exoplanet is almost the same size as our planet, it probably doesn’t have any atmosphere left. It orbits so close to its star that radiation from the star is intense enough to strip away any atmosphere. As co-author Julien de Wit, University of Liège alumnus, MIT professor and co-director of the SPECULOOS Northern Observatory, noted:

In such an environment, the presence of an atmosphere around the planet is highly unlikely.

That’s bad news for habitability, but not necessarily for science overall, as de Wit noted:

The fact that this planet has no atmosphere could be a plus in several respects. For example, it could enable us to learn a great deal about ultracool dwarf stars, which in turn will make possible more in-depth studies of their potentially habitable planets.

‘Cold’ star but hot planet

The star SPECULOOS-3 is similar in size to Jupiter. It is also 10 times less massive, more than twice as “cold” and 100 times less luminous than the sun, with an average temperature of 4,700 degrees Fahrenheit (2,600 degrees Celsius). Yet the planet is so close, it still receives about 16 times more energy than Earth does from the sun.

Astronomers designed SPECULOOS specifically to search for Earth-sized rocky planets around ultracool dwarf stars. It’s still not an easy task, however. Gillon said:

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.

Future observations

SPECULOOS-3 b might also be a good target for the James Webb Space Telescope. A former researcher at the University of Liège, Elsa Ducrot, now at Paris Observatory, said:

With the JWST, we could even study the mineralogy of the planet’s surface!

Gillon added:

This discovery demonstrates the ability of our SPECULOOS-North observatory to detect Earth-sized exoplanets suitable for detailed study. And this is just the beginning! Thanks to the financial support of the Walloon 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 planets.

Steve B. Howell at NASA Ames Research Center in California also added:

We’re making great strides in our study of planets orbiting other stars. We have now reached the stage where we can detect and study Earth-sized exoplanets in detail. The next step will be to determine whether any of them are habitable, or even inhabited.

Bottom line: Astronomers have discovered a new Earth-sized exoplanet orbiting the ultracool dwarf star SPECULOOS-3. But any atmosphere it had is probably long gone.

Source: Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3

Via University of Liège

Via NASA

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