Planet’s perfect storm

Artist’s impression of H209458b and its Sun-like star

Astronomers have measured a superstorm in the atmosphere of a distant planet. This artist’s impression shows the Jupiter-like planet and its Sun-like host star.

  • HD209458b is a planet 150 light-years from Earth
  • 60% the mass of Jupiter, with similar carbon content
  • Fierce winds detected, driven by high temperatures

Astronomers studying a planet 150 light-years from Earth have detected a huge, heat-driven storm raging in its atmosphere.

HD209458b is about 60% as massive as Jupiter, orbiting a Sun-like star at a distance of only 1/20th that of the Sun to the Earth.

The planet always keeps one side facing its star, making that side very hot, while the other is much cooler. In fact, the intense heat from the star makes the planet’s dayside around 1,000 degrees Celsius.

“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 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 10,000 km per hour,” says Ignas Snellen, who led the team of astronomers.

High-precision observations

HD209458b was the first exoplanet (ie. one that orbits a star other than our Sun) to be found “transiting”. Every 3.5 days the planet moves in front of its star, blocking a small portion of the starlight during a three-hour period.

During these events, a tiny fraction of the starlight filters through the planet’s atmosphere, leaving an imprint.


Enclosures of the four main VLT telescopes

A team of astronomers from the Leiden University, the Netherlands Institute for Space Research (SRON), and MIT in the United States, used the European Southern Observatorys’ (ESO) Very Large Telescope and its powerful CRIRES spectrograph to detect and analyse these faint fingerprints, studying the planet for about five hours as it passed in front of its star.

CRIRES enabled the astronomers to measure the spectrum of carbon monoxide gas in the planet’s atmosphere to high precision. “This high precision allows us to measure the velocity of the carbon monoxide gas for the first time using the Doppler effect,” says team member Remco de Kok.

Similar to Jupiter and Saturn

The astronomers achieved several other firsts, including directly measuring the velocity of the planet as it orbits its 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 have 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.”

Adapted from information issued by ESO / L. Calçada.

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