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Ice crystals in Earth's atmosphere sometimes align just right to create various striking visual effects, from a halo around the moon, to bright spots called sundogs on either side of the sun in a winter sky, or a rainbowed pillar, called a crown flash, above a storm cloud.

Similar phenomena can appear in the skies over some exoplanets of the "hot Jupiter" variety, a common type of gaseous giant that always orbits close to its host star, Cornell astronomers have found. On WASP 17b, a hot Jupiter exoplanet, 10,000 mile per hour winds could align particles in clouds made of quartz and other crystalline mineral aerosols, creating conditions in which polarizing dust could interact with starlight in the same way aligned ice crystals interact with sunlight on Earth.

"Just like the alignment of ice crystals in Earth's atmosphere produces observable phenomena, we can observe the alignment of silicate crystals in hot Jupiter exoplanets," said Elijah Mullens, M.S. '24, doctoral student in astronomy and co-author of the study.

"" was published in The Astrophysical Journal Letters. In the paper, Mullens and co-author Nikole Lewis, associate professor of astronomy in the College of Arts and Sciences, propose that the conditions are right in this exoplanet's atmosphere for the wind to align silicate crystals—a process called mechanical alignment—creating visual effects.

The idea of mechanical alignment was put forward in 1952 by Cornell astronomy professor Tommy Gold to explain what aligns dust in the interstellar medium (ISM), Lewis said. Gold proposed that gas movement aligned dust particles, as if air were blowing on them. His mechanical alignment theory has fallen out of favor for ISM dust particles; researchers now say it's more likely that magnetic fields and radiative torques, where starlight heats one side, make particles align.

"Now we see that the 1952 proposal doesn't work for the interstellar medium, but it probably works for a hot Jupiter exoplanet, a very hot planetary atmosphere with high-speed winds," said Lewis, whose expertise is in atmospheric dynamics of these exoplanets. "When we started looking at planetary atmospheres, in particular these hot Jupiters, it occurred to me that with 10,000 mile per hour winds zipping around in these very dense atmospheres, surely the grains would align."

Lewis and Mullens came up with this idea when they were both on the team that used the James Webb Space Telescope (JWST) to find evidence for quartz nanocrystals in the high-altitude clouds of WASP-17 b, a hot Jupiter exoplanet 1,300 light-years from Earth, reported in 2023.

"We didn't expect to see quartz crystals in a hot Jupiter atmosphere," Lewis said. "We were predicting something completely different."

Quartz crystals are tiny: at 10 nanometers across, 10,000 could fit side-by-side across a human hair. And they have an elongated shape, like boats, said Mullens. In a wind, the crystals behave like a group of boats on a river with a strong current.

"If you put a bunch of these crystals in very strong winds like we expect on hot Jupiters, they're going to align themselves with the wind like boats in a current," he said.

But even if they don't align horizontally with the wind, as proposed in this paper, Mullens said, the crystals are susceptible to being arranged in some way—perhaps vertically, or with electric fields, or even randomly—that create visual effects from interactions with the light from its star.

Researchers can see these effects with JWST, an infrared telescope. They can't take photographs of WASP 17b because it is so far away, Lewis said, but "if we were able to take a picture of WASP 17b at optical wavelengths and resolve the disk of the planet, we would see these types of sundog features."

Both on Earth and an exoplanet, visual effects reveal much about what's going on in the atmosphere, said Mullens.

"Other than being pretty, these effects can teach us about how crystals are interacting in the atmosphere. It's really information-rich, just as on Earth where the atmospheric conditions need to be a certain way for them to be horizontally oriented to produce a sundog," he said. "If we see something similar in a hot Jupiter, we can be able to tell something about how the crystals are interacting with local forces."

Mullens will continue to study particle directionality on WASP 17b very soon; he is principal investigator on a proposal for further observations of the exoplanet, accepted by JWST for the coming year.