Kepler sets sights on Earth-like planets

By Andrea Thompson

updated 10:58 a.m. ET March 3, 2009

If Friday's launch goes according to plan and successfully lobs NASA's new Kepler space telescope into orbit, the mission stands to potentially change the way we look at the universe.

Kepler is designed to turn its eye on thousands of stars in our own Milky Way galaxy and look for signs of Earth-sized planets orbiting in a region conducive to supporting life.

"Kepler will push back the boundaries of the unknown in our patch of the Milky Way galaxy, and its discoveries may fundamentally alter humanity's view of itself," said Jon Morse, director of NASA's Astrophysics Division at the agency's headquarters in Washington, D.C.

The probe is slated to launch on Friday from Cape Canaveral Air Force Station in Florida aboard a Delta 2 booster. NASA delayed the mission by one day for extra rocket checks after the loss of another probe just after its launch last week.

The $600 million Kepler mission is named for Johannes Kepler, the 17th century German scientist who pioneered the fields of optics and planetary motion.

"Now, 400 years later, we're using his discoveries in order to answer a profound and fundamental question about our place in the universe: Are there other Earth-like planets out there?" Morse said.

Kepler will use an unprecedented combination of light detectors (adding up to about 95 million pixels) to capture the subtle shifts in light that characterize an extrasolar planet orbiting its star. For comparison, a high-end digital camera on Earth might have 10 megapixels, but Kepler's detectors add up to 95-megapixel array, mission researchers said.

Looking for Goldilocks
Over the last two decades, scientists have spotted more than 300 extrasolar planets circling other stars in our Milky Way galaxy. Most of these planets have been about the size of Jupiter or larger, making it unlikely they would harbor life. But those aren't what Kepler is aiming to find.

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"We're interested in planets like that of Earth, rocky planets in an orbit where life might be possible," said principal investigator for Kepler science William Borucki of NASA's Ames Research Center in Moffett Field, Calif.

Kepler will be pointing its 0.95-meter diameter telescope and array of 42 charge-coupled devices (light-sensitive microchips also found in standard digital cameras) at a pre-selected target group of 100,000 stars.

Kepler will be looking for fluctuations in the light of each star that result from a planet transiting – or moving in front of – its parent star as seen from the perspective of Earth. These fluctuations can tell scientists how big the planet is, as well as how far away its orbit is from its parent star.

"When a planet goes across a star, it blocks some light," Borucki explained. "The bigger the planet, the more light it blocks, so we get the size of the planet from the size of the dimming."

By looking for multiple transits, the Kepler team can determine the planet's orbital period, or how long it takes the planet to circle its star. Each planet will need to be observed through at least three transits, Borucki said, to determine the period and to make sure the dimming isn't due to some other astronomical phenomenon, such as a spot on the star.

"We don't want to have false discoveries; we want to be very, very sure that when we say it's an Earth, it really is an Earth," Borucki said.

If the potential planet has a short orbital period (a few days or weeks), that means it orbits very closely to its sun. A long period (of several years) would mean it sits closer to the edge of the star's gravitational grip. In general, such extreme orbits would make the planet too hot or too cold, respectively, for life to arise there.

Borucki and the rest of the Kepler team are interested in finding a planet with a more middle-of-the-road orbit, one that is "not too hot, not too cold, but just right," he said. Such orbits, which fall in what is called the sun's "habitable zone," would mean that the temperature of the planet was mild enough for liquid water to exist on the surface.

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