Ancient White Dwarf Stars
Planet-hunting astronomers started out in the 90’s by searching for alien worlds around Sun-like stars. It made sense: the only planets we knew of at the time orbited the actual Sun, so why not look in a familiar kind of place? More recently, attention has moved to the much smaller and dimmer stars known as red dwarfs, or M-dwarfs: not only are they far more numerous than Sun-like G stars, but they’re less bright too. Any Earth-like planet in orbit around them would be less likely to be washed out in their glare and thus more likely to be spotted.
But now an even smaller and stranger class of stars has been floated as perhaps the best place to search, not only for mirror Earths, but for the existence of life itself.
According to Avi Loeb, of Harvard, and Dan Maoz, of Tel Aviv University, white dwarfs — the glowing embers left behind when stars like the Sun die — are where the action might really be. With the James Webb telescope, says Loeb, scheduled for launch as early 2018, “you could detect biomarker molecules like oxygen in just a few hours of observation time.” If Loeb and Maoz are right, in short, we could find evidence of life on other worlds in as little as five short years.
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But that’s a pretty big if, and for those who know a little astronomy, the idea sounds frankly nuts. In order to reach the white dwarf stage, a star first swells to become a red giant, incinerating any nearby planets completely (it’ll happen to Earth when our Sun dies in about 5 billion years). Then the outer layers puff off into space and the rest contracts into a white-hot charcoal briquet, no bigger than Earth, albeit far more massive.
But for those who know a little more, it’s not crazy at all. “Of course the inner planets don’t survive the red giant phase,” says Loeb, “but there are two ways to make a second generation of planets.”
The first could happen if the expanding star ripped apart a giant, Jupiter-size planet, orbiting far enough out, without completely destroying it. That material could form an orbiting disk when the star shrank back down, re-creating the sort of ring formation that produced the original planets billions of years earlier. The second: if a giant planet were left intact, its gravity could fling comets and asteroids inward, where they might ultimately accrete into an Earth-size planet.
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If they did, and if the planet circled the white dwarf at about a million miles out, it would seem bizarrely normal to an Earth visitor in some ways. “The white dwarf,” says Loeb, “would be about the same color as the Sun, and while it would be a hundred times smaller, you’d also be a hundred times closer.” Look up, and you wouldn’t notice anything different.
Of course, your year would last only ten hours. “If you went to the beach in the middle of summer,” says Loeb, “it might start to snow five hours later.” That’s what could happen, at least, if the planet were rotating. But tidal locking would keep one of its hemispheres looking perpetually toward the star, and one looking away, much as the moon always keeps the same face toward us. If life did manage to gain a foothold a white dwarf world, it might do best in the endless twilight on the border between its two sides.
If the planet did exist, and if it happened to pass in front of its star, it would block the star’s light, and the wink could easily be seen from Earth. In fact, says Maoz, “we’ve been looking here in Israel for such major eclipses of white dwarfs for some time.”
Looking for signs of life, however, wasn’t on anyone’s agenda until Maoz and Loeb began talking back in December — but those conversations produced a lot. Astronomers currently probe the atmosphere of so-called exoplanets by watching as they pass in front of their stars. When they do, starlight streams through the planets’ blankets of air, revealing the spectral imprint of atmospheric molecules.
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Those planets are tiny compared with their stars, though, so you’re looking for a very small imprint on very bright starlight. “It’s like looking at Los Angeles from a billion miles away and trying to figure out if one of the city’s 200,000 streetlights is out,” says Maoz. But in the case of white dwarfs, planet and star are about the same physical size; the planet blocks out most of the starlight, so the spectral imprint would be far easier to read. “It’s more like looking at a small neighborhood, and seeing if one out of 200 lights is out.”
All of this depends on whether white-dwarf planets exist, of course, but there is a kind of precedent: in the early 1990’s astronomers found planets around pulsars, the leftovers from stars that haven’t just died, but have burst apart in supernovas, the most violent explosions in the cosmos. If planets can form in those circumstances, white dwarfs should be no problem at all.
Still, it’s all just theoretical until someone actually finds the hard evidence. That, says Loeb, could come with the launch of the European Gaia satellite, slated to go into orbit later this year, which could locate the 500 or so white dwarfs — any number of which could be home to a planet.
“I have to admit,” says Maoz, “that I was kind of skeptical that there was enough meat here to make it worth writing a paper about. But we’ve gotten a lot of good attention. So it must have been worth saying after all.”