Forecast for alien worlds: windy, hot

Astronomers are getting clearer views of exoplanets by measuring the light change during distant eclipses.

Astronomers are getting clearer views of alien worlds. This month, a research team published the first ever "weather" map of an exoplanet (a planet outside our solar system). Another group took the temperature of an exoplanet that glows like a hot coal. And recent data from a planet-hunting satellite has led scientists to think that they can at last detect exoplanets as small as Earth.

Most of the 230-plus exoplanets so far discovered have revealed themselves by the way they tug on their parent star. It's like inferring the presence of an unseen animal by the way bushes wiggle.

Now a new detection technique is coming into its own: When viewed from Earth, 17 exoplanets pass in front of their star. This eclipse dims the star's light, revealing the planet's presence. The way light from the star system changes during the eclipse, as well as when the planet passes behind the star, helps scientists assess the spectrum of light coming from, or absorbed by, the planet itself. That, in turn, allows estimates of the planet's temperature and chemical composition.

That's how Heather Knutson at Harvard University and colleagues got what she calls "our first good look at an alien world." Labeled HD 189733b, it lies 60 light-years away in the constellation Vulpecula. The planet is a little larger and more massive than Jupiter. Two weeks ago, the scientists explained in Nature how they used about a quarter-million data points gathered by NASA's Spitzer infrared-observing satellite to map the distribution of temperature over the planet. Temperatures range from 1,200 degrees F. on the night side to 1,700 degrees F. on the sunny side. Inferring winds from the temperature distribution, they estimate that this planet has powerful jet streams that "may blow as fast as 6,000 miles per hour." Earlier study of the planet had shown no sign of water or other distinctive molecules expected to be in the planet's atmosphere. Something may be hiding them. For now, at least, astronomers have a crude "weather" map.

Meanwhile, Joseph Harrington at the University of Central Florida in Orlando and colleagues described in Nature their study of another exoplanet, one Professor Harrington calls "simply the most bizarre planet." Labeled HD 149026b, the planet is a bit smaller than Saturn. It's 279 light-years away in the constellation Hercules. At a sizzling 3,700 degrees F., it's the hottest planet known. Harrington says it's "like a hot coal in space." It's so hot that no one, including Harrington, knows what to make of it.

Europe's COROT satellite should accelerate this research into transiting exoplanets. Its instruments that observe the transiting planets demonstrate a degree of accuracy that should allow it to detect planets three times smaller than its designers had thought possible. "We can expect great discoveries in the future," says COROT project scientist Malcolm Fridlund.

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