

When humans look into outer space and its amazing distant realms, sometimes all we really want to find is someplace like home.

Another planet like Earth, that is. Soon, a new NASA telescope mission called Kepler may finally make that happen.

Set to launch March 5 from Florida's Cape Canaveral Air Force Station, the $550 million Kepler telescope is designed to detect extrasolar planets that are the same size as Earth, orbiting around stars the same size as the sun, at a range similar to Earth's distance from the sun, and with orbits of about one year, like ours.

"The whole mission was designed around this goal," said Kepler co-investigator William Cochran, an astronomer at McDonald Observatory at the University of Texas at Austin. "If we find no Earth-like planets, then we can say with great confidence that Earths like ours are rare."

Although more than 300 exoplanets have been discovered around other stars, none have been quite as small as Earth, and even the ones that come close don't usually orbit in what's called the habitable zone — the range in which temperatures would be favorable for life.

The results from Kepler could be important in trying to predict how common life, and even intelligent life, is in the galaxy. A famous calculation called the Drake equation aims to quantify how many extraterrestrial civilizations might exist in our galaxy that we could be able to communicate with.

The equation, devised in 1960 by Frank Drake of the Search for Extraterrestrial Intelligence Institute, or SETI, takes into account many factors, such as the rate of star formation in the galaxy, the fraction of stars with planets and the fraction of planets that are habitable. If Kepler can help estimate the frequency of Earth-size planets, astronomers will be a lot closer to making an accurate estimate with the Drake equation.

Kepler differs from most planet-hunting projects, which use the Doppler-shift method to search for stars that wobble due to the small gravitational pull of an orbiting planet. With existing technology, that method can only detect planets that are more massive than Earth, whose gravitational pull is large enough to tug at their parent stars with a strength we would notice.

Instead, Kepler will use a powerful optical telescope to detect the slight dimming of light that results when a planet moves between us and its star. This technique, as opposed to the Doppler method, does not depend on a planet's mass, so is better suited to reveal smaller planets.

"This could be a really fabulous mission," said UCLA astronomer Benjamin Zuckerman, who studies extrasolar planets, but has no direct involvement in the Kepler mission. "The nice thing about this mission, if it works as well as they hope, is that whatever result it gets is interesting. Either they find something, or they can put a strong limit on the frequency of Earth-sized planets."

After Kepler's launch, the telescope will begin continuously observing a single field of view, in which the brightnesses of 100,000 stars will be monitored for changes resulting from transiting planets. It will begin collecting data within days of its launch and the first results could be released by next fall.

"The number of stars we are looking at is such that if it turns out Earth-like planets are common, we should find 30 to 50 of them," Cochran told Wired.com.

If Kepler finds planets that could be Earth twins, it won't be able to detect whether or not they host life. Other missions, such as a potential NASA observatory called the Terrestrial Planet Finder or the European Space Agency's Darwin, will have to investigate those possibilities.

After its launch, Kepler is set to observe for at least three and a half years, with the possibility of extending its run if things go well. The observatory could find Earth-size planets with quick orbits within months, but detecting planets with periods closer to one year will likely happen toward the end of Kepler's mission.

"The only planets we can detect are those for which we are looking edge-on to the orbit," Cochran said. "Then for those where we are looking edge-on, the planets that we'll see most easily are the ones with the shortest periods."

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Images: NASA