Another exoplanet weather report: 8,500 km/hour winds

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Kepler 438b is an Earth-like extrasolar planet orbiting the star Kepler 438, some 470 lightyears away in Lyra. The planet has an ESI (Earth Similarity Index) of 0.88, the highest currently measured. It's an indication of how similar physically a given world is compared to Earth, and it takes into account the planet's mass, radius, density, escape velocity and surface temperature. Although Kepler 438b is similar physically the Earth, the star Kepler 438 is a flaring red dwarf, highly volatile. It is therefore quite unlikely that the planet harbors any form of life, despite its high ESI.

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Another exoplanet weather report: 8,500 km/hour winds

We're only a couple of decades removed from the first discovery of a planet orbiting a star other than our Sun. But with greater attention and some improved technology, we're increasingly able to learn things about the nature of the planets themselves. In the latest success in this area, researchers have measured the winds in an atmosphere of an exoplanet, and found that our models of exoplanet atmospheres seem to get things right.

The planet in question is called HD 189733b, and it resides a bit over 60 light years from Earth. It belongs to a class of bodies called hot Jupiters, gas giants that reside close to their host stars. In this case, it's close enough to be tidally locked—it rotates so that a single side of the planet is constantly facing its host star.

To image the atmosphere of such a planet, you have to wait for it to pass in front of its host star. Then, some tiny fraction of the star's light will pass through the atmosphere of the planet on its way to Earth. You can then identify the presence of atoms and molecules in the atmosphere when they absorb specific wavelengths of the star's light. If those wavelengths are red- or blue-shifted due to the Doppler effect, then it implies the atmosphere is moving.

In fact, earlier observations of HD 189733b had detected that some molecules in the atmosphere appeared to be moving rapidly. But that didn't directly translate to knowing what was going on with the atmosphere. After all, both the planet and star are rotating. If there's a global wind system, then net Doppler shift is complicated by the fact that the atmosphere may be moving in different relative directions on opposite sides of the planet.

To deal with all of this, the authors imaged the full transit of the exoplanet as it passed between its host star and the Earth. The imaging focused on the sodium vapor that's present in the atmosphere (remember, this is a hot Jupiter). To control for any sodium in the atmosphere of the star, they also imaged both before and after the transit.

As shown in this diagram, when the planet first starts to eclipse part of the starlight, the shift will be dominated by the winds on the leading side of the planet. Then, as the planet ends the transit on the opposite side, any shift will be dominated by the winds on the opposite side of the planet.

In fact, the authors found different shifts on the different sides of the planet. The leading edge showed a red shift of 2.3 kilometers a second. The trailing edge, in contrast, had a blueshift of 5.3 kilometers a second. (Both of these measurements had an error of about a kilometer a second.) Once the rotation of the planet was accounted for, this meant that the planet's winds are moving at a brisk 8,500 km/hour. It takes the form of an eastward equatorial jet, moving heat from the daytime side of the planet to its dark side.

And this is exactly what our models of exoplanet atmospheres suggest should be happening.

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