Sand clouds form every morning but clear up by nightfall on WASP-94A b, a well-studied gas giant in a constellation located nearly 700 light years away from Earth.

The research, which uses data from the James Webb Space Telescope (JWST), is among the first to detect cloud cycles on a Hot Jupiter exoplanet. By isolating the clouds, researchers can more accurately measure the planet’s atmosphere and provide one of the clearest pictures to date of the planet’s composition — a significant advance in planetary science.

“I’ve been looking at exoplanets for 20 years, and general cloudiness has been a thorn in our side. We’ve known for quite a while that clouds are pervasive on Hot Jupiter planets, which is annoying because it’s like trying to look at the planet through a foggy window,” said co-author and program PI, David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins. “Not only have we been able to clear the view, but we can finally pin down what the clouds are made out of and how they’re condensing and evaporating as they move around the planet.”

The results are published today in the journal Science.

To study WASP-94A b in the Microscopium constellation, Sing and his team of researchers gathered data as the planet passed directly in front of its star. Using the high-powered, space-based JWST, the researchers were able to take separate measurements of WASP-94A b’s leading edge as it started to cross in front of the star and the trailing edge as the planet completed its transit. At the leading edge, the air flows from the night side of the planet to the day side, effectively making it the morning. Air flows from day to night at the trailing edge, making it the evening.

Observations revealed that mornings and evenings on WASP-94A b have extremely different weather patterns: mornings are riddled with clouds made of magnesium silicate, a common mineral found in rocks, while the evening has clear skies.

The researchers think one of two things could be happening. Powerful winds might lift clouds high into the sky on the cooler side of the planet and then plunge downward on the hotter dayside, dragging the clouds deep into the planet’s interior and effectively burying them out of sight before sunset. Alternatively, the phenomenon may be akin to morning fog burning off on Earth, but on an extreme scale. Clouds would form in the darkness of the planet’s nightside. As they drift into the scorching heat of over 1,000 degrees on the day side, the chemicals that make up the clouds boil away, and the clouds simply vaporize.

“It was a huge surprise. People have expected some differences, like its cooler in the morning than the evening—that’s something natural that we experience here on Earth,” Sing said. “But what we saw was a real dichotomy between the weather on both sides of the planet, and huge differences in cloud coverage, and that changes our whole picture of the planet.”

Because the evenings are clear of clouds, the researchers could look to the trailing edge specifically to see what the atmosphere of the planet looked like—something the Hubble telescope could not provide.

Artistic representation of WASP-94A b, a gas giant in the Microscopium constellation. Clouds build as air flows over the dark side of the planet, reaching a large swell by daybreak. The clouds dissipate on the dayside, leaving clear skies in the early evening.  Credit:Hannah Robbins/Johns Hopkins University

“With the Hubble telescope, when we used to do this type of observation, we got an average view of the whole planet with data from the clouds and the atmosphere squished together and indistinguishable,” said first author Sagnick Mukherjee, a postdoctoral fellow at Arizona State University who was a student at Johns Hopkins and UC Santa Cruz at the time of the research. “This approach with the JWST lets us localize our observations, which helped us see the cloud cycle.”

When the researchers looked at the clear evening sky, they found that WASP-94A b was much more like Jupiter than they thought. Previously, when the clouds were averaged in, the data suggested the planet was made of hundreds of times more oxygen and carbon than Jupiter—a finding that baffled researchers given it couldn’t be explained by planet formation theory. The new data, however, shows WASP-94A b has only five times the amount of oxygen and carbon.

Hot Jupiter planets orbit much closer to their stars—closer even than Mercury to the sun—and therefore are much hotter and are exposed to more radiation. Because of their extreme environments, these planets also make good laboratories to study the chemistry and physics of cloud dynamics.

Using WASP-94 Ab as a benchmark, the team looked at eight other hot gas giants and discovered the same distinctive cloud cycle on two other worlds: WASP-39 b and WASP-17 b. Next, Sing and his team will be using data from a new large JWST program to study cloud cycling across a wide variety of exoplanets, including an eccentric gas giant planet in the habitable zone.

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