Free Astronomy Magazine January-February 2024

24 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING 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, SiO 2 ) in the clouds of an exoplanet. The spec- trum 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. For each wavelength, the amount of light blocked by the planet’s atmosphere (white circles) was calculated by subtracting the amount that made it through the atmosphere from the amount originally emitted by the star. The solid purple line is a best-fit model to the Webb (MIRI), Hubble, and Spitzer data. (The Hubble and Spitzer data cover wavelengths from 0.34 to 4.5 microns and are not shown on the graph.) The spectrum shows a clear feature around 8.6 microns, which astronomers think is caused by silica particles absorbing some of the starlight passing through the atmosphere. The dashed yellow line shows what that part of the transmission spectrum would look like if the clouds in WASP-17 b’s atmosphere did not contain SiO 2 . This marks the first time that SiO 2 has been identified in an exo- planet, and the first time any specific cloud species has been identified in a transiting exoplanet. [NASA, ESA, CSA, Ralf Craw- ford (STScI). David Grant (University of Bristol), Hannah R. Wakeford (University of Bristol), Nikole Lewis (Cornell University)] 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 sub- tracting 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 ex- pected if the clouds were made of magnesium silicates or other possible high-temperature aerosols like alu- minum oxide, but which makes per- fect sense if they are made of quartz. While these crystals are probably similar in shape to the pointy hexag- onal prisms found in geodes and gem shops on Earth, each one is only about 10 nanometers across – one-millionth of one centimeter. “Hubble data actually played a key role in constraining the size of these particles,” explained co-author Nikole Lewis of Cornell University, who