Astronomers have mapped the atmospheres of exoplanets for a while, but a close look at their night sides has proven elusive — until now. An MIT-led study has provided the first detailed look at the dark side of a “hot Jupiter” exoplanet by mapping WASP-121b’s elevation-based temperatures and water resources. Because the distant planet (850 light-years away) is tidally locked to its parent star, the differences from the bright side couldn’t be greater.
The dark side of the planet contributes to an extremely violent water cycle. Where the day side rips apart water with temperatures in excess of 4,940°F, the night is cool enough (at most “only” 2,780°F) to rejoin them to water. The result hurls water atoms around the planet at over 18,000 km/h. This dark side is also cool enough to have clouds of iron and corundum (a mineral found in rubies and sapphires), and you can see rains of liquid gems and titanium as steam from the day side cools.
The researchers collected the data using Hubble Space Telescope spectroscopy for two orbits in 2018 and 2019. Many scientists have used this method to study the light sides of exoplanets, but the observations on the dark side required detecting tiny changes in the spectral line indicative of water vapor. This line helped scientists create temperature maps, and the team ran these maps through models to help identify likely chemicals.
According to MIT, this is the first detailed study of an exoplanet’s global atmosphere. This comprehensive look should help explain where hot Jupiters like WASP-121b can form. And while a Jovian world like this is clearly too dangerous for humans, more thorough studies of exoplanet atmospheres could help in the search for truly habitable planets.
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