Supersonic winds observed on exoplanet

Posted by Emma Woollacott

Astronomers have for the first time observed a massive storm on a planet outside our solar system.

The 'hot Jupiter' exoplanet HD209458b doesn't have the balmiest of climates. Observations show that carbon monoxide gas is streaming at supersonic speeds from the extremely hot day side to the cooler night side of the planet.

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

HD209458b is around 60 percent of the mass of Jupiter, and orbits a sun-like star 150 light-years from Earth towards the constellation of Pegasus.

Circling at a distance of only one twentieth the Sun-Earth distance, the planet keeps the same face to its star at all times. It thus has a surface temperature of about 1,000 degrees Celsius on the hot side - but 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," says team member Simon Albrecht.

HD209458b was the first exoplanet to be found transiting: every three and a half days, the planet moves in front of its host star.

The teram used ESO's Very Large Telescope and its powerful CRIRES spectrograph to observe the planet for about five hours as it passed in front of its star.

"CRIRES is the only instrument in the world that can deliver spectra that are sharp enough to determine the position of the carbon monoxide lines at a precision of 1 part in 100,000," says team member Remco de Kok. "This high precision allows us to measure the velocity of the carbon monoxide gas for the first time, using the Doppler effect."

The astronomers achieved several other firsts. They directly measured the velocity of the exoplanet as it orbits its home star. "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. 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," says co-author Ernst de Mooij.

Also for the first time, the astronomers 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," says 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."

The full paper is here.