Ever since satellites began to peer at Earth from deep space, they’ve spotted a glimmering mystery: glistening, star-like sparkles. But Earth’s a planet, not a star—so why does it twinkle to far-off observers? That question vexed scientists, including famed astronomer Carl Sagan, for years—but now, reports The New York Times’ Nicholas St. Fleur, a team of NASA researchers finally figured out why.

In a new study in the journal Geophysical Research Letters, researchers explain the secret behind Earth’s seeming sparkle from space: ice.

Deep Space Climate Observatory (DISCOVR), a satellite designed to alert scientists of mass coronal ejections from the sun, hangs out in space about a million miles from Earth. It’s spotted the strange glints from our home planet since it began making observations in 2015.

The glints were first recorded in a 1993 paper from Carl Sagan and his colleagues, who were examining images taken of Earth by the Galileo spacecraft as it headed toward Jupiter. At the time, the scientists noted that the flashes seemed to happen over water.

When DISCOVR launched, writes St. Fleur, the public began to ask Alexander Marshak, the paper’s author, about the glints. He discovered Sagan’s paper, but realized that the flashes in the Galileo photographs weren't limited to bodies of water. Intrigued, he worked with a team to study a year’s worth of data from DISCOVR to find the flashes’ origins.

The team looked at over 800 flashes on images taken by DISCOVR, taking latitude, angles, and oxygen absorption in Earth’s troposphere into consideration. They narrowed down their source to sunlight, then discovered that they matched up with the locations of cirrus clouds. These wispy clouds are made up of ice crystals that form in the upper troposphere. And the team thinks that horizontal ice particles inside the clouds reflect light from the Sun that can be spotted even from deep space.

That means that the twinkle is pretty different from the one that humans spot on stars. Those twinkles occur because of atmospheric turbulence on Earth that refracts starlight, creating the illusion of a shifting shape.

Earth’s distinctive glint, on the other hand, is due to its water—and the technique could one day be used to spot other water-rich planets. In a press release, Marshak says that he’s working to figure out how common the horizontal particles really are in a bid to use them to find out even more about how Earth interacts with its own far-away star.