NATIONAL HARBOR, MD. — For the first time, astronomers have taken a direct look at an exoplanet’s insides.
An exoplanet about 800 light-years away is spilling its guts into space, and new observations with the James Webb Space Telescope, or JWST, have let astronomers read the entrails, astronomers report this week at a meeting of the American Astronomical Society.
“If this is true, it’s super cool,” says astronomer Mercedez López-Morales of the Space Telescope Science Institute in Baltimore, who was not involved in the new work. “For the first time you can study directly what the interior of an exoplanet is made of. That’s exciting.”
The planet, a Neptune-sized world called K2 22b, was discovered in 2015. The planet sits scorchingly close to its star, completing an orbit in just nine hours. It is too small to be detected itself, but it periodically emits clouds of opaque dust that form a cometlike tail, blocking less than 1 percent of the host star’s light.
Astronomers soon realized that the dust was probably cooled magma from the planet’s interior. That trail of planet viscera offered a unique opportunity to figure out the chemical composition of an exoplanet’s mantle.
Getting insight into any planet’s mantle, even the Earth’s, is challenging, says study coauthor Jason Wright of Penn State. “When nature gives you a gift like that, you have to take it.”
Wright and colleagues observed K2 22b with JWST’s sensitive mid-infrared spectrometer in April 2024. Different minerals in the dust emit specific wavelengths of light, letting the team figure out what the planet is made of.
The dust doesn’t seem to be pure iron, which is what scientists would expect if the planet was a bare core with no mantle or crust surrounding it. “There is still meat left on the bones, so to speak,” said astronomer Nick Tusay in a January 14 talk at the meeting.
But something in the dust emitted light that was hard to link to any specific material, Tusay said. The researchers first checked to see if the dust grains were magnesium oxide and silicon monoxide, which are expected in mantle material. But those minerals didn’t fit the data.
Surprisingly, the dust looks most like nitric oxide and carbon dioxide from vaporized ices, Tusay says. “If that’s true, what we’re looking at is a snowball disintegrating,” he says. That’s hard to explain for a planet so close to its star. “It’s just so weird and unexpected.” He has requested more JWST observing time to find out more. He and his team have also reported their results in a paper submitted January 14 to arXiv.org.
López-Morales agrees that more observations are needed to confirm the planet’s composition. “It’s very preliminary, very promising, definitely needs more data,” she says. Observing the handful of other known disintegrating exoplanets would be interesting, too.
A newly discovered disintegrating planet might be the best place to start. A planet discovered with the space-based TESS telescope in October is emitting a dust cloud so big, it extends halfway around its host star in a 9-million-kilometer-long horseshoe, astronomer Marc Hon of MIT reported at the meeting on January 15. This is the closest disintegrating planet to Earth yet discovered, so its contents will be even more clear in JWST data.
“We’ve proved we can do it with K2 22b,” Tusay says. “This one will be better.”