Exoplanet Has Clouds Made Of Rock Crystals

Researchers using NASA’s James Webb Space Telescope have detected evidence for rock crystals or quartz in the high-altitude clouds of WASP-17 b, a hot Jupiter exoplanet 1,300 light-years from Earth. The detection, which was made possible thanks to Webb’s Mid-Infrared Instrument, marks the first time that this mineral common on Earth has been spotted on an exoplanet.

Silicates—minerals rich in silicon and oxygen—make up the bulk of Earth and the Moon, as well as other rocky objects in our solar system. Magnesium-rich silicates like olivine and pyroxene are common in meteorites and asteroids and have been detected in dust clouds across the galaxy and in the atmospheres of exoplanets and brown dwarfs. But so far, pure crystalline SiO₂ or quartz was known only from Earth.

“We were thrilled!” said Dr. David Grant, a researcher at the University of Bristol and first author of the discovery. “We knew from Hubble observations that there must be aerosols—tiny particles making up clouds or haze—in WASP-17 b’s atmosphere, but we didn’t expect them to be made of quartz.”

With a volume more than seven times that of Jupiter and a mass less than one-half Jupiter, WASP-17 b is one of the largest and puffiest known exoplanets. This, along with its short orbital period of just 3.7 Earth-days, makes the planet ideal for transmission spectroscopy : a technique that involves measuring the filtering and scattering effects of a planet’s atmosphere on starlight.

Webb observed the WASP-17 system for nearly 10 hours, collecting more than 1,275 brightness measurements of 5- to 12-micron mid-infrared light as the planet crossed its star. By subtracting the brightness of individual wavelengths of light that reached the telescope when the planet was in front of the star from those of the star on its own, the team was able to calculate the amount of each wavelength blocked by the planet’s atmosphere.

What emerged was an unexpected “bump” at 8.6 microns, a feature that would not be expected if the clouds were made of magnesium silicates or other possible high temperature aerosols like aluminum oxide, but which makes perfect sense if they are made of quartz.

While these crystals are probably similar in shape to the pointy hexagonal prisms found in geodes and gem shops on Earth, each one is only about 10 nanometers across—one-millionth of one centimeter.

Unlike mineral particles found in clouds on Earth, the quartz crystals detected in the clouds of WASP-17 b are not swept up from a rocky surface. Instead, they originate in the atmosphere itself.

“WASP-17 b is extremely hot—around 1,500 degrees Celsius (2,700°F)—and the pressure where they form high in the atmosphere is only about one-thousandth of what we experience on Earth’s surface,” explained Grant. “In these conditions, solid crystals can form directly from gas, without going through a liquid phase first.”

On Earth, with a lower average surface temperature, water vapor can change directly into ice crystals—forming snowflakes and hoar frost— based on the same physical process.

“These beautiful silica crystals tell us about the inventory of different materials and how they all come together to shape the environment of this planet,” explained co-author Hannah Wakeford, also from the University of Bristol.

Understanding what the clouds are made of is crucial for understanding the planet as a whole. WASP-17 b is tidally locked, with one hemisphere constantly facing its host star and displaying a very hot day side and cooler night side, likely fueling a constant stream of quartz clouds around the planet.

On the planet’s night side, with temperatures falling beneath the melting temperature of SiO₂, clouds composed of quartz crystals form. Strong winds, driven by the extreme temperature difference between the two hemispheres, then transport the crystals from the night side to the hotter day side, where they are vaporized again.

The study “JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b” was published in The Astrophysical Journal Letters (2023). Additional material and interviews provided by Laura Betz and Christine Pulliam for nasa.gov.

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