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.