In the ongoing quest to find life elsewhere in the universe, it helps to have a role model. Presently, however, there's just one to choose from: Earth.
"It is terra-centric of us," said planetary scientist Sara Seager. "It's like that story of a person who loses their keys on a dark sidewalk and looks for them under the street light because that is the only place he can see."
Seager, with the Massachusetts Institute of Technology, is among dozens of researchers in the hunt for Earth-like worlds. She spends her time modeling what the atmospheres and interiors of planets outside our solar system might look like. Many of her ideas come from the home planet.
"If we get data, we want to know what it means," she said in an interview with Discovery News.
Twinkle, Twinkle
For example, at a planet-hunters conference in France last month, researchers reported that from the perspective of space, light from Earth twinkles as clouds pass in and out of view.
"A distant extraterrestrial observer would see Earth as a point source of light that varies in brightness in a repeating, predictable pattern, just like spots on a spinning ball," Science magazine reported in an article last month about the research, which was headed by Enric Palle of the Institute of Astrophysics of the Canary Islands.
If similar patterns were discovered on an extrasolar Earth, scientists might be able to look for what would appear to be variations in the planet's rotation, a phenomenon actually due to clouds.
On Earth, clouds indicate the presence of water vapor, and water, as far as scientists know, is a key ingredient for life.
This sort of detective work is what scientists searching for habitable planets have available. Direct imaging of large planets, let alone smaller Earth-sized worlds, is not possible with the telescopes and technologies that exist today.
The Next Pale Blue Dot
Even with the sophisticated observatories in the planning stages, such as NASA's Terrestrial Planet Finder, scientists' best hope is to discern a pale blue dot.
Their most important tool will not be a camera, but a spectrograph, which can split light bouncing off a target planet into individual wavelengths, much like visible light can be broken into a rainbow array of colors. Chemicals in a planet's atmosphere will absorb particular wavelengths, resulting in dropouts, like black lines in the rainbow.
"So from afar, we can see that there's water vapor, for example, which may be an indication of water on the planet's surface," Seager explained. "We can detect oceans, maybe weather."
After water, oxygen would be the next key find. Earth's atmosphere is rich with oxygen due to plant life. Uninhabited sister planet Venus, in comparison, has just a trickle.
"You can't generate that much oxygen by geologic processes," Seager said. "Life produces the same thing that can be produced naturally but, there's lots more of it."
Earth From a Distance
Among Seager's projects is a recycled asteroid probe now on an extended mission to hunt for extrasolar planets as it makes it way toward a comet. NASA on Thursday released one of the first offerings from Deep Impact's second life: a movie of Earth.
"It helps the search for other life-bearing planets in the universe by giving insights into how a distant, Earth-like alien world would appear to us," said University of Maryland astronomer Michael A'Hearn, the lead scientist for the Deep Impact extended mission, called EPOXI.
For example, in the video, sunlight can be seen bouncing off Earth's oceans.
"Similar glints to be observed from extrasolar planets could indicate alien oceans," said Drake Deming, with NASA's Goddard Space Flight Center in Greenbelt, Md.
The team also found eye-popping reflections of the planet's plant-covered continents in near-infrared imagery taken by Deep Impact.
"They're brighter in that wavelength," Seager observed. "I didn't really appreciate how that makes the continents stand out until I saw it."
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