Astronomers hunt for 'exoplanets'

This could be a banner year in the search for alien planets. Earlier this month, a new European satellite began its mission to spot planets as they orbit in front of their stars. Meanwhile, a research team has demonstrated the power of that technique by using Hubble Space Telescope data to trace activity in the outer atmosphere of an alien world. It's the first time such meteorological details have been gathered from a planet in another star system.

Astronomers have found 209 "exoplanets," the name they give to alien worlds. More could be reported at any time. Observers have found most of them by studying how a planet's gravitational tugging affects the movement of its parent star. They have caught only a handful of planets transiting stars. Europe's COROT satellite is designed to turn reliance on luck into a systematic search.

Astronomers can learn a lot about a star and a planet by watching changes in a star's brightness. COROT can detect these in minute detail. Its name stands for Convection, Rotation, and Transit. The convection and rotation part refers to the ripples so-called "star quakes" send across a star's surface, changing its brightness. Astronomers can determine a star's mass and chemical composition from such data. They call this asteroseismology.

The transit part of the name refers to the rich data harvest the satellite can gather when a planet passes in front of a star. The brightness change reveals the size and presence of the planet, the way starlight passing through the planet's atmosphere reveals its composition and weather. Gilda Ballester and colleagues at the University of Arizona in Tucson have used this technique to detect what Dr. Ballester calls "the details of how a planet loses its atmosphere."

The first planet detected transiting its star is known only by a number: HD 209458b. It's located 150 light-years from Earth in the constellation Pegasus. The Hubble telescope has accumulated a lot of data on this planet. Ballester's team has been mining the Hubble archive from 2003. The planet is what astronomers call a "hot Jupiter," meaning it's a lot like our Jupiter but is 20 times closer to its star. Analysis of Hubble data has shown that the planet's atmosphere contains oxygen, carbon, sodium, and a lot of hydrogen in its outer layer, which was ejecting a gaseous tail.

On Feb. 1, the team reported further analysis of that outer layer in the journal Nature. Ballester says it's "a transition zone where the temperature skyrockets from about 1,340 degrees Fahrenheit to about 25,540 degrees."

That's way hotter than our sun. The hydrogen is boiling off that planet's atmosphere at 10,000 tons a second. Fortunately, the planet is large enough to last another 5 billion years or more even at that loss rate.

About 10 to 15 percent of known exoplanets are hot Jupiters. Ballester says that what they are learning from the Hubble data "could yield insights into the atmospheres of other hot Jupiters."

COROT should provide a flood of relevant new data. Its observations also can detect small exoplanets, including rocky worlds only a few times larger than Earth. It should monitor around 120,000 stars during its 2-1/2-year mission.

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