Earth-Like Planets May Abound in the Milky Way

Maybe Frank Drake was right. Nearly half a century ago, the American astronomer postulated that, based on pure statistical probability, the Milky Way could be teeming with Earth-like planets. Now observations of formerly sunlike stars called white dwarfs suggest that the overwhelming majority of them once harbored at least one rocky world. And because sunlike stars could account for up to half of the Milky Way's population of several hundred billion suns, that means hundreds or even thousands of civilizations might inhabit our galaxy.

The question of how many rocky worlds exist in the galaxy has perplexed astronomers for the better part of a century. Even now, technology hampers the search. Astronomers are years away from being able to image another Earth directly. The two methods of detecting extrasolar planets, nicknamed "wobble and blink," involve plotting tiny shifts in a star's motion caused by the gravitational tug of its orbiting planets, and catching the slight dimming in a star's light that occurs whenever a planet passes between the star and an observer's telescope. Both methods have revealed hundreds of Jupiter-like planets, but not a counterpart to Earth—though a few rocky giants have been spotted.

But tomorrow at a meeting of the Royal Astronomical Society in Glasgow, United Kingdom, a team of researchers will present a new way to estimate how many rocky planets could be out there. The study centers on white dwarf stars. These dying suns once shone like our own, but late in their 9-billion-year life spans they ballooned into red giants, stars with diameters up to 200 times that of our sun. (If this happened in our solar system, the sun would expand to beyond Earth's orbit.) Then, gradually, the bloated stars shriveled to only half of their original size, slowly dimming into oblivion and surrounded by huge, thin atmospheres.

Those atmospheres can provide an easy-to-read signal that rocky planets once orbited the dying stars, according to the researchers. The team studied the spectra, or chemical signatures, of the light from 146 white dwarfs located within a few hundred light-years of Earth. Among those stars, 109 exhibited spectra indicating that heavier elements such as calcium were present in their atmospheres. Rocky planets are the only likely sources for these heavy elements, so the spectra show that the stars must have swallowed such planets during their expansive, red-giant stage.

Based on the data, the team extrapolates that at least 3.5% of all sunlike stars across the Milky Way currently harbor rocky planets. By a further rough calculation, that means the galaxy has held as many as a billion rocky worlds at one time or another. A small fraction of these, in turn, could have been Earth-like, meaning they met such criteria as harboring water and existing within a habitable distance from their suns.

The study reinforces the idea that the formation of planets around stars "is a common outcome," says planetary scientist Jonathan Fortney of the University of California, Santa Cruz. So common, he says, that the number of stars harboring rocky planets is "probably much higher" than the value of 3.5% estimated by the authors.

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