Rock clouds on Hot Jupiter 700 light-years from Earth form in the morning and disappear by evening

Scientists have discovered a planet where rock clouds form in the day and vanish by night. WASP-94A b is a distant Hot Jupiter which was spotted by the James Webb Space Telescope. The weather shifts from one extreme to another within a matter of a few hours from our perspective, something that would typically take days or weeks on Earth. Rock-forming minerals gather early morning to form clouds, start dissipating as the atmosphere heats up and completely vanish by evening. This giant planet orbits extremely close to its star and is located nearly 700 light-years from Earth in the constellation Microscopium. It is too far for regular, clear observations, and so astronomers turned the JWST towards it. The telescope captured the subtle changes in starlight as the planet crossed in front of its star. The findings were published in the journal Science.

JWST noticed that WASP-94A b has a daily cloud cycle, something which is believed to be common among hot Jupiter planets. This is the clearest detection yet of this activity on such a body. The team separated cloudy regions from clearer parts of the atmosphere to get data on the planet’s chemistry.

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"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. The Professor of Earth and Planetary Sciences at Johns Hopkins said that not only is it a clear 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."

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How JWST studied the Hot Jupiter

As the planet passed in front of its star, JWST measured its leading edge at the start of the transit and its trailing edge as it moved away. Both edges represented different times of day on the planet. In the first instance, the air was observed moving from the nightside into the dayside - morning conditions. At the trailing edge, air moves from day to night, representing evening. WASP-94A b’s mornings contain clouds made of magnesium silicate, a rock-forming mineral, but these clouds are not seen during the evening. The team proposed two possible explanations. One is that strong winds may push clouds high into the atmosphere on the cooler side of the planet. They are then driven downward on the hotter dayside, hiding them deep inside the planet before evening.

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Another possibility is that it could be similar to how morning fog vanished from Earth, except that the conditions are a bit more extreme. Clouds may form in the darkness of the nightside and then vaporise as they enter the dayside, with temperatures exceeding 1,000 degrees Celsius. “It was a huge surprise...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," Sing said.

Hot Jupiters are intensely heated and exposed to strong radiation. The team examined eight other hot gas giants and found the same type of cloud cycle on two more planets, WASP-39 b and WASP-17 b.