U.S. Water News Online
WASHINGTON -- Scientists taking their first "sniffs of air"
from planets outside our solar system are a bit baffled by what they
didn't find -- water.
One of the more basic assumptions of astronomy is that the two
distant, hot gaseous planets they examined must contain water in
their atmospheres. The two suns the planets orbit closely have
hydrogen and oxygen, the stable building blocks of water. These
planets' atmospheres -- examined for the first time using light
spectra to determine the air's chemical composition -- are supposed
to be made up of the same thing, good old H2O.
But when two different teams of astronomers used NASA's Spitzer
Space Telescope for this new type of extrasolar planet research, they
both came up dry, according to research published in the recent
edition of Nature and the online version of the Astrophysical Journal
The study of one planet found hints of fine silicate-particle
clouds. Research on the other planet found no chemical fingerprints
for any of the molecules scientists were seeking.
"We had expected this tremendous signature of water ... and it
wasn't there," said the study leader for one team, Carl Grillmair of
the California Institute of Technology and Spitzer Science Center.
"The very fact that we've been surprised here is a wake-up call. We
obviously need to do some more work."
Grillmair's colleague, Harvard astronomy professor David
Charbonneau, said these surprising "sniffs of air from an alien
world" tell astronomers not to be so Earth-centric in thinking about
"These are very different beasts. These are unlike any other
planets in the solar system," Charbonneau said. "We're limited by our
imagination in thinking about the different avenues that these
atmospheres take place in."
Our own solar system has two planets without water in the
atmosphere, Grillmair noted -- Mercury, which doesn't have an
atmosphere, and Venus, which is a different type of planet from the
huge gaseous ones that would be expected to have the components of
water in the air.
So far, scientists have found 213 planets outside our solar
system, but only 14 have orbits that make it possible for this type
of study -- only eight or nine of those are close enough to see.
Grillmair's team studied the closest, which goes by the catchy name
HD 189733b. It's a mere 360 trillion miles from Earth in the
constellation Vulpecula. The other planet, HD209458b, is about 900
trillion miles away in the constellation Pegasus and it's the one
with the strange silicate cloud.
So where did the water go?
Maybe it's hiding, scientists suggest. The water could be under
dust clouds, or all the airborne water molecules have the same
temperature, making it impossible to see using an infrared
spectrograph. Or maybe it's just not there and astronomers have to go
back to the drawing board when it comes to these alien planets.
The other finding on the more distant of the two planets seems to
indicate the atmosphere is full of silicon-oxygen compounds, said
study lead author L. Jeremy Richardson of NASA's Goddard Space Flight
"They'd be like dust grains and they would form clouds,"
Richardson said. And that cloud of silicates could be blocking the
space telescope from measuring lower-lying water, Richardson and
other scientists said.
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