Could dying white dwarf stars be home to habitable planets?

In the search for Earth-like worlds, astronomers look for conditions that are roughly the same as those on our planet. But the assumption that most Earth-like planets will be found around stars like our Sun might not necessarily be true.

As we recently discussed, it's exciting times in the search for Earth-like planets, with the latest estimates suggesting our galaxy alone is home to two billion habitable worlds. And that figure only considers planets found around stars like our Sun, while far more numerous red dwarf stars could also be home to habitable planet.

Well, it might be time to add yet another kind of star to the exoplanet-hunting list. White dwarfs, the ultra-dense degenerate form of stars too small to explode in a supernova, might actually be capable of supporting life, according to Eric Agol of the University of Washington at Seattle. A good 97 percent of all stars in the galaxy, including our own sun, will end up as white dwarfs.

Agol calculates that many white dwarfs can sustain surface temperatures of roughly 5000 Kelvin for about three billion years. Planets would have to be much closer to these dense, tiny stars to soak up that warmth - about a hundredth the distance between Earth and the Sun - but three billion years is enough time for life to emerge, if perhaps not intelligent life.

There's a rather big pro and an absolutely gigantic con to this idea. On the plus side, any Earth-sized planets orbiting that close to a white dwarf would be very easy to spot because its star is so tiny. But here's the problem - white dwarfs only come into existence after the original star undergoes a massive expansion into a red giant, vaporizing any planets that happen to be within the radius of expansion.

For instance, we know that when the Sun dies and becomes a red giant in about five billion years' time, its expansion will destroy Mercury, Venus, Earth, and probably Mars. That's quite a bit further out than a hundredth the distance between Earth and the Sun, so any potentially habitable planet would have to migrate to its new position once the red giant contracted into its white dwarf stage.

That isn't exactly impossible, and a lot of the strange exoplanet sights we've already seen with NASA's Kepler telescope appear heavily dependent on planetary migration. The good news, according to Agol, is that it won't take a lot of work for us to detect these planets. He suggests it will only take a network of twenty ground-based telescopes, each about a meter in size, undertaking a two-year survey of white dwarfs to find at least half a dozen planets. That's a pretty small outlay for a real chance at discovering Earth-like planets, even if the skies of these white dwarf worlds would look incomprehensibly different.

arXiv via Technology Review. Image via National Geographic.