[Editors] MIT: Extrasolars' light guides atmosphere research

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT: Extrasolars' light guides atmosphere research
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For Immediate Release
WEDNESDAY, FEB. 21, 2007, 1:00 P.M. EDT
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

IMAGES, PHOTO AVAILABLE


CAMBRIDGE, Mass.--So far, astronomers have discovered about 200 
planets outside our solar system, known as "extrasolar" planets. Very 
little is known about most of them, but for the first time, 
scientists have obtained new information about the atmospheres of two 
such planets by splitting apart the light emitted from them.

Sara Seager, MIT associate professor of earth, atmospheric and 
planetary sciences, is part of a research group based at Goddard 
Space Flight Center that studied a planet about 904 trillion miles 
from Earth, known as HD 209458b. The researchers used NASA's Spitzer 
Space Telescope to capture the most detailed information yet about an 
extrasolar planet.

Seager's team is one of three that are reporting spectral 
observations of extrasolar planets this week. Two groups studied HD 
209458b, and one studied another planet in a different solar system. 
The work by Seager's team is reported in the Feb. 22 issue of Nature.

Astronomers often learn about distant objects, such as stars and 
galaxies, by studying the composition of light emitted by them, 
Seager said. But extrasolar planets are much dimmer than stars and 
thus far more difficult to study.

Light from extrasolar planets is "very, very hard to measure because 
the stars are so bright and the planets are faint. This planet is 
right at the edge of what we can detect with this telescope," said 
Seager, who arrived at MIT in January to start a program devoted to 
studying extrasolar planets.

The study appearing in Nature was led by L. Jeremy Richardson of the 
Goddard Space Flight Center near Baltimore, Maryland. The three 
studies mark the first time a telescope has captured enough light to 
detect traces of molecules in an extrasolar planet's atmosphere.

"This is an amazing surprise," said Spitzer Project Scientist Michael 
Werner of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. 
"We had no idea when we designed Spitzer that it would make such a 
dramatic step in characterizing exoplanets."

Studying light spectra only works with so-called "transit 
planets"-planets whose orbits carry them in front of their sun when 
viewed from our solar system. HD 209458b, discovered in 1999 in the 
constellation Pegasus, was the first of 14 such planets that have 
been found.

Only a few of those planets, including HD 209458b, are bright enough 
for the spectral studies to yield useful data. A research group at 
Caltech is reporting spectral observations of another such planet, 
known as HD 189733b. Both planets are classified as "hot Jupiters," 
meaning they are large and gaseous, like Jupiter.

Temperatures on HD 209458b range from 1100 to 1600 degrees Kelvin, 
and the star is so close to its sun that it takes only three and a 
half days to complete its orbit.

 From previous observations, scientists already knew that HD 209458b 
had sodium, hydrogen, helium and carbon in its atmosphere. They also 
expected it to have water vapor, but the Goddard spectral analysis 
did not show any signs of water vapor in the atmosphere. Scientists 
did not find traces of water vapor in the atmosphere of HD 189733b 
either.

"That doesn't mean water vapor's not there, but it means the 
atmosphere is behaving differently than expected," Seager said.

The Goddard team's other major finding was evidence of sandy 
particles known as silicates in the atmosphere of HD 209458b. NASA 
scientists hypothesize that clouds of those particles could be 
blocking emissions from water vapors.

Another team that studied HD 209458b, based at JPL obtained similar results.

The Goddard team recorded its data during two eclipses of HD 209458b, 
each of which lasted for about three hours. By subtracting the light 
that was emitted by the sun alone from the light of the planet and 
sun together, the researchers obtained a spectrum of light from the 
planet itself.

After the infrared light was captured, it was separated into its 
component wavelengths, the same way a prism diffracts light into a 
rainbow. Signatures at different wavelengths represent emissions from 
different chemical compounds present in the atmosphere.

The new results offer hope of finding and analyzing other planets, 
including smaller, rocky planets like Earth.

"It's very hard to find Earth-like planets because they're too small. 
But that's the ultimate goal," said Seager.

Seager's team's research was funded by NASA, the Goddard Center for 
Astrobiology, the Spitzer Theory Program and the Carnegie Institute 
of Washington.

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Written by Anne Trafton, MIT News Office