When
it comes to life as we know it, nothing is more important than liquid water.
Now scientists have devised a way to spot water on distant planets that can
only barely be seen now, which in turn could reveal whether they might be able
to support life.
In
the past two decades, astronomers have detected more than 300 planets orbiting
alien stars. Although most of these exoplanets
are gas giants similar to Jupiter, powerful space telescopes such as the one
aboard NASA's recently launched Kepler Mission will make it easier to detect
smaller rocky exoplanets similar to Earth.
Seen
from dozens of light years away, an Earth-like
exoplanet will appear in telescopes as little more than a pale dot. Now a
team of astronomers and astrobiologists has come up with a method to tell
whether such a planet harbors liquid water, using NASA's Deep Impact spacecraft.
The
probe was set
to watch Earth on two separate days, March 18 and June 4, 2008. At the
time, Deep Impact was between 17 million and 33 million miles from our planet
and directly above the equator. It measured the light intensity from our planet
in seven bands of visible light, from longer redder wavelengths to shorter
bluer wavelengths.
The
scientists analyzed small deviations from the Earth's average color caused by
surface features such as clouds and oceans rotating in and out of view. This
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,
explained researcher Nicolas Cowan, an astronomer and astrobiologist at the
University of Washington in Seattle.
The
light coming from the Earth was summed up as just as a single pixel. Since
clouds cover half our planet, for the most part that dot looked very gray.
However, as each day progressed, the clouds rotated in and out of view,
revealing the surface underneath, "and the color of that little speck of
light changed by some small amount, say 10 or 20 percent," Cowan told SPACE.com.
Cowan
and his colleagues found two dominant colors emerged, one at redder
wavelengths, interpreted as landmasses, and the other at bluer wavelengths, judged
to be oceans.
"You
could tell that there were liquid oceans on the planet," Cowan said.
Although
some non-habitable planets such as Neptune can also appear blue, in the case of
Neptune this color is likely caused by methane in the atmosphere. Cowan noted
there are clues that one can use to tell a watery planet from other kinds of
blue planet. For instance, Neptune "looks blue from every angle, the same
blue all the way around," he said. "For Earth, the blue varies from
one place to another, which indicates that it's not something in the
atmosphere."
Scientists
have detected water
in the atmosphere of hot extrasolar gas giants in the past three years,
"but you can't claim those are nice places to live," Cowan said.
"If you found water on a rocky planet that is actually in the
habitable zone, not too close or far from the sun so that any water is
entirely frozen or vaporized, that would be key, since liquid water seems to be
the thing needed for life."
Cowan
and his colleagues are scheduled to detail their findings in the August issue
of Astrophysical Journal.
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