Free Astronomy Magazine January-February 2024

25 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING D epiction of ten known hot Jupiters. The images are to scale with each other. HAT-P-12b, the smallest of them, is approximately the size of Jupiter, while WASP-17b, the largest planet in the sample, is almost twice the size. The planets are also depicted with a variety of different cloud properties. [ESA/Hubble & NASA] leads the Webb Guaranteed Time Observation (GTO) program de- signed to help build a three-dimen- sional view of a hot Jupiter at- mosphere. “We know there is silica from Webb’s MIRI data alone, but we needed the visible and near-in- frared 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-17 b are not swept up from a rocky sur- face. Instead, they originate in the atmosphere itself. “WASP-17 b is ex- tremely hot – around 2,700 degrees Fahrenheit (1,500 degrees Celsius) – and the pressure where the quartz crystals form high in the atmos- phere 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.” 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 (H 2 O) and carbon dioxide (CO 2 ). “If we only consider the oxy- gen that is in these gases, and neg- lect to include all of the oxygen locked up in minerals like quartz (SiO 2 ), we will significantly underes- timate the total abundance,” ex- plained Wakeford. “These beautiful silica crystals tell us about the inven- tory 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,” said Grant. 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.” !