Feeling the heat of an extrasolar planet

Astronomers have for the first time measured the temperature variation between the lit and unlit sides of a planet outside the solar system—a difference that’s, literally, night and day.

TWO FACED. Sequence (left to right) shows the hot, bright side of the extrasolar planet Upsilon Andromedae b facing Earth and rotating away at a distance of 40 light years. B. Hansen, JPL/NASA

Researchers used NASA’s infrared Spitzer Space Telescope, which measures the heat emitted from distant objects, to study a massive extrasolar planet that lies 40 light-years from Earth. This so-called hot Jupiter, known as Upsilon Andromedae b, orbits its parent star at only about a tenth of the distance that Mercury resides from the sun.

Joe Harrington of the University of Central Florida in Orlando and his colleagues found that the temperature difference between the icy, dark side and the fiery, bright side of the planet is about 1,400°C.

The huge variation comes about, the researchers theorize, because one side of the planet always faces toward its star, while the other side faces away. The same side of the moon likewise always faces Earth. Unlike the moon, however, Upsilon Andromedae b is a giant ball of gas. Harrington and his collaborators describe their study online and in the Oct. 27 Science.

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