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White Dwarfs, Habitable Zones and Other Earths

Astronomers have ignored white dwarfs in their search for exoplanets. That may have been a mistake, according to a new study of white dwarf habitable zones

  • March 17, 2011

Although black holes and neutron stars get all the attention as the ultimate fate of stars, most will never make it that far. Some 97 per cent of the stars in our galaxy are not massive enough to form either.

Instead, astronomers believe they will end their days as white dwarfs, hot dense lumps of inert matter in which all nuclear reactions long ago burnt out.

These stars are about the size of the Earth and supported against gravity by the Pauli exclusion principle which prevents electrons occupying the same state at the same time.

The only radiation they emit is thermal heat as they cool, so it’s easy to imagine that these objects are of little interest to astrobiologists. And as it turns out, most searches for exoplanets have focused on nearby stars like our own.

Today, Eric Agol at the University of Washington in Seattle points out that planet hunters may be missing a trick. He says that white dwarfs could be good targets for exoplanet searches.

He points out that they are as common as Sun-like stars, that the most common ones have a surface temperature of about 5000 K and that this should produce a habitable zone at distances of about 0.01 AU for periods in excess of 3 billion years. That’s long enough for something interesting to have emerged on these bodies.

What’s more, any Earth-sized planet orbiting at this distance ought to be easy to spot as it passes in front of the tiny disc of a white dwarf.

There is a caveat, however. As stars age, they form red giants that engulf everything within a radius of about 1 AU. So any planet orbiting a white dwarf in the habitable zone would have to have migrated there after the white dwarf formed.

That’s a little discouraging but it’s not entirely impossible. Many theories of solar system formation assume that planet migration plays an important role.

Agol goes on to calculate many of the properties of these other Earth’s, which turn out to be surprisingly similar to our own. “Inhabitants of a planet in the [habitable zone] will see their star as a similar angular size and color as we see our Sun,” he says.

On the other hand, the short orbit and the possibility of tidal locking mean these planets will probably have a permanent day and and night side.

But what is most exciting about Agol’s work is that the deep transits in front of the parent star should make these planets easy to detect. “Earth-sized or even smaller bodies could in principle be detectable with ground-based telescopes,” says Algol. In fact, he reckons a network of twenty 1-metre sized telescopes systematically surveying the sky over 2 years could find half a dozen planets.

Which means there’s an outside chance that the first Earth-like planet could be found orbiting a white dwarf.

Ref: arxiv.org/abs/1103.2791: Transit Surveys For Earths In The Habitable Zones Of White Dwarfs

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