Researchers have used JWST to observe WASP-121b, a tidally locked hot Jupiter exoplanet best known for the heavy metals in its atmosphere. This study illuminates the conditions on the planet’s night side and provides new evidence for metallic clouds.
A Noteworthy Exoplanet
WASP-121b seems to pop up in the news each time astronomers point a telescope at it. This hot Jupiter exoplanet zips around its host star every 30 hours with one side permanently facing the star, resulting in daytime temperatures hot enough to vaporize gold. Previous observations have found evidence for metals like iron, nickel, and vanadium floating in the planet’s atmosphere

A representation of an exoplanet’s phase curve. Click to enlarge. [ESA]

A broadband light curve for WASP-121 and WASP-121b obtained with JWST NIRSpec (grey circles) and the best-fitting model (orange line). Click to enlarge. [Adapted from Mikal-Evans et al. 2023]
A Year on WASP-121b as Seen by JWST
In October 2022, a research team led by Thomas Mikal-Evans (Max Planck Institute for Astronomy) used JWST to stare at the star–planet system for about 1.5 Earth days — about the length of one day or one year on WASP-121b. The team used JWST’s Near Infrared Spectrograph (NIRSpec) in a special observing mode designed for bright targets, allowing them to collect data for 99% of their observing time.
The light curve shows a deep dip when the planet crosses in front of the bright star, a shallower dip when the planet passes behind the star, and a gentle curve marking when both the star and the planet are fully visible.
Metallic Clouds at Night, Astronomers Delight

Model predictions for the planet-to-star emission (orange and blue lines and symbols) and observed values (black diamonds). The two colors show the results for different metallicities. The orange and blue symbols show the model results binned to match the NIRSpec bandpasses. [Mikal-Evans et al. 2023]
Speaking of the planet’s night side: the team measured WASP-121b’s night side temperature to be about 1000K (1,340℉/727℃). While this may sound scorching hot, it’s actually cool enough for certain metallic compounds thought to be common in hot Jupiter atmospheres to form liquid droplets — in other words, WASP-121b might have metallic clouds at night!
In addition to teasing out the details of WASP-121b’s phase curve, this study showcased the capability of JWST’s NIRSpec instrument. Next, the team plans to study WASP-121b’s phase curve as a function of wavelength to learn more about the planet’s nigh tside atmosphere.
Citation
“A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation,” Thomas Mikal-Evans et al 2023 ApJL 943 L17. doi:10.3847/2041-8213/acb049