Is Anybody Out There?

The universe teems with planets, and science has found hundreds of them. But the true prize — a warm, watery, Earth-like world — remains elusive

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Artwork by Lynette Cook / Courtesy of the National Science Foundation

This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star only 20 light-years away from Earth. The large planet in the foreground is the newly discovered GJ 581g, an Earth-size planet that orbits in the star's habitable zone.

In the currency of the cosmos, stars come cheap. There are hundreds of billions of them in our galaxy alone. The true prizes, as astronomers have always known, are planets. It took earthly observers millennia to discover the ones circling our own little sun. Whether any existed around other stars remained a mystery until 1995, when the first so-called exoplanets were found.

The biggest question on people's minds when the discovery of the new worlds was announced was whether they could harbor life. The answer: not even close. They were too big, too close to their stars or both. In the decade and a half since, with nearly 500 exoplanets under their belts, astronomers still haven't found an Earth-size planet in a star's habitable zone—also known as the Goldilocks region, where things are not too hot, not too cold, but just right for life. It looked as though they'd finally done it a few weeks ago with the announcement of a planet called Gliese 581g identified by a team of American astronomers. But as other scientists have studied the data, they've raised serious questions about whether the planet exists at all.

Disappointing as that has been, errors are to be expected in the planet-hunting game simply because the work is so hard. At multiple-light-year distances, even the largest planets cannot be seen with telescopes. Instead, scientists must use the back-and-forth gravitational wobbles, or radial velocity, a planet imposes on its parent star to infer its existence. Measure that movement and you can determine the planet's mass. Another method, known as transiting, occurs when a planet passes in front of its star, blocking a tiny bit of the light the star emits. By measuring the dimming, astronomers can calculate how large the blocking object is. A final approach, gravitational microlensing, involves analyzing how light from one star is bent as it travels around a planet orbiting another star closer to Earth.

The Kepler space telescope, which has been orbiting the sun since 2009, uses transiting to hunt for exoplanets, keeping its sensors fixed on about 100,000 sunlike stars. If it detects dimming in one of them, and if that dimming happens about once every year, it means the planet has an orbit like Earth's—and we're smack in the middle of the habitable zone. The Kepler scientists will make their first major reveal in February, announcing the planets they've found so far. The sheer number of stars out there makes some Earth-like worlds all but inevitable. The search will then be on for any Earth-like life upon them.