Webb telescope discovers a distant planet that isn't quite round

Astronomers discovered that WASP-17b has an uneven atmosphere, with extreme temperature differences between its morning and evening sides.

Planets appear as perfectly round disks when they cross in front of their stars, and astronomers have long measured them that way. But a new analysis of the giant exoplanet WASP-17b shows that its atmosphere doesn't follow a perfect circle. One side bulges farther into space than the other.

On the side of the planet where day turns to evening, the atmosphere extends about 1.8 times as far out as on the morning side. It also runs roughly 1,200°F (660°C) hotter. The measurement is the first to follow an exoplanet atmosphere from east to west and also from its equator up to its poles.

The team used the James Webb Space Telescope. Lili Alderson, a postdoctoral fellow in the Department of Astronomy at Cornell University, led the work. Hannah Wakeford and David Grant of the University of Bristol, who developed the method, worked with her.

"This is proof of concept that planets are in fact 3D objects, and that consistently, as they go around their star, what we predict should be happening is happening," Alderson said. "To move beyond computational exercises and see it for real is a different matter."

Webb maps a planet's atmospheric edge

WASP-17b is a gas giant more than 1,000 light-years away, nearly twice as wide as Jupiter with half its mass. It circles its star every 3.7 days, close enough that its atmosphere runs at about 2,700°F (1,500°C). Its atmosphere is unusually puffed up, so starlight has a lot of gas to pass through.

When a planet crosses its star, a little starlight filters through the thin ring of atmosphere around its edge. Each gas absorbs particular colors, so the dip in starlight at each wavelength tells astronomers what that ring holds. Astronomers usually average the whole ring as one circle.

The team instead fitted the planet's outline as a shape free to differ from a circle, using software called Harmonica that Grant and Wakeford wrote. Where the atmosphere is taller, the dark outline reaches farther from the planet's center.

Webb watched WASP-17b cross its star in March 2023 with three instruments, covering light from the near-visible to the mid-infrared. Fitting a single wavelength band took about two weeks on 40 computer cores, and there were 24 bands. The result was a separate spectrum for each angle around the edge.

The evening side runs hotter

On a planet like this one, the same side always faces the star. Winds race along the equator and carry heat from the day side toward the night side. So the edge where day turns to evening is warmer than the edge where night turns to morning.

The morning side came out at about 1,100°F (590°C) and the evening side at about 2,280°F (1,250°C). Hotter gas spreads out more, so the evening side's atmosphere reaches higher. Around the edge, the planet's apparent size differs by about 0.9% to 1.6% of its radius, roughly 700 to 1,300 miles (1,200 to 2,100 kilometers).

The morning-to-evening gap is larger than the gaps found on three other hot planets in earlier studies. Those ranged from about 180°F to 500°F (100°C to 280°C), and all three planets are cooler than WASP-17b.

In an interview with Earth.com, Wakeford said the map changes what the data can show. "This allows us to actually track the local weather around the planet," she said. For the first time, the team can follow how temperature changes across a planet's poles.

Water and carbon dioxide remain uniform

Temperature isn't the only thing the map can read. Water vapor and carbon dioxide both absorb strongly in Webb's range, and the team measured how much of each was present at every angle.

Both gases turned out to be spread evenly around the edge. Computer models had predicted that. The amounts were easier to measure on the hot evening side, where the taller atmosphere makes each gas's signal larger, than on the cooler morning side.

"It was also nice to confirm that important molecules like water and carbon dioxide are pretty consistent all the way around the limb," Alderson said.

Clouds were harder. In one set of models, the best fits had clouds on the morning side and none from the poles around to evening. A second set couldn't pin down any cloud properties.

The planet's temperature range spans the point where silicate clouds are expected to give way to salt clouds. So different kinds of clouds could exist on different parts of the planet.

WASP-17b behaved as predicted

Asked what surprised her most, Wakeford named the lack of surprises. "We were surprised by how well behaved the planet seems to be," she told Earth.com. The measurements matched the team's predictions, she said, and earlier measurements made with other techniques.

A well-behaved atmosphere wasn't a given. WASP-17b orbits its star backwards, Wakeford said, which means "something rather catastrophic happened in its past to knock it out of alignment in its system." Even so, its atmosphere moves as the models say it should.

She said no single view of the planet is enough. A day-side map, the glow of the night side, and the full day-side spectrum all add to the new map. The whole set was then compared with a wide range of models.

Future Webb observations will examine clouds

The team assumed the planet's northern and southern halves mirror each other, which may not hold for other planets. The mid-infrared bands are broad enough that they may be hiding changes around the edge. A hint that methane increases toward the poles, as some models predict, isn't strong enough to count yet.

A new round of Webb observations in the mid-infrared, already approved, could sharpen the view of clouds. The team has run a simpler version of the technique on a second hot Jupiter, HD 209458b, and is now looking at how WASP-17b changes with angle in more detail. Wakeford said she hopes that, on other planets, the method will eventually track individual storms.

For now, the uneven outline of WASP-17b shows how much astronomers can already learn about the weather on a planet more than 1,000 light-years away.

The full study was published in the journal Nature Astronomy.

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