Seen from a distance of light years through telescope an Earth-like exoplanet looks like a “pale blue dot,” the term coined by the late astronomer Carl Sagan.
Using instruments aboard the Deep Impact spacecraft, a team of astronomers and astrobiologists has devised a technique to tell whether such a planet has liquid water, a major constituent for supporting life.
“Liquid water on the surface of a planet is the gold standard that people are looking for,” said Nicolas Cowan, researcher, University of Washington.
The researchers from UW, MIT, Goddard Space Flight Center studied small colour deviations of two colours red and blue from surface features like clouds and oceans rotating in and out of view from the spacecraft. They interpreted the red as land masses and the blue as oceans.
The scientists made maps of Earth in 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, the lead author of the paper 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.