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Spacecraft Studies Earth to Help Spot Other Worlds

Irene Klotz, Discovery News

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June 1, 2009 -- A robotic probe sailing out in space to look for planets beyond the solar system has its first target in sight -- Earth.

The idea behind observing the home planet is to give scientists an understanding of how an alien Earth may look in the data collected by future telescopes, chemical-analyzing spectrographs and other instruments.

"It's basically an extra tool to have in our tool belt as we go looking for exoplanets," said Nicolas Cowan, with the University of Washington.

Cowan and colleagues have been using NASA's Deep Impact spacecraft to study Earth from millions of miles away. Originally dispatched to blast a hole into comet Tempel 1 so scientists could study the comet's interior, Deep Impact was given a follow-on assignment to visit comet Hartley 2 in November 2010 and search for extrasolar planets along the way.

Cowan focused on understanding how an ocean-bearing world like Earth, with continents interspersed with water, would impact the overall reflectivity of light in seven select wavelengths, particularly those in the near-infrared.

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They found that as Earth completed a 24-hour rotation, the change in brightness varied by about 30 percent up and down. The shift is caused by the planet's reflectivity as the sun alternatively shines on oceans and then continents.

"The key is to look at how the planet changes over time," Cowan told Discovery News.

Clouds were not found to be a significant contributor to the change in brightness because they typically do not completely dissipate or form within a 24-hour period.

"You could imagine some weird planet where the clouds change very quickly and then you wouldn't be able to use this technique. You'd be hosed," Cowan said.

Another show-stopper would be if a planet's water were in one hemisphere and its mass of land on the other, as the total amount of light reflected from its parent star would stay stable throughout a rotation.

Cowan says that variable brightness -- particularly in the wavelengths where light reflected off water and land show up most vividly -- might be an important clue that an alien planet shares Earth's proclivity for water, and perhaps life.