Astronomers have announced the discovery of an extrasolar planet not much larger than Earth—the smallest exoplanet yet found. Although the world orbits too close to its sun to sustain life, the finding is a milestone in the quest to find out how common Earth-sized, habitable planets really are. It also shows that, with some luck and some innovative new technology, astronomers could be announcing the discovery of a habitable Earth-like exoplanet within a few years.
Today's announcement, made at the winter meeting of the American Astronomical Society in Seattle, Washington, had been foreshadowed. Last year, team members operating the Kepler telescope orbiting the sun announced that they had found so many new exoplanets less than half the size of Jupiter that Earth-sized exoplanets must be abundant.
Now Kepler has found the much-anticipated first rocky, Earth-sized exoplanet. It did it by staring for months on end at the same 150,000 stars in the constellation Cygnus. Kepler's 1-meter-diameter telescope, hooked up to a sensitive light-measuring instrument, is capable of detecting the dimming of a star as a planet orbits in front of it—even if the star dims by only 0.01%. That's like detecting the dimming of 10,000 light bulbs when one burns out, noted Kepler deputy science team leader Natalie Batalha of NASA's Ames Research Center in Mountain View, California.
The star, designated Kepler-10, dimmed 0.015% every 0.84 day, revealing a planet—dubbed Kepler-10b—orbiting only 1/20th as far from its star as Mercury, the innermost planet in our solar system, orbits the sun. Kepler researchers could refine their picture of this exoplanet far more than usual because the star's light is bright and relatively free of disruptions such as sunspots. That let them bring in two other kinds of observations: ground-based telescopic observations of the infinitesimal color changes of the star induced by the planet's gravitational tugging and Kepler measurements of "starquakes" that reveal more about the nature of the star.
The better understood the star is, the greater the confidence in the description of the exoplanet. With a diameter 1.42 times that of Earth's and a density 1.6 times Earth's, Kepler-10b is the best characterized exoplanet yet, says Kepler team member Sara Seager of the Massachusetts Institute of Technology in Cambridge. That precision is what makes Kepler-10b the first unquestionably rocky planet orbiting another star, Batalha said. The size and mass of the only other competitor, Corot 7-b, is too poorly known because its star flickers with too much activity.
Kepler-10b may be Earth-sized, but it's hardly Earth-like. Being so close to a sunlike star, one side is permanently facing its star and always has a temperature of about 1833 K, hotter than most lavas on Earth. Even if there were some cranny somewhere that maintained more life-tolerant temperatures, says Seager, the planet's great age and the star's howling stellar wind suggest that any water it ever had was blown away long ago.
The next Kepler milestone will be Earth-sized exoplanets far enough from their stars to have the right temperature for liquid water, an essential ingredient for life. For sunlike stars, that will take a couple of more years of observations, says Kepler team member Alan Boss of the Carnegie Institution for Science in Washington, D.C. It will also require a particularly suitable star, he says, as well as some more precise Earth-based observations. A fainter, cooler star with a closer planet might bring the announcement of an Earth twin even closer.
*This item has been corrected. It was originally reported that Kepler-10b has a density 8.8 times Earth's instead of 1.6 times Earth's density.