http://www.sciencedaily.com/releases/2007/12/071221105611.htm

To Curious Aliens, Earth Would Stand Out As Living Planet

ScienceDaily (Dec. 24, 2007) - With powerful instruments scouring the
heavens, astronomers have found more than 240 planets in the past two
decades, none likely to support Earth-like life.

But what if aliens were hunting life outside their own planet? Armed with
telescopes only a bit bigger and more powerful than our own, could they peer
through the vastness of space and lock in onto Earth as a likely home to
life?

That's the question at the heart of paper co-authored by a University of
Florida astronomer that appeared recently in the online edition of
Astrophysical Journal. The answer, the authors say, is a qualified "yes."
With a space telescope larger than the Hubble Space Telescope pointed
directly at our sun, they say, "hypothetical observers" could measure 
Earth's
24-hour rotation period, leading to observations of oceans and the chance of
life.

"They would only be able to see Earth as a single pixel, rather than
resolving it to take a picture," said Eric Ford, a UF assistant professor of
astronomy and one of five authors of the paper. "But that could be enough
for them to identify our planet as one that likely contains clouds and
oceans of liquid water."

This research may sound whimsical, but it has a serious goal: to provide a
road map for Earth-bound astronomers trying to study Earth-like planets - a
task expected to become possible in coming decades as more powerful
telescopes come on line, said Enric Palle, the lead author of the paper and
an astronomer with the Instituto de Astrofisica de Canarias.

For humans or curious aliens, observing planets is challenging for a number
of reasons - habitable planets all the more so. The planet can't be too
close or too far away from its star, or its surface would scald or freeze.
And, it must have a protective atmosphere like Earth's.

Most planets found so far are much larger than Earth, which means they are
likely hot gas planets similar to Jupiter, a profoundly uninhabitable place
with no solid surface and atmosphere composed largely of hydrogen and
helium.

But astronomers are beginning to plan how future space telescopes could
directly detect planets much closer to Earth's size and proximity to the
sun. One challenge: To figure out how to use a planet's light to recognize
if its surface and atmosphere are Earth-like.

For Ford and his colleagues, the answer lies in probing how the Earth would
appear to outside or alien observers.

Astronomers have long recognized that even a large telescope would need to
observe Earth for several weeks to collect enough light to identify
chemicals in the planet's atmosphere. During these observations, the
brightness of the Earth would change, primarily because of clouds rotating
into and out of view. If astronomers could measure Earth's rotation period,
then they would know when a given part of the planet was in view. The hitch
was that astronomers were unsure whether Earth's seemingly chaotically
changing cloud patterns would make it impossible for alien observers to
determine this rotation rate.

Based on data retrieved from satellite observations of Earth, Ford and his
colleagues created a computer model for the brightness of the Earth,
revealing that on the global scale Earth's cloud cover is remarkably
consistent - with rain forests usually turning up cloudy, arid regions
clear, and so on. As a result, extraterrestrial astronomers who watched
Earth for a period of several months would notice repeating patterns - a bit
like watching the spots on a spinning ball come into view and then
disappear. From those repeating patterns, they could then deduce Earth's
24-hour rotation period, Ford said.

That done, the "E.T." astronomers could infer that anomalies in the pattern
were caused by changing weather patterns, most prominently, clouds, he said.
Although some uninhabitable planets are extremely cloudy, the repeated
presence and absence of clouds indicates active weather. On Earth, this
variability results in water turning from gas to a vapor and back again, so
finding similar variability on another planet would be a reasonable
indication of liquid water.

"Venus is always covered in clouds. The brightness never changes," Ford
said. "Mars has virtually no clouds. Earth, on the other hand, has a lot of
variation."

Not only that, but observers could likely also infer the presence of
continents and oceans from Earth's changing light pattern.

The research will be useful to astronomers designing the next generation of
space telescopes because it provides an outline of the capabilities required
for studying the surfaces of Earth-like planets, Ford said. He said it
appears that zeroing in on Earth-like planets orbiting the nearest stars
would require a telescope at least twice the size of the Hubble Space
Telescope. Ford said he hopes that his research will help to motivate an
ever larger space telescope that could search for Earth-like planets around
many stars.

The other authors of the paper are P. Montañés-Rodríguez and M. Vazquez,
both of the Instituto de Astrofisca de Canarias in Spain, and Sara Seager,
of the Massachusetts Institute of Technology. The IAC and UF are partners in
the construction of the Gran Telescopio Canarias, a 10-meter telescope in
the Canary Islands, which will start operations in 2008.

The research was funded in part by a Ramon y Cajal fellowship for Palle, by
a Hubble fellowship and UF for Ford, and by a NASA grant for Seager.

Adapted from materials provided by University of Florida.

-- 

Ken

"Buddhism elucidates why we are sentient."
"Buddhism follows thought throughout the Universe."
"Karma means that you don't get away with anything."