Hubble's New Weather Report for Alien World: Sunny and 3,000 Degrees

Astronomers make a detailed temperature map of a world they can’t even see.
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An illustration of changing temperatures on the Jupiter-size exoplanet WASP-43b.


Hubble Space Telescope observations have allowed scientists to create the first detailed map of an exoplanet's atmosphere, showing how temperatures vary across the alien world's perpetually illuminated dayside and how they change with altitude.

And the international astronomy team behind the observations managed to create the map without ever seeing the exoplanet directly.

Known as WASP-43b, the planet is about twice as massive as Jupiter and is about 260 light-years from Earth. (Related: "Explosion in Planet Discoveries Intensifies Search for Life Beyond Earth.")

The new temperature map of the planet, described in the current issue of Science, serves as a curtain-raiser for future NASA telescopes' potential in the search for signs of life in the atmosphere of planets orbiting nearby stars. The international team that worked on the map, led by astronomer Kevin Stevenson of the University of Chicago, also measured water, seen as a necessary ingredient for life, at different layers in the atmosphere of WASP-43b. (Related: "Goldilocks Worlds: Just Right for Life?")

"This result is exciting both because it was technically difficult and because it has turned out to be such a rich source of information about the planet's atmosphere," says Caltech's Heather Knutson, an expert on exoplanet atmospheres who wasn't part of the observation team.

This isn't the first time scientists have taken an exoplanet's temperature, and just a few weeks ago astronomers reported that they'd detected water vapor in the atmosphere of a Neptune-size world. But in order to make the highly detailed temperature map, the team had to monitor the planet continuously through three full orbits of its star.

Fortunately, the planet circles so close to its star that each orbit—WASP-43b's "year"—takes just under 20 hours. Even so, keeping Hubble trained on one spot for that long is difficult for the telescope's managers, because it frequently passes through the South Atlantic Anomaly, a region of high radiation in Earth's Van Allen belts that can damage electronics. "We thought it would be impossible," says Laura Kreidberg, also at the University of Chicago and a co-author of the Science paper. "But [the managers] worked hard with us to make it work."

Roaster Planet

Even the sharp-eyed Hubble couldn't see WASP-43b as an object distinct from the bright star it orbits. But it could see the combined light of planet and star.

The planet is "tidally locked," so the same side always faces the star, just as the same side of the moon always faces Earth. And it's so close to the star that its dayside temperature hovers around 3,000 degrees Fahrenheit (1,649 Celsius), making the planet shine with its own light. The perpetually dark nightside stays at about 1,000 degrees Fahrenheit (538 Celsius).

The planet moves, moreover, in an edge-on orbit that brings it directly between Earth and its star and then takes it directly behind the star. When the planet is hidden by the star, Hubble sees only starlight. At other times, it sees the light of both the star and the planet.

Complicating matters, WASP-43b has phases during which the amount of light glinting off the planet varies. When it shows its dark side to us, just before passing in front of the star, a tiny crescent appears. Just before going behind the star, it shows the dayside of a nearly full disk.

Space Weather

By subtracting the light of the star from Hubble's pictures, an already known quantity, the astronomers could calculate how much light had come from the planet alone.

To go beyond that and create the weather map, they used the waxing and waning of the planet. As Hubble saw more and more longitudinal slices of the dayside atmosphere, the scientists gauged how much additional energy there was. That allowed them to calculate how much heat each band of longitude across the alien world was contributing to the whole.

"This gives us information about the dynamics of the atmosphere," says Stevenson, "and about how heat is being redistributed from the dayside to the nightside."

Not much crosses from the dayside to the nightside, it turns out, which he finds surprising.

Rainbow Connection

The astronomers measured the amount of water on WASP-43b by smearing its light into a spectrum—a rainbow, essentially—and looking for dark lines that marked places where light was being absorbed by water vapor in the atmosphere.

Ultimately, the detection of water on an exoplanet hints that life could be present, but so far water has been discovered only on planets that are too hot and too massive to support biology as we know it. (See "The Hunt for Life Beyond Earth.") In this case, says Kreidberg, the water observations are valuable because they give clues as to how WASP-43b might have formed, which in turn helps us understand the formation of gas-giant planets in our own solar system.

These observations are also valuable, says Caltech's Knutson, "because this is our first taste of the science we might be able to do with the James Webb Space Telescope." When it launches in 2018, this designated successor to Hubble will be able to study the atmosphere of planets much smaller and more Earthlike than even Hubble can see.

"This," says Kreidberg, "is fantastic preparation."

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