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Researchers using global robotic telescopes have discovered an Earth-sized planet, SPECULOOS-3 b, orbiting an ultracool red dwarf in Milky Way. Tidal-locked and possibly devoid of an atmosphere due to intense radiation, this planet offers new insights into long-lived red dwarfs, which are expected to be among the last burning stars in the universe.

Our galaxy is a treasure trove of red stars. In fact, more than 70% of the stars in the Milky Way are M dwarfs, also known as red dwarfs. These stars are cool and dark compared to our Sun, but they often blast orbiting exoplanets with high-energy radiation, especially early in their lives. And these “lives” continue a long time. Stars like our Sun burn for about 10 billion years before becoming hungry red giants, gobbling up any planets too nearby. M dwarfs continue to burn for 100 billion years or more, perhaps offering a foothold for life and an even longer window for life to develop.

An international team using robotic telescopes around the world recently spotted an Earth-sized planet orbiting an ultracool red dwarf, the darkest and longest-lived star. When the universe turns cold and dark, these will be the last stars to burn.

The discovery

The exoplanet SPECULOOS-3 b is about 55 light-years from Earth (really close when you consider the cosmic scale!) and about the same size. One year there, one revolution around the star takes about 17 hours. The days and nights may never end, however: The planet is thought to be tidally locked, so that the same side, known as the day side, always faces the star as the Moon faces the Earth. The night side would be locked in endless darkness.

An artist’s concept of the exoplanet SPECULOOS-3 b orbiting its red dwarf star. The planet is large around Earth, while its star is slightly larger than Jupiter – but much more massive. Credit: Lionel Garcia

Exploring the ultra-cool dwarves

In our corner of the galaxy, ultracool dwarf stars are ubiquitous. They are so faint that their planetary population is largely unexplored. The SPECULOOS (Search for Planets EClipsing ULTra-coOL Stars) project, led by Michael Gillon of the University of Liege, Belgium, is designed to change that. Extremely cool dwarf stars are scattered across the sky, so you have to observe them one by one for weeks to get a good chance of finding transiting planets. For this you need a special network of professional telescopes. This is the concept of SPECULOOS.

“We designed SPECULOOS specifically to probe nearby ultra-cool dwarf stars in search of rocky planets,” Gillen said. “With the SPECULOOS prototype and the crucial help of NASA With the Spitzer Space Telescope, we discovered the famous TRAPPIST-1 system. That was a great start!”

Gillen is the lead author of the paper announcing the planet’s discovery, published on May 15, 2024 in Natural astronomy. The project is a truly international endeavor in partnership with the universities of Cambridge, Birmingham, Bern, MIT and ETH Zurich.

The star SPECULOOS-3 is thousands of degrees colder than our Sun, with an average temperature of about 4,760 F (2,627 C), but it blasts its planet with radiation, meaning it likely has no atmosphere.

Seeing the star, let alone the planet, is a feat in itself. “Even though this particular red dwarf is more than a thousand times dimmer than the Sun, its planet orbits much, much closer to Earth, heating the surface of the planet,” said co-author Kathryn Clark, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory in Southern California.

Fun facts

• While the planet is large around Earth, its star is slightly larger than Jupiter – but much more massive.
• The planet receives almost 16 times more energy per second than Earth receives from the Sun.
• Did you catch the cookie link? The planet-finding program SPECULOOS shares its name with the tampered confection. Both are from Belgium. Sweetie!

The next steps

SPECULOOS-3 b is an excellent candidate for follow-up observations by the James Webb Space Telescope. Not only can we learn about the potential for an atmosphere and the mineralogy of the surface, but it can also help us understand the stellar neighborhood and our place in it.

“We are making great strides in our study of planets orbiting other stars. We have now reached the stage where we can discover and study Earth-sized exoplanets in detail. The next step will be to determine whether any of them are habitable or even habitable,” said Steve B. Howell, one of the planet’s discoverers at NASA’s Ames Research Center in Silicon Valley.

For more information on this discovery:

Reference: “Discovery of an Earth-sized exoplanet orbiting a nearby ultracool dwarf star SPECULOOS-3” by Michael Gillen, Peter P. Pedersen, Benjamin W. Rackham, Georgina Dransfield, Elsa Ducrot, Khalid Barkawi, Artem Y. Burdanov, Urs Schrofenegger, Yilen Gómez Maqueo Chew, Susan M. Lederer, Roi Alonso, Adam J. Burgasser, Steve B. Howell, Norio Narita, Julien de Wit, Brice-Olivier Demory, Didier Queloz, Amaury HMJ Triaud, Laetitia Delrez, Emmanuël Jehin, Matthew J. Hooton, Lionel J. Garcia, Clàudia Jano Muñoz, Catriona A. Murray, Francisco J. Pozuelos, Daniel Sebastian, Mathilde Timmermans, Samantha J. Thompson, Sebastián Zúñiga-Fernández, Jesús Aceituno, Christian Aganze, Pedro J. Amado . Akihiko Fukui, Roman Gerasimov, Murad Gachui, Maximilian N. Guenther, Samantha Hassler, Yuya Hayashi, Kevin Heng, Renyu Hu, Taiki Kagetani, Yugo Kawai, Kiyoe Kawauchi, Daniel Kitzman, Daniel DB Kohl, Monica Lendl, John H .Livingston, Xintong Liu, Eric A. Meyer Valdez, Mayuko Mori, James J. McCormack, Felipe Murgas, Prajval Niraula, Enrique Pale, Ilse Plauchu-Frein, Rafael Rebollo, Laurence Sabin, Yannick Shacky, Nicole Schanche, Frank Celsis, Alfredo Sota, Manu Stalport, Matthew R. Standing, Keivan G. Stassun, Motohide Tamura, Yuka Terada, Christopher A. Theissen, Martin Turbet, Valérie Van Grootel, Roberto Varas, Noriharu Watanabe, and Francis Zong Lang, 15 May 2024, Natural astronomy.
DOI: 10.1038/s41550-024-02271-2