CfA Release No.: 06-25
September 14, 2006

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ASTRONOMERS REVEAL FIRST ALIEN I.D. CHART

Washington, DC- It is only a matter of time before astronomers find an
Earth-sized planet orbiting a distant star. When they do, the first
questions people will ask are: Is it habitable? And even more importantly,
is there life present on it already? For clues to the answers, scientists
are looking to their home planet, Earth.

Astronomers Lisa Kaltenegger of the Harvard-Smithsonian Center for
Astrophysics (CfA) and Wesley Traub of NASA's Jet Propulsion Laboratory and
CfA, propose using Earth's atmospheric history to understand other planets.

"Good planets are hard to find," said Kaltenegger. "Our work provides the
signposts astronomers will look for when examining truly Earth-like worlds."

Geologic records show that Earth's atmosphere has changed dramatically
during the past 4.5 billion years, in part because of life forms developing
on our planet. Mapping what gases comprised Earth's atmosphere during its
history, Kaltenegger and Traub propose that by looking for similar
atmospheric compositions on other worlds, scientists will be able to
determine if that planet has life on it, and if so, that life's evolutionary
stage.

To date, all extrasolar planets have been studied indirectly, for example
by monitoring the way a host star wobbles as the planet's gravity tugs it.
Only four extrasolar planets have been detected directly, and they are
massive Jupiter-sized worlds. The atmosphere of one of these worlds was
detected by another CfA scientist, David Charbonneau, using NASA's Spitzer
Space Telescope. The next generation of space-based missions such as
NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin will be able to directly
study nearby Earth-sized worlds.

Astronomers particularly want to observe the visible and infrared spectra
of distant terrestrial planets to learn about their atmospheres.

Particular gases leave signatures in a planet's spectrum, like fingerprints or DNA
markers. By spotting those fingerprints, researchers can learn about an
atmosphere's composition and even deduce the presence of clouds.

Today, Earth's atmosphere consists of about three-fourths nitrogen and
one-fourth oxygen, with a small percentage of other gases like carbon
dioxide and methane. But four billion years ago, no oxygen was present.
Earth's atmosphere has evolved through six distinct epochs, each
characterized by a particular mix of gases. Using a computer code developed
by Traub and CfA colleague Ken Jucks, Kaltenegger and Traub modeled each of
Earth's six epochs to determine what spectral fingerprints would be seen by
a distant observer.

"By studying Earth's past, we can learn about the present state of other
worlds," Traub explained. "If an extrasolar planet is found with a spectrum
similar to one of our models, we potentially could characterize that
planet's geological state, its habitability, and the degree to which life
has evolved on it."

To better understand these time periods, or "epochs," and to put them into
perspective, one can scale the Earth's 4.5-billion-year history down to one
year, attaching dates beginning with January 1 - the date the Earth formed.

EPOCH 0 - February 12

At Epoch 0 (3.9 billion years ago), the young Earth possessed a turbulent,
steamy atmosphere composed mostly of nitrogen, carbon dioxide and hydrogen
sulfide. The days were shorter and the Sun was dimmer, shining as a red orb
through our orange brick-colored sky. The one ocean that covered our entire
planet was a muddy brown that absorbed bombardment from incoming meteors
and comets. Carbon dioxide helped warm our world since the infant Sun was
a third less luminous than today. Although no fossils survived from this time
period, isotopic signatures of life may have been left behind in Greenland
rocks.

EPOCH 1 - March 17

About 3.5 billion years ago (Epoch 1), the planet landscape featured
volcanic island chains poking out of the vast global ocean. The first life
on Earth was anaerobic bacteria - bacteria that could live without oxygen.
These bacteria pumped large amounts of methane into the planet's
atmosphere, changing it in detectable ways. If similar bacteria exist on
another planet, future missions like TPF and Darwin could detect their
fingerprint in the atmosphere.

EPOCH 2 - June 5

About 2.4 billion years ago (Epoch 2), the atmosphere reached its maximum
methane concentration. The dominant gases were nitrogen, carbon dioxide,
and methane. Continental landmasses were beginning to form. Blue-green
algae began pumping large amounts of oxygen into the atmosphere. Big
changes were about to happen.

"I'm sorry to say the first signs of E.T. probably won't be a radio or TV
broadcasts; instead, it could be oxygen from algae," lamented Kaltenegger.

EPOCH 3 - July 16

Two billion years ago (Epoch 3), these first photosynthetic organisms
shifted the atmosphere's balance permanently-they produced oxygen, a highly
reactive gas that cleared out much of the methane and carbon dioxide, while
also suffocating the anaerobic, methane-producing bacteria. In doing so,
the planet's atmosphere gained its first free oxygen. The landscape now
was flat and damp. With volcanoes smoking in the distance, brilliantly
colored pools of greenish-brown scum created a sheen on the stench-filled
water. The oxygen revolution was fully underway.

"The introduction of oxygen was catastrophic to the dominant life on Earth
at that time; it poisoned it," Traub said. "But at the same time, it made
multicellular life, including human life, possible."

EPOCH 4 - October 13

At 800 million years ago, the Earth entered Epoch 4, with continuing
increases in oxygen levels. This time period coincides with what is now
known as the "Cambrian Explosion." Beginning 550 to 500 million years ago,
the Cambrian Period is a significant marker post in the history of life on
Earth: It is the time most major animal groups first appear in the fossil
records. The Earth was now covered with swamps, seas and a few active
volcanoes. The oceans were teaming with life.

EPOCH 5 - November 8

Finally, 300 million years ago in Epoch 5, life had moved from the oceans
onto land. The Earth's atmosphere had reached its current composition of
primarily nitrogen and oxygen. This was the beginning of the Mesozoic
period that included the dinosaurs. The scenery looked like Jurassic Park
on a Sunday afternoon.

EPOCH 6 - December 31 (11:59:59)

The intriguing question that remains is: What would Epoch 6, the time
period humans occupy today, look like? Could we detect the telltale signs
of alien technologies on distant worlds?

As the general consensus builds among scientist that human activity has
altered Earth's atmosphere by inputting carbon dioxide as well as gases
like Freon, could we identify the spectral fingerprints of those
byproducts on other worlds? Although Earth-orbiting satellites and balloon
experiments can measure these changes here at home, detecting similar
effects on a distant world are beyond even the capabilities of upcoming
programs like Terrestrial Planet Finder and Darwin. It will take gigantic
flotillas of future space-based infrared telescopes to be able to
accomplish those measurements.

"As daunting as this challenge sounds," said Kaltenegger, "I do believe in
the next few decades we will know whether or not our little blue world is
all alone in the Universe or if there are neighbors out there waiting to
meet us."

This research was funded by NASA.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

For more information, contact:

David Aguilar
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
617-495-7463
cpulliam@cfa.harvard.edu