Extra-Stormy Weather: Exoplanet Atmosphere Roils with Superspeed Winds

A new look at a well-observed extrasolar planet reveals winds whipping through its upper atmosphere at 7,000 kilometers per hour
Extrasolar planet with its host star

ESO (L.Calcada)

A long-studied planet orbiting a star 150 light-years away has been given a new look, thanks to a novel method of studying extrasolar planets from Earth.

The planet, which goes by the unmemorable name of HD 209458 b, became in 1999 the first world spotted as it passed in front of its host star, an event known as a transit that reveals the fortuitously aligned planet's presence through the slight dimming of the star. Even though astronomers cannot see a transiting planet directly—its presence is inferred by shifts in the host star's apparent brightness and confirmed by other effects—they can track the spectrum of starlight through the object's orbit to isolate contributions from the planet.

In the more than 10 years since the discovery of HD 209458 b researchers have identified several molecules in its atmosphere, including water vapor, methane and carbon dioxide. Now a study in the June 24 Nature, based on high-resolution measurements of carbon monoxide in the planet's upper atmosphere, is providing a new look at the planet, revealing that winds tear through the atmosphere at several thousand kilometers per hour. (Scientific American is part of Nature Publishing Group.)

As the gas giant HD 209458 b passes in front of its host star (known as HD 209458), the planet blots out a small fraction of the star's light, and an even smaller fraction illuminates the planet's atmosphere. The study's authors used a high-resolution spectrograph on the Very Large Telescope (VLT) in Chile to identify the spectral signature of carbon monoxide in the atmosphere through the molecule's characteristic absorption of starlight.

With the resolution of the VLT's spectrograph, the researchers were even able to detect a Doppler shift in the carbon monoxide signal as the planet proceeded through its transit, a process during which it drew slightly closer to Earth before retreating once more behind its host star. The Doppler shift allowed the team to estimate the planet's orbital velocity of 140 kilometers per second—an independent verification of similar figures attained by other means. Combined with measurements of the parent star's orbital motion in response to the gravitational tug of its companion planet, the researchers were also able to derive from Newton's law of gravitation a mass estimate for both planet and star. HD 209458 b, they found, is 64 percent the mass of Jupiter, and the star HD 209458 is nearly identical in mass to the sun; both figures mesh well with preexisting estimates.

Astronomers have sought such fine-grained spectra since the first planet orbiting a sunlike star was discovered in 1995, says lead study author Ignas Snellen, an astronomer at Leiden University in the Netherlands. (That planet, 51 Pegasi b, does not transit; its presence was inferred by gravitational effects.) Snellen says the difference in his team's research was the superior resolving power of the VLT spectrograph, which came on line in 2006.

Beyond independently confirming the parameters of the HD 209458 system, the spectra provided a tantalizing glimpse into the dynamics of the giant planet's atmosphere. As the planet marched steadily along its orbital path the carbon monoxide appeared to be on the move as well. "We see this clear change in velocity" of HD 209458 b, Snellen says. "There's also an offset—the gas during the transit seems to be moving toward us."

The carbon monoxide appears to be flowing at two kilometers per second, or roughly 7,000 kilometers per hour, driven by temperature differences between the planet's hemispheres. "Very very high in the atmosphere there is a kind of superwind blowing from the dayside to nightside," Snellen says. The two sides of HD 209458 b most likely have very different temperatures, because the planet experiences robust irradiation due to its close proximity to the host star—roughly one twentieth the distance between Earth and the sun—and because the planet appears to be tidally locked, exposing the same hemisphere to its star at all times. (Tidal locking is the same mechanism that ensures that only one side of the moon is ever visible from Earth.)

Theoretical models predicted winds of that magnitude for HD 209458 b, but observational glimpses of such atmospheric action are exciting nonetheless, says astronomer Mark Swain of the NASA Jet Propulsion Laboratory in Pasadena, Calif. "The preliminary detection of this zonal wind is really special," he says, adding that more work is needed to boost the signal-to-noise level on the finding.

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