'Superstorm' rages on exoplanet

Astronomers have measured high-speed winds in the atmosphere of a planet orbiting a distant star.

Data on carbon monoxide gas in the atmosphere show that it is streaming at fierce speeds from the planet's hot day side to its cool night side.

Writing in Nature, a team detected longitudinal winds of roughly 2km/s (7,000km/h) in the atmosphere of a "hot Jupiter" planet.

Hot Jupiters are gas giants that orbit very close to their parent stars.

The planet HD209458b orbits a star in the constellation Pegasus, some 150 light-years away. It circles this star at around one-eighth the distance Mercury orbits the Sun.

This means the exoplanet is heated intensely by its parent star, and has a surface temperature of about 1,000C on its hot side.

But as the planet always has the same side to its star, one side is searing, whereas the other is much cooler.

"On Earth, big temperature differences inevitably lead to fierce winds, and as our new measurements reveal, the situation is no different on HD209458b," said co-author Simon Albrecht, from the Massachusetts Institute of Technology (MIT) in Cambridge, US.

Ignas Snellen, who led the team of astronomers, commented: "HD209458b is definitely not a place for the faint-hearted.

The researcher, based at Leiden University in the Netherlands, added: "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 10,000km per hour."

The researchers used the European Southern Observatory's Very Large Telescope (VLT) and its powerful CRIRES spectrograph to detect and analyse the tiny fraction of starlight that filtered through the planet's atmosphere as it passed in front of its star.

The chemicals in the atmosphere, such as carbon monoxide, produce distinctive fingerprints, known as absorption lines, in this spectrum of stellar light.

Team member Remco de Kok, from the Netherlands Institute for Space Research (SRON), said CRIRES was uniquely equipped to deliver precise measurements of the carbon monoxide lines.

"This high precision allows us to measure the velocity of the carbon monoxide gas for the first time using the Doppler effect."

The Doppler effect, or Doppler shift, is the change in the wavelength of the planet's light spectrum caused by its motion towards or away from Earth.

The authors were able to use the Doppler shift to obtain a direct measurement of the exoplanet's mass.

The effect was used to calculate the velocity of the exoplanet as it orbited its home star - its orbital velocity.

Once the orbital velocity of HD209458b was known, the masses of both the star and planet could be calculated using Newton's law of gravity.

Mercedes Lopez-Morales, from the Carnegie Institution of Washington, US, who was not connected with the study, said: "Measuring spectral lines from molecules in the atmosphere of an exoplanet and using them to derive the planet's mass directly is a remarkable achievement.

"Equally striking is the fact that the observations have been made using ground-based telescope facilities. Until a few months ago, such challenging precision measurements were thought to be possible only from space-based platforms."

The astronomers also measured how much carbon is present in the atmosphere of this planet.

"It seems that H209458b is actually as carbon-rich as Jupiter and Saturn. This could indicate that it was formed in the same way," said Dr Snellen.

"In the future, astronomers may be able to use this type of observation to study the atmospheres of Earth-like planets, to determine whether life also exists elsewhere in the Universe."

Paul.Rincon-INTERNET@bbc.co.uk