Astronomers have discovered an Earth-sized planet that is receiving so much radiation that its atmosphere has long since eroded away, leaving it barren. Life as we know it cannot exist on this sizzling world, but astronomers are interested in it for another reason: for the first time, they may be able to see the geology of a planet outside our solar system.
The newly discovered exoplanet, called SPECULOOS-3 b, is a rocky planet about 55 light-years from Earth. It zooms around its host star every 17 hours, but the days and nights on this planet are endless. Astronomers suspect that the planet is tidally locked to its star, just as the moon is to Earth. One day side always faces the star, while the night side is locked in eternal darkness.
Telescope observations show that frequent radiation from the exoplanet’s star, a 7-billion-year-old red dwarf the size of Jupiter, the planet toasts to Venus-like temperatures. So any atmosphere the planet easily escaped into space long ago, leaving behind an airless, sizzling ball of rock, astronomers reported in the new study, published May 15 in the journal Nature Astronomy.
“Life as we know it could not have originated on the planet’s surface – atmosphere or not – because it cannot contain large amounts of water in liquid form,” says lead author of the study. Michael Gillonan astronomer at the University of Liege in Belgium told LiveScience. “It looks like a barren rock planet Mercury.”
Related: James Webb telescope detects unique atmosphere around ‘Hell Planet’ in a distant galaxy
Although SPECULOOS-3 b is not friendly to life, the astronomers say the planet is close enough to Earth for detailed follow-up studies of its chemical composition, which should reveal whether the planet was ever geologically active. Observations have already been planned with the James Webb Space Telescope (JWST), for example, will be able to confirm whether volcanoes have erupted on the planet. That would reveal how rocky planets like SPECULOOS-3 b form around faint, lightweight stars and whether some of them could be favorable to life despite being close to their stars.
The researchers “searched intensively” for planetary siblings of SPECULOOS-3 b in the same star system but found none, Gillon said. He noted that these additional planets may exist, but are simply too small or too far from their host star to be seen.
A hot planet around a cool star
Gillon and his colleagues discovered SPECULOOS-3 b using a network of six telescopes spread across Chile, the Canary Islands and Mexico since 2011. That network is called the Search for Planets Eclipsing Ultra-Cool Stars, or SPECULOOS, and shares its name with a Belgian spiced shortbread that is traditionally given to children every December 6 on the occasion of Sinterklaas.
The main goal of the project is to detect rocky planets orbiting ultra-cool dwarf stars, whose small size makes it easier for telescopes to detect orbiting planets. Not only are they thousands of degrees cooler than the sun and hundreds of times weaker, but they also burn their fuel slowly and ultimately live much longer: about 100 billion years. (The sun will be about 10 billion years old when it dies in 4.5 billion years.)
“These are expected to be the last stars left shining in the universe,” study co-author Amaury Triaudsaid a professor of exoplanetology at the University of Birmingham in England in a rack. Their extra-long lifespans provide favorable opportunities for life to emerge on planets within their systems, the researchers say.
However, their extreme weakness makes them difficult to study. To discover SPECULOOS-3 b, a SPECULOOS robotic telescope in Mexico observed continuous noticeable dips in the host star’s light for five nights in 2021. The first indications of the orbiting new planet then emerged and were confirmed a year later, according to the study.
“If there is no atmosphere, there would be no blue sky or clouds – it would just be dark, like on the surface of the moon,”” co-author of the study Benjamin Rackhamsaid a research scientist at MIT in a separate article statement from MIT. “And the ‘sun’ would be a large, purple-red, spotted and exuberant star that would look about 18 times bigger when the sun is in the sky in front of us.”
SPECULOOS-3 b is the ninth planet found by the project, and the team expects to discover many more in the coming years, Gillon said. Like the planets previously discovered by the project – including a family of seven in the well-known TRAPPIST-1 system, some of which are considered potentially habitable – the newly discovered SPECULOOS-3 b is “an excellent target for JWST,” Gillon said.
“With this world we could actually start exoplanetary geology,” study co-author Julien de Wit, an assistant professor of planetary sciences at MIT, said in the MIT statement. “How cool is that?”