Stars are so bright and planets so dim by comparison that astronomers can’t study alien worlds directly. They’ve learned plenty through indirect observations, though—that one so-called exoplanet is half-shrouded in clouds; that another may be a water world; that yet another is probably a lovely azure blue, albeit hellishly hot and windy, with rainstorms of molten glass.
But a new report in Science may be the most impressive yet: by looking at the light of a burned-out star, astronomers working in the United Kingdom and Germany have found evidence of a water-rich asteroid that was destroyed by the star a million years ago, or even less—the merest blink of an eye in cosmic terms. “It’s the same sort of object,” says lead author Jay Farihi, “that might have brought water to Earth when our solar system was forming.”
No solar system is forming in this case: the star in question, known as GD 61, about 170 light-years from Earth, died about 200 million years ago. First it swelled into a bloated behemoth known as a red giant, then it shrank into a glowing ember—a white dwarf. The same thing will happen to the Sun in five billion years or so, and when it does, Mercury and Venus will be toast—literally. “Most theorists believe the Earth will survive, though,” says Farihi, although it will become uninhabitably hot. And nobody doubts that Mars, Jupiter and the outer planets will remain intact.
The same goes for the asteroid belt, and with a giant planet like Jupiter in the neighborhood, asteroids will occasionally be flung inward to be swallowed by what used to our Sun. That’s just what Farihi and his colleagues think must have happened with GD 61. They’ve got a couple of lines of evidence. The first: infrared observations show that the white dwarf is surrounded by a disk of material, presumably created when some sort of solid body ventured too close and was torn to shreds by the dead star’s still-powerful gravity.
Second, there’s a surprising abundance of oxygen in the white dwarf’s outer layers. The oxygen could have come from plain old rocks: it’s present in the silicates that make up most of Earth’s bulk, for example. But there’s too much oxygen for that explanation to be the whole story. It could have come from carbon monoxide or carbon dioxide, but, says Farihi, “we don’t see any carbon,” effectively ruling out that idea.
The most plausible explanation, they decided, was a pulverized asteroid whose remnants are raining down from the disk of orbiting debris onto the white dwarf’s surface and “polluting its atmosphere,” as Farihi puts it. By the scientists’ calculations, the asteroid was about the size of Vesta, the second-largest asteroid in our own solar system, measuring 326 mi. (525 km) across. Many asteroids are relatively dry, but some are surprisingly water-rich, including Vesta’s big sister, Ceres. The pulverized asteroid falling onto GD 61 belongs in the latter category, with about a quarter of its mass thought to come in the form of water.
Figuring out the composition of an asteroid 170 light-years from Earth is no mean feat—especially when the asteroid no longer exists in any recognizable form. But what’s more exciting to Farihi is that this shows that the same sort of object that may have delivered life-sustaining water to Earth may be common in other solar systems. “One of the burning questions,” he says, “is ‘how often does this happen?’” There’s one obvious way to find out, but, he says, “we’ll definitely be expanding our search.”