This artist's concept shows what the exoplanet WASP-17 b could look like. NASA, ESA, CSA, Ralf Crawford (STScI)
A transmission spectrum of the hot gas giant exoplanet WASP-17 b captured by MIRI (Webb's Mid-Infrared Instrument) on March 12-13, 2023, reveals the first evidence for quartz (crystalline silica, SiO2) in the clouds of an exoplanet. The spectrum was made by measuring the change in brightness of 28 wavelength-bands of mid-infrared light as the planet transited its star. Webb observed the WASP-17 system using MIRI's low-resolution spectrograph for nearly 10 hours, collecting more than 1,275 measurements before, during, and after the transit. NASA, ESA, CSA, Ralf Crawford (STScI)
Researchers have detected for the first time evidence of quartz nanocrystals in the high-altitude clouds of WASP-17 b, a hot Jupiter exoplanet 1,300 light-years from Earth.
Quartz, also known as silica, exists everywhere: it's found in beach sands, building stones, gem shops, and computer chips embedded in all technology from laptops to washing machines.
But its distant discovery, led by astrophysicists at the University of Bristol, is astounding and only possible thanks to the unique power of the largest telescope in space. NASA's JWST is giving unprecedented insights into the alien worlds of exoplanets like WASP-17 b from a distance of more than seven million billion miles. The findings provide vital new information about the composition of their atmospheres and weather.
Lead author David Grant, Research Associate at the University of Bristol, said: "We were thrilled! 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."
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, and are extremely common across the galaxy. But the silicate grains previously detected in the atmospheres of exoplanets and brown dwarfs appear to be made of magnesium-rich silicates like olivine and pyroxene, not quartz alone – which is pure SiO2.
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 centimetre, around 10,000 times smaller than the width of a human hair.
The results from this team, published today in the journal Astrophysical Journal Letters and involving researchers from NASA's Ames Research Center and NASA's Goddard Space Flight Center, puts a new spin on our understanding of how exoplanet clouds form and evolve.
Co-author Dr Hannah Wakeford, Associate Professor in Astrophysics at the University of Bristol, said: "We fully expected to see magnesium silicates. But what we're seeing instead are likely the building blocks of those, the tiny 'seed' particles needed to form the larger silicate grains we detect in cooler exoplanets and brown dwarfs."
With a volume more than seven times that of Jupiter and a mass less than one-half of 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.
JWST 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.
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.
David added: "WASP-17 b is extremely hot – around 2,700 degrees Fahrenheit (1,500 degrees Celsius) – 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."
"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-17 b are made primarily of hydrogen and helium, with small amounts of other gases like water vapor (H2O) and carbon dioxide (CO2).
Dr Wakeford said: "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 (SiO2), 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. Given the planet is tidally locked with a very hot day side and cooler night side, it is likely the clouds circulate around the planet, but vaporize when they reach the hotter day side.
David added: "The clouds are likely present along the day/night transition (the terminator), which is the region that our observations probe. The winds could be moving these tiny glassy particles around at thousands of miles per hour."
WASP-17 b is one of three planets targeted by the JWST Telescope Scientist Team's Deep Reconnaissance of Exoplanet Atmospheres using Multi-instrument Spectroscopy (DREAMS) investigations, which are designed to gather a comprehensive set of observations of one representative from each key class of exoplanets: a hot Jupiter, a warm Neptune, and a temperate rocky planet. The MIRI observations of hot Jupiter WASP-17 b were made as part of GTO program 1353.
Paper
'JWST-TST DREAMS: Quartz Clouds in the Atmosphere of WASP-17b' by David Grant, Nikole K. Lewis, Hannah R. Wakeford et al in The Astrophysical Journal Letters
