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Kepler mission to hunt for planets just our size

The US will launch the Kepler spacecraft Friday to look for other Earths.

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The one-ton Kepler observatory boasts a 1.4-meter (4-1/2 foot) diameter mirror and a push-the-envelope camera, according to James Fanson, project manager for the Kepler mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. A typical digital camera has roughly eight million to 10 million individual picture elements, or pixels, on its light detector. Kepler’s camera boasts 95 million pixels.

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The camera won’t take pictures of the stars it monitors, however. Instead, it will measure changes in starlight as an orbiting planet slips in front of its host star. It’s a tough measurement to make – akin to measuring the dimming effect of a flea passing in front of a car’s headlight, Dr. Fanson says.

The amount of dimming yields the planet’s radius. The elapsed time between each dimming episode yields its orbital period. From this data, and from a knowledge of the parent star’s traits, astronomers can calculate the planet’s mass, its density, and whether it falls in the habitable zone.

Not all solar systems will be oriented in ways that allow the telescope to detect planet “transits” across the faces of stars. So Kepler’s “transit” technique for picking out planets has dictated the large number of stars the telescope must monitor.

Tracking 100,000 stars
Picking the targets, 100,000 sun-like stars, was no cake walk. Astronomers spent five years methodically measuring traits of 4.5 million stars in Kepler’s planned field of view.

Out of the resulting 100,000-star catalog, Kepler scientists estimate that perhaps only 10 percent have planets with orbital periods short enough to allow for repeated detections within Kepler’s 3-1/2 year primary mission length. Some 0.5 percent of the 100,000 stars are expected to reveal planets orbiting at Earth-like distances.

That still means there is potential to find hundreds of Earth-like planets orbiting in that sweet spot. The solar systems range in distance from around 50 light-years away to some 3,000 light-years or more.

The architecture of solar systems
But finding planets at habitable distances from their suns does not alone a livable planet make.

Missions to Mars, Jupiter and its moons, and Saturn have yielded locations where simple forms of life might have gained a foothold and may still exist today, notes Renu Malhotra, a planetary scientist at the University of Arizona in Tucson. Jupiter and Saturn in particular fall outside the traditional “habitable” range of orbits.

Solar-system architecture is critical to habitability, she explains, adding that she expects Kepler, as well as follow-up observations from ground-based telescopes, to reveal in more detail how planets are distributed in these new solar systems.
Earth’s moon, for instance, has stabilized Earth’s tilt and limited the periodic wobble in Earth’s spin. This reduces the extremes at which climate could change over periods ranging from seasons to geologic time scales.

Our solar system’s order of planets has led to regions such as Kuiper Belt beyond Neptune or the asteroid belt between Mars and Jupiter, where relics from the solar system’s birth remain in relatively stable, tidy orbits. Without such shepherding, they could hurtle chaotically, boosting the risk of pummeling the rocky inner planets.

As planetary scientists appear ready to step across their Great Divide, Alan Boss, with the Carnegie Institution for Science in Washington, D.C., sums up the mood.

“It’s quite an exciting time to be alive,” he says.