If you're looking for Earthlike planets around other stars places about the size and temperature of our own planet, where life could in theory be found it might seem like a letdown to stumble instead on a world bigger than Jupiter, hotter than molten iron and, with a density like that of Styrofoam, the most insubstantial planet ever seen. But when NASA astronomer Bill Borucki stood before a packed audience at this week's meeting of the American Astronomical Society in Washington to announce the discovery of Styrofoam World, along with four other huge, hot planets, he didn't seem even slightly disappointed.
Borucki heads the Kepler Mission, a space-based planet-hunting telescope that went into solar orbit last spring to search for distant worlds like our own. While the first five worlds detected are nothing like Earth, nobody expected them to be. What's important, Borucki declared, was that "these five new exoplanets come from the first six weeks of data." An additional eight months of Kepler observations are already in the can and awaiting analysis, meaning many more planets are undoubtedly lurking on hard drives at the NASA Ames Research Center in California, where Kepler is headquartered. "We're going home to lots of presents still unopened," says Natalie Batalha, a San Jose State University astronomer on the Kepler team.
Still, even these planetary finds are unlikely to be exact copies of Earth, and for a very simple reason. Kepler spots faraway planets by watching them transit, or pass in front of, their stars, blocking out a little bit of light and making the star slightly dimmer. The five planets just announced orbit very close to their suns, which is the reason they're so ridiculously hot. That proximity also means they move very fast, completing three or even more transits in the first round of observations which is just the kind of data stream the Kepler team prefers. "We want to see at least three transits to be absolutely sure," says Borucki.
The dimming caused by an Earth-size planet would be easy enough for Kepler to notice too, and such smaller planets most likely exist. But to be in an orbit where the temperature is balmy enough to support life, the planet has to be about as far from its star as Earth is from the sun, making its orbit about a year long; and that, by definition, requires at least two years after the initial detection. "Have patience," said Jon Morse, director of NASA's astronomy and physics division, to the assembled crowd.
For many scientists, just watching the data come in is a pleasure all by itself. "Kepler is working so amazingly well," says Berkeley's Geoff Marcy, a champion planet hunter in his own right and a member of the Kepler science team, "that the light curves [that is, the dips in light caused by transiting planets] look like they come from a textbook, not a real instrument."
It's that precision, plus the fact that Kepler is staring, unblinking and unceasingly, at 150,000 mostly sunlike stars in the Milky Way, that makes astronomers confident that Earths are in Kepler's future. As for any signs of life on those Earths, their detection will undoubtedly have to wait for future telescopes that can image the planets directly, probing their atmospheres for gases that hint at biological activity.
Still, Kepler has had some tantalizing news on the biology front too. While looking for planets, the probe has been taking note of the behavior of the stars themselves. Our sun is remarkably steady, without dramatic changes in warmth and brightness that might have prevented the emergence and evolution of life and Kepler now reports that two-thirds of the sunlike stars it's monitoring are no more active than the sun at its most turbulent. Lots of stable suns could mean at least a handful of promising Earths and those, in turn, could mean living company for our own still lonely world.