Free Astronomy Magazine January-February 2024

23 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING on a paper published in the Astro- physical Journal Letters . “We knew from Hubble observations that there must be aerosols – tiny particles mak- ing 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 gal- axy. But the silicate grains previously detected in the atmospheres of ex- oplanets and brown dwarfs appear to be made of magnesium-rich sili- cates like olivine and pyroxene, not quartz alone – which is pure SiO 2 . The result from this team, which also includes 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. “We fully expected to see magnesium silicates,” said co-au- thor Hannah Wakeford, also from the University of Bristol. “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. T his artist’s concept shows what the exoplanet WASP-17 b could look like. [NASA, ESA, CSA, Ralf Crawford (STScI)]