|Watery world heading for a white dwarf smack-down (Illustration: Mark A. Garlick/space-art.co.uk/University of Warwick/University of Cambridge)|
Zombie star caught feasting on a soggy asteroid
by Jacob Aron
The remains of a water-rich asteroid have been found splattered across the face of a white dwarf star about 150 light years away. The discovery marks the first time that we have seen chemical evidence for water on rocky worlds orbiting other stars, supporting the idea that the galaxy hosts many life-friendly exoplanets akin to Earth.
The find was made on a stellar corpse similar to what our sun will become when it dies. That hints that some watery rocks in our solar system could survive the sun's end and be a resource for space-faring humans of the future.
Water, one of the key ingredients for life as we know it, has previously been spotted in the atmospheres of gassy, Jupiter-like exoplanets. We expect it to exist on some of the rocky, Earth-like planets found in the habitable zone around other stars, the region warm enough for liquid water to exist. But for now alien oceans are purely theoretical.
"It's one thing to just hypothesise, it's another to actually find evidence," says Jay Farihi at the University of Cambridge.
His team used the Hubble Space Telescope to examine light from the white dwarf star GD 61. This object is the corpse of a star that ballooned into a red giant as it was dying, then shed its outer layers of gases to expose its dense core. Dwarf stars have high surface gravity, which causes heavier elements to sink to the centre, leaving an upper atmosphere of just hydrogen and helium.
But like a handful of other white dwarfs, the atmosphere of GD 61 is polluted with heavier elements, which suggests that new material must be steadily falling onto the star. In addition to hydrogen and helium, the team detected oxides of magnesium, aluminium, silicon, calcium and iron.
They also found an excess of oxygen that could not be explained by these materials. "The only other chemically viable molecule at that point is water," says Farihi.
Infrared observations confirm the extra elements are coming from a disc of heated material orbiting the white dwarf, probably the leftover pieces of a rocky object like an asteroid that was destroyed as the star died. The team calculate that this body would have been 26 per cent water by mass, making it similar to Ceres, a dwarf planet and the largest asteroid in our solar system.
Move to Mars
Despite its vast oceans, Earth is just 0.02 per cent water by mass, suggesting that our planet formed dry and was then bombarded by water-laden asteroids. Finding the remains of a soggy space rock around GD 61 means the same thing could have happened in this star system, says Farihi. "It had the ingredients to deliver water to the surface of planets that were built in that system."
It is good to have confirmation that water and rock exist together around other stars, says Michiel Hogerheijde of Leiden Observatory in the Netherlands, who was not involved in the research. "It tells us water content must survive throughout the star's whole evolution."
We don't yet know if any full-sized planets survived the death of the star that resulted in GD 61. But the discovery of water is potential good news for any humans still hanging around our solar system in 5 billion years, when the sun will meet a similar fate. Chances are the red giant sun will consume Mercury, Venus and Earth, but our descendants could move elsewhere and mine asteroids for water, says Farihi. "We'd need to be at least living on Mars, if not further out."
Journal reference: Science, DOI: 10.1126/science.1239447