Today's exoplanet weather: 1,000°C, glass rain, 8,700 km/h winds

Fancy a weekend break on HD 189733b?

Those readers who enjoy complaining about the weather might like to consider a short weekend break on 'hot Jupiter' exoplanet HD 189733b, where conditions will give them something to really moan about.

Studies of HD 189733b – discovered in 2005 transiting the star HD 189733, which lies some 63 light years from Earth in the constellation Vulpecula – have revealed that the planet has a mass of around 13 per cent greater than Jupiter's, but that it's 180 times closer to its sun than our own gas monster.

That, and the fact that it's tidally locked, mean that temperatures on the permanently sun-facing side hit 1,000°C, and a positively balmy 650°C on the dark side.

That's according to a 2007 observation by NASA's Spitzer Space Telescope, although later work by Swiss astronomers suggested temperatures in the upper atmosphere might reach a scorching 3,000°C.

Whatever the case, it's bloody hot, but the "mild" (as NASA puts it) temperature variation between the planet's lit and unlit sides mean that winds are likely distributing heat around the planet.

NASA speculated in 2007 that such currents "might rage across the surface at up to 9,600 kilometers per hour (6,000 miles per hour)".

As it turns out, the space agency wasn't far off, because scientists from the University of Warwick have now calculated the figure to be 8,700km/h (5,400mph).

Using data from the HARPS (High Accuracy Radial velocity Planet Searcher) in La Silla, Chile, a team led by Tom Louden of the uni's astrophysics group produced an atmospheric velocity map as the planet transited its star.

The technique involved measuring velocities on both sides of HD 189733b "using high resolution spectroscopy of the Sodium absorption featured in its atmosphere".

Louden explained: "As parts of HD 189733b's atmosphere move towards or away from the Earth the Doppler effect changes the wavelength of this feature, which allows the velocity to be measured."

The researchers knocked together a charmingly crude graphic to further explain how they got their results:

Diagram showing the exoplanet at three positions during a transit

They elaborated:

The planet is shown at three positions as it crosses its parent star. The changing background illumination allows us to separate absorption from different parts of the planetary atmosphere.

By measuring the Doppler shift of the absorption we are able to measure wind velocities.

The blue-shaded region of the atmosphere is moving toward the Earth at 12,000mph (19,300km/h), while the red-shaded region is is moving away from the earth at 5,000mph (8,000km/h).

After correcting for the expected spin of the planet we measure a wind velocity of 5,400mph (8,700km/h) on the blue side, indicating a strong eastward wind flow from the heated day side to the night side of the planet.

So, it's pretty breezy, and if that hasn't put you off booking a hotel for a couple of nights, be aware that HD 189733b is a vivid blue, due to silicates in the atmosphere, which, given the temperature, might fall as molten glass rain. Be sure to pack plenty of sunscreen an a reinforced tungsten umbrella.

The Warwick Uni research – Spatially resolved eastward winds and rotation of HD 189733b – will be published by The Astronomical Journal Letters. ®




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