Science and Nature

The researchers used the Hubble Space Telescope’s Cosmic Origins Spectrograph to observe GD 61 and split its light into constituent colors, revealing the chemicals it contains. They found magnesium, iron, silicon and other heavy elements, which wouldn’t exist naturally on the surface of a white dwarf, suggesting that they fell onto the star from an orbiting object. The researchers also found a huge excess of oxygen—an amount, they say, that indicates the asteroid polluting the star’s surface was originally composed of 26 percent water. That’s pretty wet—Earth, by contrast, is only 0.02 percent water. “This work marks the first detection of water-rich rocks in exoasteroids, and is an important step in developing a comprehensive picture of exoplanetary systems,” says Kevin France of the University of Colorado at Boulder, who wasn’t involved in the research.

The find could be significant, because theorists think Earth, having formed too close to the sun for water to survive, got its oceans from just such large, wet asteroids that impacted it long ago. “We’ve got the same kind of object which probably delivered Earth’s oceans, and we found this around another star,” says research leader Jay Farihi at the University of Cambridge in England. The discovery, he says, is a step in the quest to find habitable worlds, and maybe even life, beyond Earth. “This goes beyond planets in the habitable zone. We have some actual chemistry that tells you the ingredients for habitable planets were there.”

Some experts aren’t convinced that the oxygen found on the surface of the white dwarf is a clear sign that water existed on an orbiting asteroid, however. “The link of the pollution of a white dwarf to the inventory of water in an earlier planetary system is a very interesting scientific question still under investigation,” says exoplanet researcher Lisa Kaltenegger of Harvard University and the Max Planck Institute for Astronomy in Germany, who was not involved in the research. Claire Moutou, another exoplanet specialist at the Laboratory of Astrophysicsof Marseille in France, agreed. “I find the analysis/conclusions of the paper reasonable, as far as the amount of oxygen available to lie in H2O molecules is concerned. The interpretation of the origin of this water content is more speculative.”

The scientists behind the project, which is detailed in the October 11 issue of Science, say they took pains to verify that the chemicals they see really do prove the destroyed asteroid had water. They observed the star GD 61 in many wavelengths through many telescopes, including NASA’s Spitzer Space Telescope and two instruments on the W. M. Keck Observatory in Hawaii, along with Hubble. “The authors seem to have done a careful job of cataloguing the elements and searching for reasons to explain away the oxygen excess,” says debris disk expert John Debes of the Space Telescope Science Institute in Baltimore. “The detection of hydrogen in addition to the oxygen is a really convincing signature of water.” The finding sheds light on how planets form and evolve, adds Brice-Olivier Demory, an exoplanet researcher at the Massachusetts Institute of Technology who also was not involved in the research. “This is a startling result strengthening the fact that water can be found in a very diverse range of environments.”