3. Planetpalooza

A new crop of alien planets raises the bar on bizarre.

By Stephen Ornes|Wednesday, December 12, 2007

Its name doesn’t exactly roll off the tongue, but HD 189733b is the planet of the year. A gas giant orbiting a yellow dwarf star roughly 63 light-years away, HD 189733b is the first exoplanet—short for extrasolar planet—for which astronomers have been able to produce a weather map. That map was created in May by a team led by Heather Knutson at Harvard University.

Culled from infrared spectrographic data captured by NASA’s Spitzer Space Telescope, the weather map shows that the atmosphere of HD 189733b is riven with supersonic winds and has highs of around 1,700 degrees Fahrenheit. The planet appears to be too hot and violent to support anything like life as we know it, but now that astronomers know how to study the atmosphere of one exoplanet, they are ready to try extending the technique to other, potentially more inviting worlds. “This is not something we thought we’d be able to do for 10 or 20 years,” Knutson says. “It’s exciting to see if we can do it for smaller planets.”

HD 189733b was in the spotlight for another reason as well. Two months after Knutson’s team published the weather map, a group of European, Asian, and American astronomers, also using Spitzer data, made an equally pioneering observation: The atmosphere of that blistering, blustery giant contains the telltale spectrographic signature of water vapor—the first convincing evidence of extrasolar H2O. Although the atmosphere of this type of planet was expected to have water, the discovery showcased astronomers’ new ability to detect it, a necessary step in finding extraterrestrial life.

“Water is one of the most ubiquitous molecules there is,” says Greg Laughlin, an astronomer at the University of California at Santa Cruz who develops theoretical models of extrasolar planets. “It would have been incomprehensible if it hadn’t been there. What’s so exciting about those spectra is the fact that we can actually measure atmospheric properties around other stars.”

Astronomers light up when they talk about HD 189733b. It’s one of the closest known “transiting exoplanets,” one that passes directly across the line of sight between its host star and Earth. During the transit, it partially blocks light from the star. The starlight that streams around the planet as it passes conveys valuable information—like the planet’s size and the chemical makeup of its atmosphere—to the telescopes and spectrometers of Earth-bound stargazers.

HD 189733b wasn’t the only exoplanet to make the news in 2007: A fiery, fluffy behemoth called TrES-4 became the new largest known world in the universe. TrES-4 and other oddball planets among the 250 or so discovered in the past 12 years are challenging, even revolutionizing, astronomers’ assumptions about how planets and planetary systems form. “We weren’t imaginative enough,” says David Charbonneau, a Harvard astronomer and DISCOVER’s 2007 Scientist of the Year. “The galaxy was much more imaginative about making planets than we were.”

This year’s findings regarding HD 189733b, the weather and the water, suggest that the hunt for extrasolar Earth-like planets—and thus for life outside our solar system—has turned a corner. Technology is finally catching up to wishful thinking, and astronomers now need to find a nice rocky planet to test it out.

In April, they nearly had their chance. An exoplanet introduced that month, Gliese 581c, surprised astronomers not by its weirdness but by its familiarity. Unlike the Jupiter-size HD 189733b, Gliese 581c is only slightly larger than Earth. Its size suggests that, also like Earth, it is a rocky planet. It orbits its star in the so-called Goldilocks zone, a swath of space not too hot and not too cold, where an Earth-like planet would receive a similar measure of energy from it.

Despite the early hype, however, Gliese 581c is no “alien Earth”: It is probably more like Venus, with a runaway greenhouse effect. The planet is not believed to transit its star, precluding astronomers’ mapping its weather anytime soon. Gliese 581c nevertheless marks a milestone in the quest to find life in the cosmos, Laughlin says, and other candidates for the alien Earth can’t be far in the future.

“It was exactly what the field has been pointing toward, and as time goes on, we’re finding lower and lower mass planets,” Laughlin says. “In the next year or two we will have a planet that you can point to with a straight face and say that it is potentially habitable.”

Planet hunters are scrambling to be the first to find it. Charbonneau and his team are building a network of rocky planet–seeking telescopes in Arizona; Laughlin is part of a group constructing robotic stargazers in California. Earlier this year, the European Space Agency’s COROT satellite found its first exoplanet, and in 2009, NASA’s $550 million Kepler mission, the most sensitive planet seeker to date, is scheduled to blast off.

According to Geoff Marcy, whose team of planet hunters used the Doppler method to find 28 exoplanets in the past year alone, there could be hundreds of millions of planets to find. “Our survey shows that 10 percent of [stars in the Milky Way] have planetary systems, but that is an underestimate,” Marcy says. “If we extrapolate down to Earth-size planets, there could be something like 100 billion planetary systems.”

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