A Planet that Runs Hot and Cold

Thanks to NASA's infrared Spitzer Space Telescope, astronomers have made the first measurement of the difference between an extrasolar planet's daytime and nighttime temperatures. They found that the innermost of three known planets orbiting the star Upsilon Andromedae is 1,400°C (2,550°F) hotter on its dayside.

"This is a seminal result and will change the way we think about hot Jupiters," says team member Sara Seager (Carnegie Institution of Washington). The group, led by Brad Hansen (University of California, Los Angeles), reports its results in Science for October 27, 2006.

The term "hot Jupiter" stems from the planet's proximity to its host star. It orbits Upsilon Andromedae every 4.6 days at a distance of only 6% the Earth–Sun separation. The searing inferno of the nearby star raises the planet's daytime temperature to 2,000 to 2,500 Kelvins (3,100° to 4,000°F). The large temperature swing means that gas radiates its energy before atmospheric currents can circulate the heat to the nightside. In contrast, Jupiter has nearly the same daytime and nighttime temperature.

Spitzer observed the star over a five-day period and found that its infrared emission (a measure of heat) brightened and dimmed in sync with the planet's orbital motion — a result of the planet showing different hemispheres to Spitzer as it went around the star. The planet is so close to Upsilon Andromedae that it must be tidally locked, meaning it always shows the same hemisphere to the star, just as the Moon shows only one face to Earth. The Spitzer observations do not reveal the actual temperatures, just the difference between the two hemispheres.