Scientists believe six billion years from now, alien astronomers studying the burned out remains of our Sun may come to the same conclusion. As with the white dwarf GD 61, the Sun is destined to end its life by first expanding into a Red Giant then shedding its outer layers and contracting into a super-dense glowing ember just a few thousand kilometres in diameter. The dying Sun will radiate what is left of its heat over billions of years before finishing its life as a dead, cold, "black dwarf". Astronomers studying the light emitted by GD 61, located 150 light years away from the Earth, detected an abundance of "rocky" elements such as magnesium, silicon and iron. They also found oxygen in quantities that indicated a very large amount of water.

Only a water-rich massive asteroid, or minor planet, can explain the observations, say the scientists. The giant rock, at least 90 kilometres in diameter, would have been drawn in by the white dwarf's powerful gravity and ripped apart. If such an asteroid existed in the system, it is highly likely that rocky, water-covered, Earth-like planets did too. Since only 0.02 per cent of the Earth's mass consists of water, the oceans that cover its surface were probably delivered by impacts from watery asteroids and comets. "The finding of water in a large asteroid means the building blocks of habitable planets existed - and maybe still exist - in the GD 61 system, and likely also around substantial number of similar parent stars," Dr Jay Farihi, from the Institute of Astronomy at Cambridge University, said.

"These water-rich building blocks, and the terrestrial planets they build, may in fact be common; a system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces. "Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system." The research appears in the latest issue of the journal Science. Asteroids are essentially the building blocks of rocky planets. Water would have accounted for more than a quarter of the mass of the one believed to have orbited GD 61. In our own Solar System, the giant asteroid Ceres contains a similar proportion of water in the form of sub-surface ice.

The tell-tale excess oxygen in the dust and debris surrounding GD 61 was detected using powerful spectrograph techniques that look for chemical signatures in light. Loading Astronomers made the observations using the Hubble Space Telescope and the large Keck telescope on Hawaii. PA