Astronomers are probably just a few years from the first-ever finding of an Earth twin outside our solar system, that is, a planet roughly the size of Earth orbiting at a similarly temperate distance from a sunlike star. The idea is that finding Earths outside the solar system just might be a first step toward finding alien life. Earth has been exceptionally supportive of life-forms, so perhaps a similar planet would also be habitable.

NASA's Kepler spacecraft is leading this charge, and has already sniffed out hints of planets that look remarkably Earth-like, at least from Kepler's distant vantage point; it will take a few more years of observations to solidify these tentative findings. The spacecraft is keeping tabs on more than 150,000 stars to find out how often Earth-like worlds form around sunlike stars.

But sunlike stars may not be the only game in town, as far as habitability is concerned. In a study in the April 20 issue of The Astrophysical Journal Letters, astronomer Eric Agol of the University of Washington in Seattle raises the point that habitable exoplanets could also be found orbiting compact stellar remnants known as white dwarfs, which are plentiful in the universe. And if habitable worlds do exist around white dwarfs, Agol notes, it would be possible to detect them with modest-size telescopes on Earth, whereas finding Earth-like planets around sunlike stars requires a space-based telescope like Kepler, which cost NASA $600 million.

Kepler looks for tiny, periodic dips in a star's brightness that might be caused by a planet passing across the face of the star and blocking some of its starlight, a sort of eclipse known as a transit. But those dips are extremely subtle—the transit of an Earth-like planet in front of a sunlike star dims the star by about 0.01 percent. With a much more compact white dwarf, an Earth-like world would blot out far more of its light. Agol calculates that an Earth twin in a potentially habitable orbit would block almost 50 percent of a white dwarf's emitted light during a transit. A change of that magnitude would be detectable even with a one-meter telescope on the ground. For a true Earth analogue orbiting a sunlike star, on the other hand, the 0.01 percent dip can only be identified from space, outside the distorting, turbulent veil of Earth's atmosphere.

The question is, can habitable planets even exist around a white dwarf? A planet would have to be exceedingly close to a white dwarf, which has exhausted its nuclear fuel and is quite dim, to receive enough heat to maintain liquid water on its surface. That wouldn't be a problem—Kepler and other exoplanet searches have found many planets in very tight orbits—except for the fact that before a star becomes a white dwarf, it swells up as a red giant and swallows all the planets nearby. (That is the fate that may await Earth in a few billion years' time when the sun becomes a red giant.) Perhaps planets could re-form after the ballooning red giant has retreated to become a much smaller white dwarf, or perhaps planets distant enough to escape immolation could migrate inward after the red giant phase ends. If such planets could take up orbits at the right distance from a white dwarf—about 0.5 percent to 2 percent the distance between Earth and the sun—they might have billions of years of temperate, potentially habitable conditions, according to Agol.

An even wilder idea is that dark matter, rather than starlight, could provide enough heat to make a planet livable. In a paper posted to the physics preprint Web site March 25, Dan Hooper and Jason Steffen of Fermi National Accelerator Laboratory in Batavia, Ill., note that large planets in dark matter–rich regions of the universe could gravitationally capture a substantial number of dark matter particles, which would mutually annihilate on contact. With a few optimistic assumptions of how dark matter behaves—remember that no one really knows what it is—the energy released in dark matter annihilations could provide a steady source of heat that might allow liquid water to persist on the surface of such a planet. "On these rare planets," Hooper and Steffen wrote, "it may be dark matter rather than light from a host star that makes it possible for life to emerge, evolve, and survive."

For the time being, astronomers have plenty of work to do following up Kepler's leads for possible planets around ordinary stars. But if nothing else, the new studies help illustrate that we don't really know the boundaries of what makes a planet habitable. With just one example of a livable world to draw from—our own—it makes sense to look for Earth-like planets first. But there might be an almost unimaginable combination of factors in this vast universe of ours that could render other kinds of worlds habitable as well.

Photo of ancient white dwarfs in the globular cluster M4: NASA and H. Richer (University of British Columbia)