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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