Half-Hot, Half-Cold Planets Have Supersonic Jet Streams

Anne Minard
for National Geographic News
October 20, 2008
Jupiter-like exoplanets—planets outside our solar system—have supersonic jet streams that transport heat from their sunny side to their dark side, a new study says.

These gas giants orbit extremely close to nearby stars.

"Because these planets are so close to their stars, we think they're tidally locked, with one side permanently in starlight and the other side permanently in darkness," said lead study author Adam Showman, a planetary scientist of the University of Arizona.

"So, if there were no winds, the dayside [the side of a planet in sunlight] would be extremely hot, and the nightside would be extremely cold."

But when the winds pick up, they bring scorching heat—sometimes even on the cool side—that's hotter than anything seen in our own solar system.

The exoplanets "are pretty crazy places. Expect supersonic winds and dayside temperatures hot enough to melt lead and rocks," Showman said in a statement.

(Related: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)

The work was presented recently at the annual meeting of the American Astronomical Society's Division for Planetary Sciences, and will appear in an upcoming issue of the Astrophysical Journal.

Extreme Heat

Showman and colleagues combined Spitzer and Hubble space telescope observations with computer models to puzzle together weather and climate patterns on the gas giants.

About 300 Jupiter-like planets have been discovered around stars, and, in most cases, astronomers know their masses and orbits.

For a handful of the brightest planets, space-based telescopes such as Spitzer have sent back images that reveal surface temperatures.

Those images are revealing "alien" climate regimes, researchers say.

"These planets are 20 times closer to their star than Earth is to the sun, and so they are truly blasted by starlight," Showman said.

Their dayside temperatures reach up to 3,000 degrees Fahrenheit (1,648 degrees Celsius).

Fierce Winds

But one planet is tipping researchers off to a different point of view.

The planet HD 189733b is 63 light-years from Earth in the constellation of Vulpecula.

Its star, HD 189733, is visible with binoculars from Earth, but only the most powerful space telescopes can see the planet.

Here, the nightside temperature exceeds 1,300 degrees Fahrenheit (704 degrees Celsius).

Researchers had proposed that winds carry heat from the dayside to the nightside and warm it up—but were unable to explain how.

Showman and colleagues performed computer simulations that, for the first time, coupled weather activity with a realistic representation for how starlight is absorbed and how heat is lost to space.

The models suggest that to carry the heat the planet must have jet streams with speeds reaching 7,000 miles (11,265 kilometers) per hour.

"You're talking about winds fast enough to carry you in a hot air balloon from San Francisco to New York in 25 minutes," Showman said.

The computers predict that the winds move predominantly from west to east, pushing the heat away from the region that's receiving the most starlight.

"According to the observations, the hottest region on the planet is not 'high noon' but eastward of that by maybe 30 degrees of longitude," Showman said.

"Our simulations are the first to explain why that phenomenon occurs."

Still Learning

Weather studies on exoplanets are constantly evolving, said Alfred Vidal-Madjar, an astrophysicist at the Institut d'Astrophysique de Paris in France who was not involved with the new study.

"Everyone knows [it] is certainly not the final word," he said.

Vidal-Madjar said true tests of the models will come from studies of exoplanet atmospheres, particularly when they pass in front of their parent stars and become backlit relative to Earth- or space-based telescopes.

David Charbonneau, an astrophysicist with the Harvard-Smithsonian Center for Astrophysics, is a co-author on the new paper.

He said it's remarkable "that we are actually able to study the weather patterns on planets orbiting other stars.

"In that sense, they are beginning to feel much more like the planets of [our] solar system, with distinct personalities that we have come to know and love over time."

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