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Hubble takes the temperature of an extreme exoplanet

Jenny Winder, News Writer
Oct 12, 2014, 17:06 UTC

Sen—The most detailed global map yet of the glow from a turbulent planet outside our Solar System has revealed its air temperatures and water vapour.

Astronomers have mapped the temperatures at different layers of the planet's atmosphere and traced the amount and distribution of water vapour. 

The Hubble Space Telescope's observations show the exoplanet, called WASP-43b, is a world of extremes, where seething winds rage at the speed of sound from a 3,000-degree-Fahrenheit “day” side, hot enough to melt steel, to a pitch-black “night” side with plunging temperatures below 1,000 degrees Fahrenheit.

First discovered in 2011, WASP-43b is located 260 light-years away. The planet is too distant to be photographed, but because its orbit is observed edge-on to Earth, astronomers detected it using the 'transit' method—when regular dips in the light of its parent star are observed as the planet passes in front of it.

“Our observations are the first of their kind in terms of providing a two-dimensional map on the longitude and altitude of the planet’s thermal structure that can be used to constrain atmospheric circulation and dynamical models for hot exoplanets,” said team member Kevin Stevenson of the University of Chicago.

A ball of mostly hydrogen gas, there are no surface features on the planet that can be used to track its rotation. Only the severe temperature difference between the day and night sides can be used by a remote observer to mark the passage of a day on this world.

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Temperature map of exoplanet WASP 43b. The white-colored region on the daytime side is 2,800 degrees Fahrenheit. The nighttime side temperatures drop to under 1,000 degrees Fahrenheit. Image credit: NASA/ESA

WASP-43b is about the same size as Jupiter, but is nearly twice as dense. The planet is so close to its orange dwarf host star that it completes an orbit in just 19 hours. It is also gravitationally locked so that it keeps one hemisphere facing the star, just as our Moon keeps one face toward Earth.

Scientists combined two previous methods of analyzing exoplanets in an unprecedented technique to study the atmosphere of WASP-43b. They used spectroscopy, dividing the planet’s light into its component colours, to determine the amount of water and the temperatures of the atmosphere. By observing the planet’s rotation, the astronomers also were able to precisely measure how the water is distributed at different longitudes.

“The planet is so hot that all the water in its atmosphere is vaporized, rather than condensed into icy clouds like on Jupiter,” said team member Laura Kreidberg of the University of Chicago.

Water that has precipitated out of the upper atmospheres of cool gas giant planets like Jupiter is locked away as ice. But on “hot Jupiters,” water is in a vapour that can be readily traced.

In order to understand how giant planets form astronomers want to know how enriched they are in different elements. The team found that WASP-43b has about the same amount of water as we would expect for an object with the same chemical composition as our Sun, shedding light on the fundamentals about how the planet formed. The team next aims to make water-abundance measurements for different planets.