An international team of astronomers has reprocessed observations of the exoplanet WASP-121b collected by the NASA/ESA Hubble Space Telescope in the years 2016-19. The resulting dataset allowed them not only to analyse the planet’s atmosphere but also to monitor its changing state across several years. They found clear evidence that the observations of WASP-121b varied over time. Sophisticated modelling demonstrates that such variations could be explained by weather patterns in the exoplanet’s atmosphere.
Observing exoplanets, lying beyond our own Solar System, is challenging both because of their huge distance from Earth and because they orbit much bigger and brighter stars. Typically, astronomers need to combine data from several observations to strengthen the signal to get enough information to make confident deductions about the exoplanet’s atmosphere. This process yields an averaged picture of the atmosphere but does not tell us whether and how it is changing in time. Studying the weather requires far more high-quality data taken over a longer period of time. Fortunately, Hubble has been active for long enough to accrue a vast data archive including multiple distinct observations of WASP-121b.
WASP-121b (also known as Tylos) is a well-studied “hot Jupiter”, that is, an inflated gas giant 880 light-years from Earth, completing a full orbit around its host star in just 30 hours. Its close proximity to the host induces tidal locking, ie. just as with the Moon and Earth the same face of WASP-121b is always presented to the star so that the star-facing hemisphere is very hot with temperatures exceeding 3000K. Spectral absorption features reveal the presence of water (H2O), dissociated hydrogen in the form H-, and refractory molecules such as TiO, VO, and FeH. The team combined four sets of archival observations made using Hubble’s Wide Field Camera (WFC3), including WASP-121b both transiting in front of and eclipsed behind the star, together with two time-varying phase curves. Each raw dataset was reprocessed in the same way, being fitted with a computationally intensive atmospheric retrieval code, employing the unique resources of DiRAC’s CSD Icelake resource at Cambridge; this permits direct comparison among the separate observations.
One of the team’s principal investigators, Quentin Changeat, an ESA Fellow based at the Space Telescope Science Institute in Baltimore, elaborates:
This dataset represents a significant amount of observing time for a single planet and is currently the only consistent set of such repeated observations. The information we extracted was used to characterise the chemistry, temperature and cloud patterns of the WASP-121b atmosphere at different times. This provided us with an exquisite picture of the planet, changing in time.
The processed data showed clear evidence that the observations varied over time. The data support apparent movement of the planet’s hottest point, which is typically offset
from the sub-stellar point, and variations in the spectral signature linked to atmospheric composition indicate a changing atmosphere.
Next, sophisticated atmospheric dynamics simulations were used to model the observed behaviour. The models suggest the results could be explained by quasi-periodic weather patterns, specifically massive cyclones that are repeatedly created and destroyed as a consequence of the huge temperature difference between the star-facing and dark sides of the exoplanet. This is a significant step towards the goal of observing weather patterns on exoplanets. Future observations with Hubble and other powerful telescopes including Webb will provide greater insight into weather on distant worlds, leading ultimately, possibly, to finding exoplanets with stable long-term climates and weather patterns.
