The planet, named SPECULOOS-3 b, is the second of its kind discovered around this type of star. It completes an orbit of the star in about 17 hours. The star is more than twice as cold as our sun and significantly less massive and luminous.

SPECULOOS-3 b is likely tidally locked, meaning the same side always faces the star, similar to the relationship between the moon and Earth.

The discovery, published on May 15, 2024, in Nature Astronomy, was made by the SPECULOOS project, led by the University of Liege, in collaboration with the Universities of Birmingham, Cambridge, Bern, and the Massachusetts Institute of Technology. SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) uses a network of robotic telescopes worldwide to search for exoplanets orbiting ultra-cool dwarf stars.

Ultra-cool dwarf stars, which constitute about 70% of the stars in the Milky Way, are faint and scattered, requiring weeks of observation to detect transiting planets.

"We designed SPECULOOS specifically to observe nearby ultracool dwarf stars in search of rocky planets that lend themselves well to detailed studies," says Michael Gillon, astronomer at the University of Liege and lead author on the paper. "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!"

While most observations were made by SPECULOOS telescopes in the Northern Hemisphere, researchers from the University of Birmingham contributed observations from the SPECULOOS South Observatory in the Atacama Desert, Chile.

Amaury Triaud, Professor of Exoplanetology at the University of Birmingham, commented, "The discovery of SPECULOOS-3 shows our worldwide network functions well and is ready to detect yet more rocky worlds orbiting very low mass stars. While ultra-cool 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 lifespan may offer opportunities for life to develop on orbiting planets.

In this case, planetary candidates were not detected by algorithms but by team members manually reviewing nightly data. Dr. Georgina Dransfield, a former PhD student at the University of Birmingham and a current postdoctoral researcher, noticed the planetary signal and alerted the team.

She said, "The small size of ultra-cool dwarfs makes it easier to detect small planets. SPECULOOS-3 b is special in that its stellar and planetary properties make it an optimal target for JWST, which is capable to get information about the composition of the rocks that make its surface."

Next steps include follow-up observations by the James Webb Space Telescope to gather important insights into the planet's surface mineralogy and potential atmosphere.

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

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