A transmission spectrum of WASP-17b — a hot-Jupiter exopanet around 1,300 light-years away — captured by the Mid-Infrared Instrument (MIRI) on the NASA/ESA/CSA James Webb Space Telescope reveals the first evidence for quartz (crystalline silica, SiO2) in the clouds of an exoplanet.
An artist’s impression of the hot-Jupiter WASP-17b. Image credit: NASA / ESA / CSA / Ralf Crawford, STScI.
WASP-17b is located approximately 1,300 light-years from Earth in the constellation of Scorpius.
First discovered in 2009, this planet orbits the F-type main sequence star WASP-17.
With a volume more than 7 times that of Jupiter and a mass less than one-half of Jupiter, WASP-17b 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.
“We were thrilled! We knew from Hubble observations that there must be aerosols — tiny particles making up clouds or haze — in WASP-17b’s atmosphere, but we didn’t expect them to be made of quartz,” said Dr. David Grant, an astronomer at the University of Bristol.
Using Webb’s MIRI instrument, Dr. Grant and colleagues 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, they were 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 nm across.
This transmission spectrum of WASP-17b was captured by Webb’s Mid-Infrared Instrument (MIRI) on March 12-13, 2023. Image credit: NASA / ESA / CSA / R. Crawford, STScI / D. Grant, University of Bristol / H.R. Wakeford, University of Bristol / N. Lewis, Cornell University.
“Hubble data actually played a key role in constraining the size of these particles,” said Dr. Nikole Lewis, an astronomer at Cornell University.
“We know there is silica from Webb/MIRI data alone, but we needed the visible and near-infrared observations from Hubble for context, to figure out how large the crystals are.”
Unlike mineral particles found in clouds on Earth, the quartz crystals detected in the clouds of WASP-17b are not swept up from a rocky surface. Instead, they originate in the atmosphere itself.
“WASP-17b is extremely hot — around 1,500 degrees Celsius (2,700 degrees Fahrenheit) — and the pressure where the quartz crystals form high in the atmosphere is only about one-thousandth of what we experience on Earth’s surface,” Dr. Grant said.
“In these conditions, solid crystals can form directly from gas, without going through a liquid phase first.”
Understanding what the clouds are made of is crucial for understanding the planet as a whole.
“Hot Jupiters like WASP-17b are made primarily of hydrogen and helium, with small amounts of other gases like water vapor and carbon dioxide,” said Dr. Hannah Wakeford, an astronomer at the University of Bristol.
“If we only consider the oxygen that is in these gases, and neglect to include all of the oxygen locked up in minerals like quartz, we will significantly underestimate the total abundance.”
“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.”
Exactly how much quartz there is, and how pervasive the clouds are, is hard to determine.
“The clouds are likely present along the day/night transition (the terminator), which is the region that our observations probe,” Dr. Grant said.
“Given that the planet is tidally locked with a very hot day side and cooler night side, it is likely that the clouds circulate around the planet, but vaporize when they reach the hotter day side.”
“The winds could be moving these tiny glassy particles around at thousands of miles per hour.”
The study was published in the Astrophysical Journal Letters.
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David Grant et al. 2023. JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b. ApJL 956, L29; doi: 10.3847/2041-8213/acfc3b
