Astronomers have found an Earth-sized planet, SPECULOOS-3 b, which has had its atmosphere stripped away by intense radiation, leaving its surface exposed. While this planet isn't suitable for life, it presents a unique chance for scientists to study the geology of an exoplanet for the first time.
SPECULOOS-3 b, located approximately 55 light-years away, is a rocky planet that completes an orbit around its star every 17 hours. The planet is tidally locked, meaning one side always faces its star while the opposite side remains in perpetual darkness. This results in a continuous day on one side and an endless night on the other.
The planet's host star, a red dwarf about the size of Jupiter and around 7 billion years old, bombards SPECULOOS-3 b with radiation. This radiation heats the planet to temperatures similar to those of Venus, causing any atmosphere it might have had to dissipate long ago, leaving behind a barren, scorching surface. These findings were published on May 15 in the journal Nature Astronomy.
Michaël Gillon, an astronomer at the University of Liège in Belgium and the lead author of the study, explained, "Life as we know it could not emerge on the surface of the planet—atmosphere or not—because it could not sustain large amounts of water in liquid form." He likened the planet to Mercury, describing it as a bare rock.
Although SPECULOOS-3 b isn't suitable for life, its proximity to Earth makes it ideal for detailed chemical analysis. Future observations with the James Webb Space Telescope (JWST) will aim to determine if the planet was ever geologically active, possibly indicating past volcanic activity. These observations could provide insights into the formation of rocky planets around faint, lightweight stars and their potential to support life.
The researchers conducted an extensive search for other planets in the SPECULOOS-3 system but found none. Gillon suggested that other planets might exist but could be too small or too distant from their host star to be detected.
SPECULOOS-3 b was discovered by a network of six telescopes located in Chile, the Canary Islands, and Mexico, as part of the Search for Planets Eclipsing Ultra-Cool Stars (SPECULOOS) project. This project aims to find rocky planets orbiting ultracool dwarf stars, which are easier to observe due to their small size and dim light. These stars, much cooler and dimmer than the sun, burn their fuel slowly, potentially lasting up to 100 billion years.
Amaury Triaud, a professor of exoplanetology at the University of Birmingham and a co-author of the study, noted that ultracool dwarf stars could be the last stars shining in the universe due to their long lifespans. Their extended lifespans provide ample time for life to develop on planets within their systems.
However, the faintness of these stars makes them challenging to study. SPECULOOS-3 b
was discovered after a robotic telescope in Mexico observed dips in the host star's light over five nights in 2021, with the findings confirmed a year later.
Benjamin Rackham, a research scientist at MIT and co-author of the study, described the planet's appearance without an atmosphere: "There would be no blue sky or clouds—it would just be dark, like on the surface of the moon. And the 'sun' would be a big, purplish-red, spotted, and flaring star that would look about 18 times as big as the sun looks to us in the sky."
SPECULOOS-3 b is the ninth planet discovered by the SPECULOOS project. The team expects to find many more such planets in the coming years. Gillon highlighted that, like the planets found in the TRAPPIST-1 system, some of which are considered potentially habitable, SPECULOOS-3 b is an excellent target for the JWST. Julien de Wit, an assistant professor of planetary sciences at MIT and co-author of the study, expressed his excitement: "With this world, we could basically start doing exoplanetary geology. How cool is that?"
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