Free Astronomy Magazine March-April 2024

26 MARCH-APRIL 2024 ASTRO PUBLISHING Its extremely close proximity to its host star means that it is tidally locked, and that the star-facing hemisphere is very hot, with tem- peratures exceeding 3000 Kelvins. The team combined four sets of archival observations of WASP-121 b, all made using Hubble’s Wide Field Camera 3 (WFC 3). The com- plete assembled dataset included observations of: WASP-121 b transit- ing in front of its star (taken in June 2016); WASP-121 b transiting be- hind its star, also known as a sec- ondary eclipse (taken in November 2016); and two phase-curves of WASP-121 b (taken in March 2018 and February 2019 respectively). The team took the unique step of pro- cessing each dataset in the same way, even if it had been previously processed by a different team. Exoplanet data processing is time consuming and complicated, but nonetheless it was worth it because it allowed the team to directly com- pare the processed data from each set of observations with one an- other. One of the principal investiga- tors of the team, Quentin Changeat, an ESA Research Fellow at the Space Telescope Science Institute, elabo- rates: “Our dataset represents a sig- nificant amount of observing time for a single planet and is currently the only consistent set of such re- peated observations. The informa- tion that we extracted from those observations was used to charac- terise (infer the chemistry, temper- ature, and clouds) of the atmos- phere of WASP-121 b at different times. This provided us with an ex- quisite picture of the planet, chang- ing in time.” After cleaning each dataset, the team found clear evidence that the observations of WASP-121 b were varying in time. While instrumen- tal effects could remain, the data showed an apparent shift in the ex- oplanet’s hot spot and differences in by NASA/ESA − Bethany Downer A n international team of as- tronomers has assembled and reprocessed observa- tions of the exoplanet WASP-121 b that were collected with the NASA/ESA Hubble Space Telescope in the years 2016, 2018 and 2019. This provided them with a unique dataset that allowed them not only to analyse the atmosphere of WASP 121 b, but also to compare the state of the exoplanet’s atmosphere across several years. They found clear evidence that the observations of WASP-121 b were varying in time. The team then used sophisti- cated modelling techniques to demonstrate that these temporal variations could be explained by weather patterns in the exoplanet’s atmosphere. WASP-121 b (also known as Tylos) is a well-studied hot Jupiter that or- bits a star that lies about 880 light- years from Earth, completing a full orbit in a very brisk 30-hour period. Hubble observes a changing exoplanet atmosphere