Since their discovery, astronomers had no idea whether, like Jupiter, the temperatures on hot Jupiters (planets outside the Solar System, large and gaseous like Jupiter, but which orbit much closer to their stars) are constant, or if temperature variation between night and day is high.

All three studied planets orbit within about five million miles (eight million kilometers) off their stars, much closer than Mercury is to our sun.

To their surprise, they discovered that heated winds on the planets blow at supersonic speeds - at a speed of 9,000 miles (14,500 kilometers) per hour - that keep a constant temperature even the night sides that never face their parent stars.

The team at the University of California, Los Angeles found that on a nearby hot Jupiter called Upsilon Andromeda b, one half was as hot as molten lava, while the other was chilled, possibly below freezing. The scientists supposed that the sunlit side was tidally locked to its stellar parent, like the moon is with Earth, so that only one planet side faced the star. Another assumption was that the planet was rapidly losing heat from its star into space, before the planetary winds could circulate it to the night side.

But a different situation can occur, as well. Using NASA's Spitzer Space Telescope, astronomers assessed the thermal brightness of the planets when their day sides faced Earth, when their night sides faced Earth and at various phases in between.

The lack of variation in the infrared brightness suggested slight temperature differences between the planets' sunned and dark sides. The planets revealed a uniform temperature of about 1,700 degrees Fahrenheit (925 degrees Celsius) and they are slightly cooler than they would have been without the circulating winds. "The cooling occurs via thermal infrared radiation, which is why the planet has a more uniform brightness", said study team member Eric Agol of the University of Washington. "The heat absorbed on the dayside is carried by fast winds, partly releasing the heat on the day side and partly on the night side."

The three planets, named 51 Pegasi, HD179949b and HD209458b, are situated at about 50, 100 and 147 light-years away from Earth, respectively. Because of their close proximity to their stars, the planets may be tidally locked to their suns. 51 Pegasi is the first extra-Solar System planet ever found, in 1995.

By now, the number of known extrasolar planets has surpassed 200, most of them being hot Jupiters. Smaller planets are difficult to detect using the current technology. "Variations in their atmospheric chemical composition of the planets or dust content could change how fast they absorb and reradiate heat, and thus whether the winds can carry heat to the night sides", said Agol.

"There has been some speculation that life might be able to exist on the cooler night-side of hot extrasolar rocky planets, but if those planets had similar weather patterns in their atmospheres as these hot Jupiters, that may not be the case," Agol said.
Image credit: NASA