JWST Caught Dual Atmospheres on the Same Planet 600 Light-years from Earth

Researchers created 3D climate models to help confirm their discovery.

Six hundred light-years from Earth, a giant planet is giving astronomers something they have never quite seen before: two very different skies on the same world.

The planet coined WASP-94A b is a “hot Jupiter,” a gas giant roughly in the same broad family as Jupiter but orbiting much closer to its star. Close enough, in fact, that the planet is tidally locked. One side always faces the star and roasts in constant daylight. The other side remains turned away in endless night.

That setup already sounds extreme. But new observations from the James Webb Space Telescope (JWST) show something stranger. The planet’s morning side and evening side do not just differ in temperature, they appear to have different atmospheres — or at least atmospheres that look very different when starlight passes through them.

On the cooler morning edge, high mineral clouds seem to block much of the chemical signal below. On the hotter evening edge, the skies appear much clearer, allowing water vapor to show up strongly in the data.

Same planet. Same atmosphere. Two very different readings.

The findings published in Science come from an international team led by Johns Hopkins, who used the JWST to study the planet during a transit, the moment when the planet passes in front of its star from our point of view.

This is one of the best tricks astronomers have for studying planets they cannot see directly. As a planet crosses its star, a tiny bit of starlight filters through the planet’s atmosphere before reaching the telescope. Molecules in that atmosphere absorb certain colors of light. Water vapor leaves one kind of fingerprint. Carbon dioxide leaves another. Methane, sodium and other chemicals leave their own marks.

That technique is called transmission spectroscopy. It has become one of the main ways scientists study exoplanet atmospheres.

Usually, though, the result is treated as one blended signal from a thin ring of atmosphere around the planet. That makes sense when the signal is faint and the planet is hundreds of light-years away. But hot Jupiters are not calm, uniform places. They are blasted by radiation on one side and cooled by darkness on the other. Their winds can race around the planet, carrying heat and clouds from one hemisphere to the next.

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The researchers wanted to know whether JWST could separate those two sides.

It turns out it could.

As WASP-94A b begins its transit, the first part of the atmosphere to cross the star is the morning edge. This is the side rotating out of the planet’s cooler night. As the transit ends, the last part of the atmosphere to leave the star is the evening edge, which has spent hours exposed to the planet’s furnace-like dayside.

The difference between those two edges showed up clearly. The morning side had a flattened spectrum, the kind of muted signal astronomers expect when high clouds block the light from reaching deeper atmospheric layers. The evening side showed a much cleaner spectrum, including a strong signature of water vapor.

JWST’s Near Infrared Imager and Slitless Spectrograph made the measurement possible. The instrument can capture near-infrared wavelengths where water and other molecules leave strong marks. It can also measure very small changes in brightness — the kind of precision needed to split a planet’s atmosphere into morning and evening halves from 600 light-years away.

Clouds part of the story

For years, scientists wondered what blocks the view in many hot Jupiter atmospheres. Some have suspected photochemical haze, a soot-like material created when stellar radiation breaks apart molecules high in the atmosphere. Others have pointed to mineral clouds, made of particles that condense when temperatures drop.

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WASP-94A b seems to favor the second explanation.

The team created 3D climate models, which suggest the clouds form on the cooler night side of the planet. Powerful winds then carry them toward the morning edge. As those clouds move into the hotter dayside, they evaporate. By the time the atmosphere reaches the evening edge, much of the cloud cover has cleared, allowing water vapor to stand out.

The temperature difference driving that cycle is extreme, hovering at approximately 260 degrees Fahrenheit (126 degrees Celsius) between the cooler and hotter sides.

Until now, astronomers have often had to simplify exoplanet atmospheres. They take a blended signal and build a model that treats the atmosphere as more or less uniform. That is not carelessness. For years, the data often left them little choice. Luckily, now the JWST is changing that.

WASP-94A b shows that a single average spectrum can hide major differences. On some planets, the average may not describe the morning side or the evening side very well. It may be a compromise that smooths over the real weather. That does not mean old studies suddenly become useless. It does mean some hot Jupiter atmospheres may need to be reexamined with more complex models, especially when clouds and heat vary sharply across the planet.