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Superstorm on an alien world

Astronomers have issued the first-ever storm warning for a planet beyond our solar system, with carbon monoxide winds in excess of 3,000 mph.

The weather report is part of a study published in this week's issue of the journal Nature that also includes the first-ever measurement of the same planet's orbital speed, which provides a new way to calculate its mass. The alien gas giant is about two-thirds as massive as Jupiter, and closer to its star than Mercury is to our similarly sized sun.

The planet HD 209458b, also known as Osiris, is already famous because it was the first world beyond our solar system known to have an atmosphere. The fresh findings add to the planet's renown - even though you wouldn't want to live there.

"HD 209458b is definitely not a place for the faint-hearted," the research team leader, Ignas Snellen of Leiden Observatory in the Netherlands, said in a news release from the European Southern Observatory. "By studying the poisonous carbon monoxide gas with great accuracy, we found evidence for a super wind, blowing at a speed of 5,000 to 10,000 kilometers per hour (3,000 to 6,000 mph)."

In a follow-up phone call, Snellen told me that's much faster than the strongest winds recorded in our own solar system - the 1,200 mph winds near Neptune's Great Dark Spot.

On HD 209458b, the super winds are generated by the temperature difference between the planet's day side and its night side. Because the planet circles its star at a distance only one-twentieth of the distance between Earth and the sun, the temperature of the upper atmosphere on HD 209458b's sun-facing side is thought to be as high as 18,000 degrees Fahrenheit (10,000 degrees Celsius) while the dark side is much cooler.

The upper layers of the atmosphere were observed rushing from the hot side to the cold side. Computer models suggest that the super-strong winds drive a complex weather pattern on HD 290458b, Snellen said.

"The winds that are lower in the atmosphere are doing something different," he told me. The models indicate that there should be a circular jetstream, moving from east to west, but Snellen said he and his colleagues could not detect such a pattern directly. Other researchers have reported that the star is blasting the planet so intensely that tons of atmospheric gases are being stripped away every second - and someday the planet may end up having no atmosphere at all.

The magic of spectroscopy
The star HD 209458 is 150 light-years from Earth in the constellation Pegasus, and the giant planet (let's call it "Osiris" from now on) is visible only as a variation in the blip of light from the star. So how were the astronomers able to find out so much about Osiris' winds?

The secret lies in spectroscopy. Astronomers can break down the wavelengths of light from a distant star to determine the chemical composition of the material that is giving off that light, using an instrument known as a spectrograph. Snellen and his team used the CRIRES spectrograph on the ESO's Very Large Telescope in Chile to observe the starlight for five hours, during a time when Osiris was passing in front.

Previous spectral observations have suggested that Osiris' atmosphere contains carbon monoxide and carbon dioxide, as well as methane and even water vapor. Snellen and his colleagues could pick up the chemical fingerprint of the carbon monoxide - but they found that the wavelengths making up that fingerprint were shifted slightly toward the blue end of the light spectrum. Such a shift is caused by the Doppler effect, suggesting that the carbon monoxide molecules were rushing toward them. By measuring the amount of "blueshift," the astronomers could calculate just how fast those molecules were moving. And that's where the 3,000-to-6,000 mph figure came from.

Studying Osiris' mass and atmosphere
A detailed breakdown of the spectral observations also produced an estimate of Osiris' orbital speed. By plugging that figure into Newton's law of gravity, the astronomers found that Osiris is about 64 percent as massive as Jupiter - which agreed with earlier estimates that used other methods. They also found that Osiris' parent star was roughly equal to the mass of our own sun.

"In general, the mass of an exoplanet is determined by measuring the wobble of the star and assuming a mass for the star, according to theory," Ernst de Mooij, a co-author of the study, explained in the ESO's news release. "Here, we have been able to measure the motion of the planet as well, and thus determine both the mass of the star and of the planet."

The astronomers also measured how much carbon was in Osiris' atmosphere. "It seems that HD 209458b is actually as carbon-rich as Jupiter and Saturn," Snellen reported. "This could indicate that it was formed in the same way. In the future, astronomers may be able to use this type of observation to study the atmospheres of Earthlike planets, to determine whether life also exists elsewhere in the universe."

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