Since the early 1990s astronomers have discovered more than 300 planets orbiting stars other than our sun, nearly all of them gas giants like Jupiter. Powerful space telescopes, such as the one that is central to NASA's recently launched Kepler Mission, will make it easier to spot much smaller rocky extrasolar planets, or exoplanets, more similar to Earth.
But seen from dozens of light years away, an Earth-like exoplanet will appear in telescopes as little more than a "pale blue dot," the term coined by the late astronomer Carl Sagan to describe how Earth appeared in a 1990 photograph taken by the Voyager spacecraft from near the edge of the solar system.
Using instruments aboard the Deep Impact spacecraft, a team of astronomers and astrobiologists has devised a technique to tell whether such a planet harbors liquid water, which in turn could tell whether it might be able to support life.
"Liquid water on the surface of a planet is the gold standard that people are looking for," said Nicolas Cowan, a University of Washington doctoral student in astronomy and lead author of a paper explaining the new technique that has been accepted for publication in Astrophysical Journal.
As part of NASA's Extrasolar Planet Observation and Characterization mission, the scientists obtained two separate 24-hour observations of light intensity from Earth in seven bands of visible light, from shorter wavelengths near ultraviolet to longer wavelengths near infrared. Earth appears gray at most wavelengths because of cloud cover, but it appears blue at short wavelengths because of the same atmospheric phenomenon that makes the sky look blue to people on the surface.
The researchers studied small deviations from the average color caused by surface features like clouds and oceans rotating in and out of view. They found two dominant colors, one reflective at long, or red, wavelengths and the other at short, or blue, wavelengths. They interpreted the red as land masses and the blue as oceans.
The analysis was undertaken "as if we were aliens looking at Earth with the tools we might have in 10 years" and did not already know Earth's composition, Cowan said. "You sum up the brightness into a single pixel in the telescope's camera, so it truly is a pale blue dot."
Since Earth's colors changed throughout the 24-hour-long observations, the scientists made maps of the planet in the dominant red and blue colors and then compared their interpretations with the actual location of the planet's continents and oceans.
"You could tell that there were liquid oceans on the planet," Cowan said. "The idea is that to have liquid water the planet would have to be in its system's habitable zone, but being in the habitable zone doesn't guarantee having liquid water."
The observations on March 18 and June 4, 2008 were made when the spacecraft was between 17 million and 33 million miles from Earth, and while it was directly above the equator. Observations from above a polar region likely would show up as white, Cowan said.
It will be some years before the launch of space telescopes capable of making similar observations for Earth-sized exoplanets, but devising this technique now could guide the construction of those instruments, he said. And while those planets will be much farther away, the technique still will be applicable.
"You will still have all the spectral information, and more importantly to us you'll still have the information so that you can see how the brightness of that speck is changing over time, Cowan said."
Co-authors are Eric Agol, Victoria Meadows and Tyler Robinson of the UW, Timothy Livengood and Drake Deming of the NASA Goddard Space Flight Center, Carey Lisse of Johns Hopkins University, Michael A'Hearn and Dennis Wellnitz of the University of Maryland, Sara Seager of the Massachusetts Institute of Technology and David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. The work was funded by the Natural Sciences and Engineering Research Council of Canada, the National Science Foundation and the NASA Discovery Program.
Cowan notes that some non-habitable planets, such as Neptune, also can appear to be blue, but the color is constant and, in the case of Neptune, likely caused by methane in the atmosphere.
"It looks blue from every angle, the same blue all the way around. If you had an ocean planet it might look like that, but you can do other tests to determine that," he said. "For Earth, the blue varies from one place to another, which indicates that it's not something in the atmosphere."
See video illustrations at http://www.nasa.gov/mission_pages/epoxi/alien_ocean.html.