Life may well exist on planets orbiting dying stars - and, if it does, there's a good chance we'll find it within the next decade, say astrophysicists.
A team from the Harvard-Smithsonian Center for Astrophysics (CfA) has found that it's much easier to detect oxygen in the atmosphere of a white dwarf's planet than on an Earth-like planet orbiting a sun-like star.
"In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs," says Avi Loeb, director of the Institute for Theory and Computation.
White dwarfs are the remnants of stars like our own, left behind when the star throws off its outer layers. A typical white dwarf is about the size of Earth, and gradually cooling, but can retain heat long enough to warm a nearby world for billions of years.
Since a white dwarf is much smaller and fainter than the sun, a planet would have to be much closer in to be habitable with liquid water on its surface. It would circle the white dwarf once every 10 hours at a distance of about a million miles.
However, such a planet would never survive the previous stage of the star's life, in whcih it swells into a red giant. The planet would have to arrive in the habitable zone after the star evolved into a white dwarf, either forming from leftover dust and gas, or migrating inward from a larger distance.
The abundance of heavy elements on the surface of white dwarfs suggests that a significant fraction of them have rocky planets. Loeb and his team estimate that a survey of the 500 closest white dwarfs would be likely to spot one or more habitable Earths.
Recent research has shown that the closest habitable planet is likely to orbit a red dwarf - but difficulties with observations will make it hard to find.
"Although the closest habitable planet might orbit a red dwarf star, the closest one we can easily prove to be life-bearing might orbit a white dwarf," said Loeb.