Kepler's on a mission to discover 'Earths'

NASA's planet-hunting spacecraft, Kepler, has blasted off aboard a rocket from Cape Canaveral.

Rocketing into the night sky late Friday, Kepler, the space agency's long-awaited $591 million space telescope, opens a new era in planet detection. In its four-year mission, planetary scientists expect the spacecraft to discover roughly 1,200 planets, more than quadrupling the number of worlds spotted orbiting nearby stars since 1995.

"I call it our planet census-taker," say NASA's Jon Morse. "Its discoveries may fundamentally alter humanity's view of itself and our place in the universe."

Says mission chief scientist William Borucki of NASA's Ames Research Center: "Kepler is designed to find hundreds of Earths. Kepler won't find E.T., but it hopes to find E.T.'s home."

Along the Orion spur

After thundering out of Cape Canaveral aboard a Delta II rocket, Kepler will follow an Earth-trailing orbit and turn its 3-foot-wide telescope towards the constellation Cygnus, the Swan, to begin four years of viewing of roughly 100,000 stars.

Looking along the "Orion spur" of our Milky Way galaxy, a spoke of stars rich with stellar objects similar to our own sun, Kepler will watch unblinkingly for "transits." These tiny dips in light — a 0.01% dimming for a planet the size of Earth — can be observed as worlds orbit in front of those stars.

Only about 10% of planets orbiting nearby stars are expected, purely by chance, to orbit at an angle accessible to Kepler's view, says planetary theorist Alan Boss of the Carnegie Institution of Washington. Already a dozen jumbo planets have been spotted by transit with telescopes on Earth. Unhindered by clouds and daylight in the depths of space, Kepler should find many more:

• About 870 Jupiter-size planets (89,000 miles wide) orbiting close in to their stars, the most easily detected transits.

• At least 50 Earth-size planets (8,000 miles wide), and possibly hundreds more if rocky planets average bigger diameters than Earth.

• A handful of planets as small as Mercury (3,000 miles wide) orbiting the more common stars that are dimmer and smaller than our sun.

"The big outcome that Kepler provides is a picture of both small, Earth-size worlds and planets really far enough away from their stars to be habitable," says astronomer Greg Laughlin of the University of California-Santa Cruz, who is not part of the project. "Kepler will provide a whole crop of them and give us the numbers to really know what's out there."

Not too hot and not too cold

Transit detection provides planetary width and, in some cases, atmospheric chemistry, results that other planet-detection methods can't, adds Boss, author of The Crowded Universe: The Search for Living Planets. He expects discovery of transit planets in the "habitable zone" — not too hot and not too cold for water — of stars will spur efforts to detect whether they possess atmospheres rich in water or methane, signatures of life.

Kepler's unblinking stare and wide viewing area differ from the Hubble Space Telescope, which can look only briefly at one nearby star at a time.

"One of the mysteries Kepler should address is whether solar systems like Earth's are normal or not," says Laughlin, an investigator on the Lick Carnegie Exoplanet Survey, which uses the better-known method of detecting gravitational wobbles of stars induced by orbiting planets to discover new worlds. Planets in our solar system follow nearly circular orbits, and the massive outer gas giants block comets from wreaking too much havoc with closer planets, including Earth.

But many of the solar systems so far detected possess planets with looping, or eccentric, orbits that swing them close to their star and then further out. One recently observed transit planet, HD 80606b, sees its temperature increase from 980 to 2,240 degrees Fahrenheit in just six hours as it whips around its star during the close-in part of its orbit.

In April 2007, European astronomers led by Stephane Udry of Switzerland's Geneva Observatory announced the detection of an Earth-size planet potentially in the habitable zone of the red dwarf star Gliese 581, about 20 light-years away. (One light-year is about 5.9 trillion miles.) That discovery relied on the gravitational wobbles method. Kepler, with its transit detections, should uncover many similar planets residing in its 3,000 light-year-deep field of view, if they are there to be found.

The discovery of many Earths "certainly may well mean that life may be common throughout our galaxy," Borucki says.

"On the other hand, if we don't find anything, that may well mean that life is very rare."

Contributing: Associated Press