Astronomers Discover Earth-Sized Planet That’s Airless Ball Of Rock

An Earth-sized planet has been discovered just 55 light years away — but don’t plan on living there. While the planet, named SPECULOOS-3b, orbits a small, cold star and shares some similarities with our own world in terms of size and rocky composition, it’s a far cry from Earth’s twin.

An international team of researchers from the Massachusetts Institute of Technology, University of Birmingham and University of Liège believes that SPECULOOS-3b is likely missing an atmosphere, making it a blistering hot, airless ball of rock. The findings, published in the journal Nature Astronomy, confirm the detection of this unique planet using a network of telescopes as part of the SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project.

What makes SPECULOOS-3b so different from Earth? It all comes down to its host star, an ultracool dwarf. These stars are thought to be the most common type of star in our galaxy, but they’re also the faintest, making them difficult to spot in the night sky. The ultracool dwarf hosting SPECULOOS-3b is about one-tenth the size of our Sun and 1,000 times dimmer. It’s more similar in size to Jupiter and is twice as cold as the Sun.

Artist's view of exoplanet SPECULOOS-3 b orbiting its star
Artist’s view of exoplanet SPECULOOS-3 b orbiting its star. The planet is as big as the Earth, while its star is slightly larger than Jupiter, but much more massive. (CREDIT: Lionel Garcia)

Despite its small size and cool temperature, the ultracool dwarf packs a punch when it comes to the planet’s surface conditions. SPECULOOS-3b orbits its star in just 17 hours, meaning one year on this planet is shorter than a day on Earth. This extremely close proximity means the planet is blasted with 16 times more radiation per second compared to what Earth receives from the Sun.

The intense and relentless exposure to radiation has likely vaporized any atmosphere the planet once had, according to the research team.

“We can say from our spectra and other observations that the star has a temperature of about 2,800 kelvins, it is about 7 billion years old — not too young, and not too old — and it is moderately active, meaning that it flares quite a lot,” says study co-author Benjamin Rackham, from MIT’s Kavli Institute for Astrophysics and Space Research. “We think the planet must not have an atmosphere anymore because it would easily have been eroded away by the activity of the host star that’s basically constantly flaring.” 

Rackham describes what it would be like to look up at the sky on the planet’s surface if there’s no atmosphere.

“If there’s no atmosphere, there would be no blue sky or clouds — it would just be dark, like on the surface of the moon,” explains Rackham. “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.”

While the planet may not be a cozy place to live, its lack of atmosphere and proximity to Earth make it an ideal candidate for further study. Scientists say that NASA’s James Webb Space Telescope (JWST) could provide more details about the planet’s surface composition, which would be a first in the field of exoplanetary studies.

“SPECULOOS-3b is the first planet for which we can consider moving toward constraining surface properties of planets beyond the solar system,” says study co-author Julien de Wit, associate professor of planetary sciences at MIT. “With this world, we could basically start doing exoplanetary geology.”

The discovery of SPECULOOS-3b is the result of a collaborative effort involving a network of six robotic, 1-meter telescopes that continuously observe the sky for signs of planets orbiting around ultracool dwarf stars. The project aims to observe about 1,600 nearby ultracool dwarf stars, as their small size makes it easier to detect planets that cross in front of them, momentarily blocking their light.

The first hints of SPECULOOS-3b were observed in 2021, but it wasn’t until 2022, when MIT’s Artemis telescope took a closer look, that the planet’s existence was confirmed.

“We think that the planet is nearly as hot as Venus, so not habitable,” concludes Rackham. “It’s not hot enough to have a lava surface. It should be solid rock. But depending on how bright that rock is, it could be recently resurfaced due to plate tectonics or volcanic activity, or it could be a planet that’s been eroded by space weathering and has a much darker surface. Going forward, we should be able to distinguish between some interesting scenarios for the surface of the planet.”

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