In a solar system over 100 light years from Earth, researchers have spotted one of the building blocks of life-sustaining planets. That building block may not sound like much: just a single asteroid containing a lot of water. But it's this same type of asteroid that may have brought water to earth when it was first being formed. And this marks the first time that such an asteroid has been detected outside our own solar system — indicating that this far-off realm may once have been home to habitable planets.

"Asteroids are the Legos that go into planets"

The findings come from research led by the University of Cambridge, and are published today in Science. "Asteroids [are] the Legos that go into planets," Jay Farihi, the paper’s lead author, tells The Verge. Thousands and thousands of asteroids come together to create a planet. But the process of putting them together is inefficient, Farihi says. "You don't use up all of the pieces."

What Farihi and his team have spotted is evidence of leftover asteroid bits still speckling that solar system. But the odds that any remaining planets there still sustain life as we know it are slim: the system's sun, GD 61, is a white dwarf — a burning out star in the final throes of life.

In fact, this white dwarf is what allowed Farihi and his team to spot the water-rich asteroid in the first place. "Unlike regular stars like our sun, [white dwarfs] act like white pieces of paper," Farihi says. Almost like dirt landing on that paper, Farihi says that "if any rocks or material other than hydrogen or helium falls onto them, we can see that very clearly."

Farihi's team analyzed the star using data from a spectrograph onboard the Hubble Space Telescope. Because a white dwarf should only have hydrogen and helium on its surface, anything else stands out as having come from someplace else. In this case, Farihi's team noticed an excess of oxygen that he says couldn't have come from a dry asteroid.

Over a quarter of the asteroid's mass was from water

The team found that over a quarter of the asteroid's mass must have been from water — a quantity that drastically eclipses the Earth's surface water mass of just .02 percent. It’s also just about identical to how much water many expect that we’ll find on Ceres, the largest asteroid in our own solar system, Farihi says.

"I think it's really awesome that we found the signs of planet pieces that can be building habitable environments and environments for life," he says. The team’s next step will be to observe other white dwarves to see how often water-rich asteroids can be spotted, which may offer a better idea of how common life-sustaining planets might be. "The real hardcore science comes from putting those objects into context."

Though Farihi can’t say what the planets in that solar system were like, he says that at least one planet must still be around. "This star definitely has planets, it has asteroids," he says. "We don't know anything beyond that, or whether there was once life or once habitable planets, but we do know it had all of the ingredients."