Sci News

Night and Day--Trillions of Kilometers Away
http://sciencenow.sciencemag.org/cgi...ll/2006/1013/2

By Govert Schilling
ScienceNOW Daily News
13 October 2006

Long-term visitors to the moon will have to cope with daily
temperatures that rise and fall up to 250 degrees Celsius. But that's
nothing compared to the day-night cycle on Upsilon Andromedae b, a
giant extrasolar planet some 40 lightyears away. Going from the
nightside to the dayside of this planet would be like jumping from an
iceberg into a volcano, according to the first temperature variation
measurements of a planet outside our solar system.

Many exoplanets are "hot Jupiters": massive balls of gas orbiting
very close to their parent star. Little wonder that they are hot,
with average temperatures as high as 1300 degrees Celsius. But even
though these blistering planets always keep the same face to their
star, astronomers had expected that the bright and dark sides of the
planet would be equally hot. That's because atmospheric jet streams
are thought to redistribute heat from the dayside to the nightside,
says theoretical astrophysicist Adam Burrows of the University of
Arizona in Tucson.

But new observations by NASA's Spitzer Space Telescope suggest
otherwise. For four-and-a-half days in February, Spitzer monitored
the combined infrared brightness of the sunlike star Upsilon
Andromedae and its gas giant companion, which can't be observed
directly. As the planet orbits the star every 4.6 days, it
alternately shows more of its sunlit face (when it is on the far side
of the star as seen from the Earth) and more of its dark hemisphere
(when on the near side). Spitzer registered a simultaneous change in
the amount of heat radiation, indicating that the two hemispheres
have wildly different temperatures. While the nightside may be just a
few hundred degrees Celsius, the planet's dayside is probably well
over 1000 degrees hotter, the team reported online yesterday in
Science.

Team member Sara Seager of the Carnegie Institution of Washington,
D.C., admits that precise temperature readings will require more
observations at a wider variety of wavelengths. But the apparent lack
of heat distribution is surprising, she says. Apparently, says
Seager, hot air on this planet cools down rapidly. The most likely
explanation is that the majority of the star's energy is deposited
high in the planet's atmosphere, where it can be quickly reradiated
into space before winds have a chance to transport it to the
nightside.

Burrows, an expert on atmospheric modeling, says the results "have
great import for the future of this burgeoning field. They promise to
vastly increase our understanding of the physical properties of
extrasolar giant planets," providing a glimpse into their origin and
evolution.



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