MIT, others ask 'What would E.T. see?'

Team analyzes how alien astronomers would study Earth

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For Immediate Release
THURSDAY, DEC. 20, 2007
Contact: Patti Richards,
MIT News Office
Phone: 617-253-8923
Email: prichards@mit.edu

PHOTOS AVAILABLE

CAMBRIDGE, Mass - As astronomers become more adept at searching for, and
finding, planets orbiting other stars, it's natural to wonder if anybody
is looking back. Now, a team of astronomers that includes a professor from
MIT has figured out just what those alien eyes might see using technologies
being developed by Earth's astronomers.

According to their analysis, among other things E.T. could probably tell that
our planet's surface is divided between oceans and continents, and learn a
little bit about the dynamics of our weather systems.

"Maybe somebody's looking at us right now, finding out what our rotation
rate is - that is, the length of our day," says Sara Seager, associate
professor of physics and the Ellen Swallow Richards Associate Professor of
Planetary Sciences at MIT.

Seager, along with Enric Palle and colleagues at the Instituto Astrofisica del
Canarias, in Spain, and Eric Ford (MIT class of 1999) of the University of
Florida, have done a detailed analysis of what astronomers here and on other
worlds could learn about a planet from very distant observations, using
telescopes much more powerful than those currently available to Earth's
astronomers. Their study, which has just been published online in the
Astrophysical Journal, will appear in the journal's print edition in April.

Most of the planets astronomers have discovered beyond the Solar System have
not actually been seen; rather, they have been indirectly observed by looking
at the influence they exert on stars they orbit. But even with the most
advanced telescopes planned by Earth's astronomers for use over the next
several years, a planet orbiting another star would only appear as a single
pixel-that is, a single point of light, with no detail except its brightness
and color. By comparison, a simple cellphone camera typically takes pictures
with about a million pixels, or one megapixel.

"The goal of [our] project was to see how much information you can extract"
from very limited data, Seager says. The team's conclusion: a great deal of
information about a planet can be gleaned from that single pixel and the way
it changes over time.

The way of analyzing the data that Seager and her co-authors studied would
work for any world that has continents and bodies of liquid on its surface
plus clouds in its atmosphere, even if those were made of very different
materials on an alien world. For example, icy worlds with seas of liquid
methane, like Saturn's moon Titan, or very hot worlds with oceans of molten
silicate (which is solid rock on Earth), would show up similarly across the
vastness of space.

However, the method depends on clouds covering only part of a planet's surface,
regardless of what each world is made of. So Titan, covered by perpetual
global smog, would not give up the mysteries of its weather or rotation,
nor would the hellishly hot Venus, with its complete shroud of clouds.

The key, the astronomers learned after sudying data from Earth's weather
satellites, is that while clouds vary from day to day, there are overall
patterns that stay relatively constant, associated with where arid or rainy
landmasses are. Detecting those repeating patterns would allow distant
astronomers to figure out the planet's rotation period because a brightening
associated with clouds above a particular continent would show up regularly
once each "day," whatever the length of that day might be. Once the day's
length is determined, then any variations in that period would reveal the
changing weather-that is, clouds in a different place than the average.

No telescope now in operation is capable of making the measurements that
Seager and her team analyzed. But planned telescopes such as NASA's Kepler,
set for launch in 2009, would be able to discover dozens or hundreds of
Earth-like worlds. Then even more advanced space observatories being
considered, such as NASA's Terrestrial Planet Finder, would allow the
followup studies to learn about these planets' rotation and weather, and
the composition of their atmospheres, Seager says.

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