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.
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.
“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.