Astronomers from MIT, the University of Liège, and other institutions have discovered a new planet orbiting a small, cold star just 55 light-years away.
While the planet shares similarities with Earth in terms of size and rocky composition, it is likely lacking an atmosphere, which sets it apart.
In a paper published today in Nature Astronomy, the researchers confirm the detection of SPECULOOS-3b, an Earth-sized, possibly airless planet found through the use of a network of telescopes as part of the SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project. This new planet orbits an ultracool dwarf star, a type of star that is smaller and cooler than the Sun. Ultracool dwarfs are believed to be the most common star type in our galaxy, though they are faint and difficult to observe in the night sky.
The ultracool dwarf star hosting the new planet is about one-tenth the size of the Sun and 1,000 times dimmer. It is more similar in size to Jupiter and has a temperature that is twice as cold as the Sun. Despite its size, the star emits an enormous amount of energy onto the planet’s surface because SPECULOOS-3b orbits it extremely closely, completing one orbit in just 17 hours. As a result, a year on this planet is shorter than a single day on Earth.
Due to it's close proximity to the star, SPECULOOS-3b is exposed to 16 times the radiation that Earth receives from the Sun every second. The research team believes this intense radiation likely vaporized any atmosphere the planet may have once had, leaving it an exposed, airless, and blistering rock.
If the planet lacks an atmosphere, scientists may soon be able to study its surface in greater detail, including determining the types of rocks present and the geological processes that may have shaped its landscape. This could include investigating whether the planet’s crust experienced magma oceans, volcanic activity, or plate tectonics in its past.
Lining up
In 2021, astronomers first detected early signs of a new planet through observations made by SPECULOOS, a network of six robotic 1-meter telescopes (four in the Southern Hemisphere and two in the Northern Hemisphere). These telescopes continuously monitor the sky for planets orbiting ultracool dwarf stars.
SPECULOOS aims to observe around 1,600 nearby ultracool dwarf stars. Since these stars are small, planets that orbit them and pass in front of them block more of their light compared to those orbiting larger, brighter stars, making it easier for astronomers to detect planets.
In 2021, one of SPECULOOS' telescopes detected a possible transit signal from a planet orbiting an ultracool dwarf star located 55 light-years away. However, the signal was inconclusive. Then, in 2022, close monitoring with MIT’s Artemis telescope brought a breakthrough.
“While the 2021 data showed some potential signals, they weren’t convincing. But the 2022 Artemis data really grabbed our attention,” says Artem Burdanov, who manages the SPECULOOS Northern Observatory at MIT. “We spotted a clear transit-like signal in the Artemis data, and once we analyzed it, we quickly decided to launch a dedicated campaign around this star. Everything just started falling into place from there.”
The team focused on the star using MIT’s Artemis telescope, the SPECULOOS network, and several other observatories. These combined observations confirmed that the star hosted a planet, which orbits it every 17 hours. Based on how much light was blocked during each orbit, the scientists estimated that the planet is about the size of Earth.
Dark like the moon
They then used these observations to estimate the star's and planet's properties. MIT’s Benjamin Rackham, who led a campaign using the Magellan telescopes in Chile and the NASA Infrared Telescope Facility (IRTF) in Hawaii, analyzed the star's light.
Because SPECULOOS-3b lacks an atmosphere and is relatively close to Earth, the team believes it is an ideal candidate for follow-up studies using NASA’s James Webb Space Telescope (JWST). JWST’s powerful capabilities should allow it to analyze the star's light and reveal more details about both the star and the planet. The team hopes to gain insights into the planet’s surface, a groundbreaking development in exoplanet research.
“We think the planet is nearly as hot as Venus, so it’s not habitable,” Rackham adds. “It’s not hot enough to have a lava surface, but it should be solid rock. Depending on the brightness of the rock, it could have recently resurfaced due to volcanic activity or plate tectonics, or it could be a planet eroded by space weathering, with a much darker surface. Moving forward, we should be able to differentiate between several intriguing possibilities for the planet’s surface.”
