An Exoplanet so Hot it has Clouds Made of Quartz

A recent study published in The Astrophysical Journal Letters used data obtained by the James Webb Space Telescope’s (JWST) Mid-Infrared Instrument (MIRI) to identify the presence of quartz nanocrystals in the upper atmosphere of WASP-17 b, an exoplanet whose mass and radius are approximately 0.78 and 1.87 that of Jupiter, respectively, and is located approximately 1,324 light-years from Earth. WASP-17 b is classified as a “puffy” hot Jupiter due to its 3.7-day orbital period, meaning the extreme temperatures could cause unique chemical processes to occur within its atmosphere, but the astronomers were still surprised by the findings.

“We were thrilled!” said Dr. David Grant, who is a researcher at the University of Bristol in the UK and lead author of the study, which is comprised of more than three dozen co-authors. “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.”

What makes this discovery unique is that exoplanets have traditionally been found to possess magnesium-rich silicates such as pyroxene or olivine, but the discovery of just quartz within an exoplanet’s atmosphere by JWST’s MIRI could provide new insights into the formation and evolution of exoplanet clouds and their respective atmospheres. Additionally, while the shape of these quartz crystals could mimic those found on the Earth, their size is strikingly different at only 10 nanometers in diameter, or one-millionth of a centimeter. For context, the average quartz crystal on Earth is a few centimeters (inches) in diameter, with the largest documented single quartz crystal being 6.1 meters by 1.5 meters by 1.5 meters (20 feet by 4.9 feet by 4.9 feet) and weighing 39,916 kilograms (88,000 pounds). But how can such crystals form in the atmosphere of WASP-17 b?

Spectroscopy data of WASP-17 b obtained by JWST’s Mid-Infrared Instrument (MIRI) between March 12-13, 2023. This data identifies the first evidence for quartz within an exoplanetary atmosphere, which also marks the first time that quartz has been found in an exoplanet, along with being the first time any specific cloud type has been identified in exoplanet using the transit method. (Credit: NASA, ESA, CSA, and R. Crawford (STScI) Science: Nikole Lewis (Cornell University), David Grant (University of Bristol), Hannah Wakeford (University of Bristol))

“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,” said Dr. Grant. “In these conditions, solid crystals can form directly from gas, without going through a liquid phase first.”

Discovered in 2009, WASP-17 b is the first exoplanet found to exhibit a retrograde orbit, meaning it orbits in the opposite direction of its star’s rotation, with a 2013 study identifying water in its atmosphere and another study finding sodium. Due to its mass being less than Jupiter but a volume greater than seven times, WASP-17 b is currently classified as one of the puffiest exoplanets ever found.

Artist illustration of a hot Jupiter’s puffed-up atmosphere. (Credit: NASA, ESA, and G. Bacon (STScI))

While quartz was identified in this most recent study, WASP-17b’s atmospheric composition mirrors traditional gas giant planets that exist both inside and outside our solar system, as it is primarily comprised of hydrogen and helium. WASP-17 b is also tidally locked with its parent star, meaning one side always faces the star. This means as the clouds circulate around the planet, they are vaporized on the dayside. However, astronomers are still trying to determine both the amount of quartz in the atmosphere along with the activity of the clouds.

“The clouds are likely present along the day/night transition (the terminator), which is the region that our observations probe,” said Dr. Grant. “The winds could be moving these tiny glassy particles around at thousands of miles per hour.”

This study was conducted as part of the Webb Guaranteed Time Observation (GTO) program, designated as GTO 1353, along with WASP-17 b being part of a larger investigation known as the Deep Reconnaissance of Exoplanet Atmospheres using Multi-instrument Spectroscopy (DREAMS) conducted by the JWST-Telescope Scientist Team, with the overall goal to conduct in-depth analyses of one exoplanet in each primary class of exoplanets: a temperate terrestrial planet (TRAPPIST-1 e, GTO 1331), a warm Neptune (HAT-P-26 b, GTO 1312), and hot Jupiter (WASP-17 b, GTO 1353).

How will these quartz crystals teach us about the formation and evolution of exoplanetary atmospheres in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!