Astronomers Detects Wind Speeds Of 5,400mph On Distant Exoplanet

Artist's impression of HD 189733b. Mark A. Garlick/University of Warwick

A team of scientists from the University of Warwick say they have made the “first weather map” of a planet outside the Solar System.

Called HD 189733b, the exoplanet is located about 63 light-years from Earth, and appears to have wind speeds of 8,690 kilometers (5,400 miles) per hour blowing from its day to its night side – west to east. That’s more than two kilometers (1.2 miles) per second, and is 20 times quicker than the fastest winds on Earth.

“This is the first ever weather map from outside of our Solar System,” said lead researcher Tom Louden in the statement. “Whilst we have previously known of wind on exoplanets, we have never before been able to directly measure and map a weather system.”

The planet is a so-called “hot Jupiter,” with temperatures approaching 1,250°C (2,280°F). It is 10 percent larger than Jupiter and orbits its star incredibly closely in just 2.2 Earth days, at a distance 180 times closer than Jupiter does the Sun. It is thought to be tidally locked, with the same side of the planet always pointed towards the star.

While winds on other exoplanets have been measured before, Louden told IFLScience that this was the first time the wind on each side of an exoplanet had been measured, giving a detailed velocity rather than an average for the whole world.

“It’s more detailed than any we have for a planet outside the Solar System,” he said. “We measured the two sides of the planet independently, and from this we can see the direction it is in [west to east].”

To measure the wind, the team watched as the planet blocked out the light of its parent star using the High Accuracy Radial velocity Planet Searcher (HARPS) at the La Silla observatory in Chile. The star is brighter in the center than at the edges, and by observing the planet at different points in its transit, the team could see how the amount of light blocked by the atmosphere changed – specifically by looking at sodium. 

A diagram of the planet's transit. Louden et al

Using high-resolution spectroscopy – analyzing the light from the planet – the team could see the amount of sodium absorption in the atmosphere. Further analysis as the planet completed its transit showed the atmosphere moving towards or away from Earth, with the Doppler effect – the stretching in frequency of a wave, similar to how a police siren changes pitch as it drives past you – being used to measure the velocity of the planet's winds.

Louden noted that this wind speed is the fastest theoretically possible for the planet, moving at the local speed of sound on the world, which works out to about seven times the speed of sound on our own.

The team thinks this same method of producing a weather map could be applied to other exoplanets, at the moment other hot-Jupiters, but possibly more Earth-like worlds in the future using more powerful telescopes.

The research will be published in The Astrophysical Journal Letters, with a pre-print available on Arxiv.

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