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Posted by baalke on June 20, 2007, 12:41 pm
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http://news-info.wustl.edu/tips/page/normal/9522.html
Scientists ponder plant life on extrasolar Earthlike planets
By Tony Fitzpatrick
Washington University in St. Louis
June 14, 2007
When we think of extrasolar Earth-like planets, the
first tendency is to imagine weird creatures like Jar Jar Binks,
Chewbacca, and, if those are not bizarre enough, maybe even the
pointy-eared Vulcan, Spock, of Star Trek fame.
But scientists seeking clues to life on extrasolar planets are
studying
various biosignatures found in the light spectrum leaking out to Earth
to speculate on something more basic and essential than the musical
expertise of Droopy McCool. They are speculating on what kind of
photosynthesis might occur on such planets and what the extrasolar
plants might look like.
Paint it black
It could be the plants are black, says Robert Blankenship, Ph.D.,
Lucille P. Markey Distinguished Professor in Arts & Sciences at
Washington University in St. Louis. But it all depends on what size
and
light intensity of star - or sun - the planet feeds off, and the
extrasolar planet's atmospheric chemistry.
Plants on extrasolar planets resembling Earth could be as black as
these
eggplants. Scientists who speculate on plant life and what might
constitute photosynthesis out there say that plant color depends on
the
size and light intensity that the planet feeds off from its star, or
sun, as well as the extrasolar planet's atmospheric chemistry.
Plants on extrasolar planets resembling Earth could be as black as
these
eggplants. Scientists who speculate on plant life and what might
constitute photosynthesis "out there" say that plant color depends on
the size and light intensity that the planet feeds off from its star,
or
sun, as well as the extrasolar planet's atmospheric chemistry.
Plants on Earth are green because of chlorophyll, which harnesses the
energy of the sun to make sugars for metabolism. But our plants aren't
completely efficient - they waste a little bit of light.
"Ideally, what you want is a black molecule that absorbs all of the
light," Blankenship said. "There could be another system developed on
an
extrasolar planet where plants are completely black if the spectrum of
light that's available to organisms is different from the light
available to organisms on Earth.
"Then, for sure, the plants will have different types of pigments
tuned
to absorb those wavelengths of light available on the other world."
Blankenship is co-author of two papers recently published in the
journal
Astrobiology. The papers detail the kinds of clues that researchers
are
looking for and explore theories of what these other worlds might be
like.
Blankenship is part of a NASA working group based at the Jet
Propulsion
Laboratory called the Virtual Plant Laboratory. He and his colleagues
are studying light that comes from stars and extrasolar planets to
infer
their composition. They can see clues that suggest the presence of
water
vapor, oxygen or carbon dioxide, for instance. One key biosignature is
the existence of disequilibrium - the simultaneous presence of things
that should not coexist on a dead world. The presence of methane and
oxygen together on an extrasolar planet, for instance, would be a
strong
smoking gun for the possible existence of life.
Life on the edge
They also are looking into the "red edge" effect. Seen at 700
nanometers
out, beyond the limit of normal human vision, this reflectance
spectrum
is a signature of the fact that there is very intense chlorophyll
absorption going on.
A third way to find extrasolar planets is to look for wobbly stars. As
a
planet - especially a massive planet - goes around the star it causes
the star to wobble a bit. The Hubble Space Telescope has found wobbly
stars.
NASA has two missions in the works designed to find possible evidence
for life on extrasolar planets. One features a space-based instrument
that will make measurements in the near infrared region; the other
measures longer wavelengths to get good biosignatures for things like
methane and oxygen.
Blankenship said that speculation about the natural world of
extrasolar
planets is at this point speculative, but that it is important to get
a
handle on what the possibilities are, how things might look, what
measurements to make and what experiments to do to conclude whether
there is life on another world.
"I think that everyone thinks that there are Earth-like ones out
there,
but very few have been detected so far," he said. "One of the things
that I've learned is that you have to free your mind from the
constraints of thinking that life elsewhere has to be like life here."
Energy on any world is critical, he said, and there has to be some
system on an extrasolar planet that involves light capture and
storage.
"When you consider another world you've got to find that life there
depends on photosynthesis in the broad sense, but it's probably not
identical to the way that photosynthesis works here," Blankenship
said.
"You'll need molecules that absorb light that are highly colored, but
whether they have the same green colors we know on Earth is unlikely."
Similarly, on Earth life depends on DNA and proteins. But out there?
"I don't think that there is anything magical about DNA in that it has
to be the same out there as here," he said. "But there has to be some
sort of information-carrying molecule - again, highly unlikely the
same
as our DNA - that has information coded in a way that allows the
ability
to transfer information. We've got proteins that do all of the dirty
work in the cell in terms of chemistry. You can imagine a different
sort
of molecule that would do that sort of chemistry. Maybe it would have
the same protein backbone with peptide bonds and so forth. But there's
no reason to think it would be comprised of the same 20 amino acids
that
we have on Earth. It's intriguing to speculate, and I think we'll know
more when we get more clues."
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