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Watching weather on Earth-like planets

A drawing of formation flying aircraft that NASA hopes will help gather information about other worlds.
A drawing of formation flying aircraft that NASA hopes will help gather information about other worlds.  

By Richard Stenger

(CNN) -- Advanced space telescopes designed to look for terrestrial-like planets could distinguish weather patterns, landforms, rotation speeds and possibly life signs on distant worlds, according to astronomers.

U.S. and European space agencies expect within decades to launch powerful orbiting observatories to scan the heavens for other planets comparable in mass and chemical composition to Earth.

Both spacecraft would use advanced spectroscopic vision to observe and analyze the spectrum of reflected light to determine whether so-called exoplanets have atmospheres similar to ours.

The technology would require highly precise observations, a daunting challenge considering that the light sources often would be extremely weak.

COSMIC ORIGINS  NASA telescopes to search for distant worlds
MESSAGE BOARD: Space Exploration  

But another method that focuses on variations in light patterns over time could provide valuable information about such worlds, including those too dim to study through spectroscopic means.

"If everything is perfect, we would want to use both techniques. But ours might be done more quickly than spectroscopy," said Eric Ford, who with Princeton University colleagues outlined his work in the Thursday edition of the journal Nature.

"Things we could identify with reasonable confidence would be the rotation period and the weather," he said.

Imagine looking at the Earth from a great distance. An observer would see great variations in the reflected light for numerous reasons: changes in cloud cover, rotating land and oceans, different levels of planet illumination, ranging from a thin crescent to an entire half.

A drawing of ESA's Darwin free-flyers
A drawing of ESA's Darwin free-flyers  

"We found that a majority of the light reflected comes from a small percentage of the surface," said Ford.

Should a Sahara desert or Asian forest come into the spotlight as a planet rotates, considerable and measurable changes in the reflected light would result, he said.

Even low-precision readings of scattered light could reveal daily and seasonal variations on terrestrial-like planets, providing insight into their meteorological and geographical characteristics.

Add basic color filters and astronomers might be able to identify the presence of life. On Earth, plants reflect infrared light much better than red light. If similar life forms exist on other worlds, the same light reflection pattern could be observed, Ford said.

When will such a search be possible? NASA hopes to launch the Terrestrial Planet Finder as early as 2011. The European Space Agency expects to send up Darwin no earlier than 2015.

But the missions, which would fly multiple spacecraft in precise formations and focus their searches between 30 to 50 light-years, require the development of many unproven technologies.

Ford, a 23-year-old graduate student, knows he may have to wait for a great while.

"I'll be happy if data comes back in my lifetime."

• NASA's Origins Program
• NASA: Terrestrial Planet Finder
• ESA: Darwin
• The University of California Planet Search Project

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