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How astronomers mapped the patchy clouds of an alien world

Image: Kepler-7b
Kepler-7b, shown in the artist's conception at left, is the first exoplanet to have its clouds mapped, thanks to data from NASA's Kepler and Spitzer space telescopes. The alien planet is 1.5 times as wide as Jupiter, which is shown at right for comparison.NASA / JPL-Caltech

High clouds in the west, clear skies in the east: That's the weather report for Kepler-7b, a sizzling Jupiter-like world that's quadrillions of miles away.

The scientific world's first map of the clouds on an extrasolar planet has been laid out in a paper accepted for publication in the Astrophysical Journal Letters. It's based on years' worth of readings from NASA's Kepler and Spitzer space telescopes.

"By observing this planet with Spitzer and Kepler for more than three years, we were able to produce a very low-resolution 'map' of this giant, gaseous planet," the paper's lead author, Brice-Olivier Demory of the Massachusetts Institute of Technology, said Monday in a NASA news release. "We wouldn't expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds."

The Kepler probe discovered Kepler-7b three years ago. Further observations from Kepler showed astronomers that there was a bright spot on the planet's western hemisphere. Then they used Spitzer's infrared observations to determine that the bright spot was due to reflected starlight rather than internal heat.

Demory's reference to low resolution should be emphasized: The "map" is little more than a division between the planet's bright and dark sides. It's not even as detailed as the murky illustration released by NASA on Monday. But it does show how data from two telescopes can be combined to learn something important about alien planets.

Image: Kepler-7b brightness
Two brightness maps, produced using different computer models, show the longitudinal brightness distribution for the planet Kepler-7b. The brighter region is thought to be covered by reflective clouds.Demory et al. via MIT

Kepler has been observing distant stars for four years, and although the spacecraft went out of service this summer, there are still reams of readings for scientists to pore through. Those readings track the subtle variations in starlight that occur when a planet passes over the disk of its parent star.

Some variations can represent changes in the brightness of a planet such as Kepler-7b — changes that might be analogous to phases of the moon or of Venus. But Demory and his colleagues had to figure out whether the changes in brightness were caused by the planet's hot glow, or by changes in the reflectivity of the planet's surface.

That's where the Spitzer data came in handy. Spitzer, which was launched 10 years ago, is an infrared telescope that can make precise temperature measurements for celestial objects — including extrasolar planets. Last year, for example, astronomers relied on Spitzer to take the temperature of a world known as 55 Cancri e, 41 light-years from Earth. 

When astronomers checked Spitzer's readings, they estimated that the planet's temperature was 1,500 to 1,800 degrees Fahrenheit (800 to 1,000 degrees Celsius). That's hot — but not hot enough to account for the light levels detected by Kepler. This gave the scientists enough confidence to claim that the differences in brightness were due to starlight being reflected by the clouds that cover a portion of Kepler-7b's surface.

"Kepler-7b reflects much more light than most giant planets we've found, which we attribute to clouds in the upper atmosphere," said Thomas Barclay, Kepler scientist at NASA's Ames Research Center. "Unlike those on Earth, the cloud patterns on this planet do not seem to change much over time — it has a remarkably stable climate."

NASA said the findings represent an early step toward techniques to analyze the atmospheres of planets more like Earth in composition and size.

"With Spitzer and Kepler together, we have a multi-wavelength tool for getting a good look at planets that are trillions of miles away," Paul Hertz, director of NASA's Astrophysics Division in Washington, said in the news release. "We're at a point now in exoplanet science where we are moving beyond just detecting exoplanets, and into the exciting science of understanding them."

Update for 8:40 p.m. ET Sept. 30: The clouds aren't the only notable feature about Kepler-7b: It's one of the lowest-density planets ever found. If the planet could be put in a bathtub, it'd float like a cork. But you're going to need a bigger bathtub: Kepler-7b is about half again as wide as Jupiter, or more than 16 times wider than Earth

Kepler-7b is estimated to lie between 1,000 and 1,400 light-years away in the constellation Lyra, orbiting its parent star at a distance of roughly 5.6 million miles (9 million kilometers, or 0.06 AU). Kepler-7b is much closer to its sun than Mercury is to ours. It's much hotter than Mercury (where temperatures get up to 800 degrees F, or 425 degrees C). It's much zippier, too: A year on Kepler-7b lasts less than five Earth days.

More about alien planets:

In addition to Demory and Barclay, the authors of "Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere" include Julien de Wit, Nikole Lewis, Andras Zsom, Sara Seager, Jonathan Fortney, Heather Knutson, Jean-Michel Desert, Kevin Heng, Nikku Madhusudhan, Michael Gillon, Vivien Parmentier and Nicolas Cowan.

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.