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Lunar eclipse tonight: How it helps the search for extraterrestrial life

The moon's ruddy color during the lunar eclipse tonight is caused by sunlight filtering through Earth's atmosphere. It's what astronomers look at when distant planets pass in front of their own stars.

By Staff writer / December 20, 2010

The full moon partly covered by clouds in Nairobi, Kenya, on Dec. 31, 2009.

Sayyid Azim/AP/File

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The lunar eclipse tonight – a total lunar eclipse for people across North America – promises to be a spectacular show, weather and coffee pot permitting.

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It will be a late, languid event. For people living east of the Mississippi, the eclipse begins well after midnight. The lunar eclipse will last about three and a half hours, with the moon falling in the depths of Earth's shadow for about an hour and twelve minutes at the height of the event.

As Earth slips between the sun and moon, changing the tint of the lunar surface from white to orange to russet and back, you're seeing the effect Earth's atmosphere is having on the color of sunlight passing through it. But the atmosphere is doing something else. It's in effect tagging the sun's rays with the chemical fingerprints of gases in the atmosphere.

Over the past two years, two teams of astronomers have been using this effect to figure out what Earth might look like as a distant, extrasolar planet orbiting another star. By analyzing the light reflected off the moon during a lunar eclipse – light that has passed through Earth's atmosphere – they have detected gases in the atmosphere that indicate the presence of organic life on the planet.

If the teams' baby steps are any indication, the techniques they are developing may be able to detect evidence of organic life imprinted in an extrasolar planet's atmosphere – at least for rocky, Earth-mass planets orbiting stars relatively close to the sun – using large Earthbound telescopes.

"It's an exciting experiment – one of the few I've seen that I wish I'd thought of myself," says Sara Seager, a physicist at the Massachusetts Institute of Technology who studies exoplanets and their atmospheres and who was not involved in either project.

"The Earth is our best laboratory; it's the only planet we know of with life," she says. "So we really want to understand what Earth would look like as an exoplanet far away."

Of special interest are planets whose orbits carry them in front of their parent stars as seen from Earth – so-called transiting planets.

These are the types of extrasolar planets NASA's Kepler spacecraft and the French Space Agency's CoRoT spacecraft currently are hunting.

Kepler in particular is searching more than 150,000 stars for Earth-mass planets in their stars' so-called habitable zones. These are regions of space close enough to a star that liquid water would be stable on the surface of a planet orbiting at that distance.

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