A combination of three wavelengths of light distinguishes Earth from among other planets in the solar system. The study may help astronomers build telescopes to pick out life-friendly extrasolar planets in the future. Wondering if any of the Milky Way's 50 billion planets are similar to Earth? A team of astronomers has figured out which colors of light future telescopes should look for.
A combination of three wavelengths -- close to colors the human eye sees as blue, green and red -- uniquely characterize Earth from among the other planets in the solar system, a new study shows.
Scientists used the mothership of NASA's recycled Deep Impact comet probe to look at light reflected off Earth, the moon and Mars. They also extrapolated from previous studies of Mercury, Venus, Jupiter, Saturn, Neptune, Uranus, the moon and Saturn's moon Titan, then ran all the data through a computer to see if there was a combination of colors unique to Earth.
"We had seven filters, from ultraviolet to infrared," Carolyn Crow, a graduate student at the University of California at Los Angeles. "We tried different combinations to see which was the best at picking at Earth."
Crow and colleagues found three wavelengths that pegged our planet, a finding that could help whittle down the rapidly growing list of extrasolar planets to those that are potentially suitable for Earth-like life.
If the rest of the galaxy is anything like the sliver of sky being surveyed by NASA's planet-hunting Kepler Telescope, the Milky Way is home to about 50 billion planets, including hundreds of millions located at the right distance from their parent stars for liquid water to exist.
Telescopes sensitive enough to directly image a planet are a long way off. Kepler works by ferreting out tiny changes in the amount of light coming from a star as orbiting planets pass around. From the telescope's point of view, so-called transiting planets block a bit of starlight.
"One of the things that have to be overcome is to be able to pick out reflected light from the planet from the light of the star. Maybe in the next 10 years or so we'll be able to do that," Crow said.
Ultimately, astronomers would like to fly telescopes sensitive enough to identify gases in an extrasolar planet's atmosphere to see if they match up with processes associated with life on Earth.
Oxygen on Earth, for example, comprises 20 percent of the planet's atmosphere by volume, but if there were no life on the planet, there would be no oxygen, said Massachusetts Institute of Technology astronomer Sara Seager.
"If we saw oxygen on a rocky exoplanet -- and with other atmosphere information -- we would be highly confident that life existed," Seager said at the American Association for the Advancement of Science meeting in Washington DC this weekend.
Wesley Traub, chief scientist for NASA's Exoplanet Exploration Program at the Jet Propulsion Laboratory in California, predicts that by 2030 scientists will have five Earth-like planets circling around nearby stars that can be scanned for life.
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