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Day and night observed on exoplanet

For the first time, scientists have directly measured day and night temperatures on a planet outside our solar system.
Exoplanet
A Jupiter-like planet orbits close to its host star in this artist's concept of extrasolar planet Upsilon Andromedae b. Scientists who measured the planet's temperature find little energy transport from the exoplanet's day to night side.
NASA/JPL-Caltech/Robert Hurt
October 12, 2006
Planetary scientists Joseph Harrington of the University of Central Florida (UCF) in Orlando and Brad M. Hansen of the University of California at Los Angeles (UCLA) are the first to directly measure an extrasolar planet's day and night temperatures. The scientists and their team used the Spitzer Space Telescope's Multiband Imaging Photometer (MIPS) to measure infrared light emitted from exoplanet Upsilon Andromedae b at five points in its 4.6-day orbit in February.

A team led by Geoffrey Marcy and Paul Butler discovered Ups And b orbiting a star 40 light-years distant in the constellation Andromeda in 1996 using the radial-velocity method. The exoplanet is dubbed a "hot Jupiter" because it orbits close to its host star at 0.059 AU (1 AU is the average Earth-Sun distance) and contains at least 0.69 Jupiter-masses. "As the planet orbits, the system gets slightly brighter and dimmer. That difference tells us the planet has a hot side and a cold side," says Harrington. Hot and cold correlate to day and night, respectively, on the tidally locked planet. The exoplanet's temperature pattern matches its known orbit.

Ups And b absorbs and reradiates energy as it orbits its host star. "Theoretical predictions suggest the hottest point on such a planet would be blown downstream by winds as strong as 6,700 miles per hour [10,800 kilometers/hour]," Harrington says. However, observations show something different. "Winds cool off before carrying much heat from the exoplanet's day side to its night side."

The scientists did not expect to observe the planet's fast cooling. This is not the case for gas-giant planets in our solar system, which do transport energy from their day sides to their night sides. Observations show Ups And b also cools off much faster than gas-giant planets in our solar system.

These observations mark the first measurement of an exoplanet that does not transit. And, for the first time, "We're studying the weather on a planet outside our solar system," says Harrington.

Results appear in the online journal Science Express October 12.
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