Extreme Winds Rule Exoplanet's Weather
They're part of a team that mapped weather on a planet beyond our solar system, a gas giant planet called HD 189733b.
"The exoplanet's wind speeds probably exceed the speed of sound," said Adam Showman of The University of Arizona Lunar and Planetary Laboratory. "And the speed of sound on these planets is 10 times faster than on Earth, so that's saying something." The speed of sound in HD 189733b's atmosphere is about 3 kilometers per second, or 6,700 mph.
Showman and Lunar and Planetary Laboratory research associate Curtis Cooper analyzed Spitzer data on planet HD 189733b using the numerical models they've been developing for exoplanet atmospheres. The planet, which is in constellation Vulpecula, is the closest known "transiting" planet. A transiting planet is seen to cross in front and behind its star when viewed from Earth. The planet is "tidally locked" to its star, so that one side always faces the star and the other side is always dark, just as the moon is tidally locked to the Earth.
A team led by Heather Knutson of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., used the Spitzer Space Telescope to measure the infrared light, or heat, as the planet orbited its sun-like star. The result is one of the first-ever temperature maps for an exoplanet. The map shows that dayside and nightside temperatures differ only by about 500 degrees Fahrenheit, ranging from 1,200 F on the nightside to 1,700 F on the dayside.
"At these high temperatures, air cools off rapidly when it moves from the dayside to the nightside," Showman said. "That relatively small temperature difference implies that fierce winds redistribute a lot of the heat."
"We need to do more detailed modeling to calculate actual wind speeds. At this stage, the numbers are all quite uncertain," he said. "However, we can be certain the speeds are fast, probably a couple of kilometers per second," or about 4,500 mph. The supersonic exoplanet winds might be as great as 10 kilometers per second, or about 22,000 mph, the UA researchers calculate.
"This isn't just the case where you need winds, but winds that are fast enough to move air from one side of the gas giant planet to the other before it has time to cool off," Showman said.