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Hubble observes changes in the atmosphere of a planet outside the solar system

An international team of astronomers compiled and reprocessed observations of the extrasolar planet WASP‑121 b collected by the Hubble Space Telescope in 2016, 2018 and 2019 and noticed weather patterns on its surface. It is a hot Jupiter type planet - that is, the size of Jupiter but very close to its sun

Artist's rendering of the planet WASP-121 b showing distinct climate patterns. Image: NASA/ESA
Artist's rendering of the planet WASP-121 b showing distinct climate patterns. Image: NASA/ESA

An international team of astronomers compiled and reprocessed observations of the extrasolar planet WASP‑121 b collected by the Hubble Space Telescope in 2016, 2018 and 2019. Unique data were thus obtained that allowed them not only to analyze the atmosphere of WASP-121 b, but also to compare the state of the extrasolar planet's atmosphere over several years. They found clear evidence that observations of WASP-121 b have changed over time. The team used sophisticated modeling technologies to show that these changes over time can be explained by weather patterns in the star's atmosphere.

Observing extrasolar planets is challenging, due to their distance from Earth and the fact that they often orbit stars that are much larger and brighter than them. This means that astronomers who have been able to observe an extrasolar planet with a sophisticated telescope like Hubble usually have to combine all their data to get enough information to draw informed conclusions about the planets' properties. Combining the observations to boost the planet's signal strength allows astronomers to build an average picture of its atmosphere, but from which it is impossible to tell if it varies. In other words, they cannot study the weather on other worlds using this averaging method. Investigating weather requires much more high-quality data, collected over a longer period of time. Fortunately, Hubble has been active for such an impressive amount of time that there is a large archive of Hubble data, sometimes with more than one set of observations of the same celestial body – and this includes the extrasolar planet WASP-121 b.

WASP-121 b (also known as Tylos) is a hot Jupiter that has been studied extensively. It orbits a star that is about 880 light years from Earth, and completes a complete cycle in a very vigorous cycle of 30 hours. Its close proximity to the host star means it is in tidal lock, and the hemisphere facing the star is very hot, with temperatures above 3000 Kelvin. The team combined four sets of archived observations of WASP-121 b, all taken using Hubble's Wide Field Camera 3. The team took the unique action of processing each of the sets in the same way, even if it had previously been processed by a different team. Processing extrasolar planet data is time-consuming and complex, but the processing paid off because it allowed the team to compare the processed data from each set of observations directly.

After cleaning each data set, the team found clear evidence that observations of WASP-121 b had changed over time. Instrumental effects may remain, but the data showed an apparent shift in the planet's hotspot and differences in the spectral signatures (which indicate the chemical composition of the atmosphere), an indication of a changing atmosphere. Next, the team used very sophisticated computational models to try to understand the observed behavior of the planet's atmosphere. The models suggested that the results could be explained by quasi-cyclical weather patterns, specifically massive cyclones that form and destroy repeatedly due to the large difference in temperatures between the star-facing and dark side of the planet. This result is a big step forward in the potential observation of weather patterns in extrasolar planets.

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