Hot Days and Cloudy Nights on a “Heavy Metal” Exoplanet

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

representation of an exoplanet's phase curve

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

One way to learn more about the atmosphere of an intriguing exoplanet like WASP-121b is by measuring its phase curve, or the total amount of light emitted and reflected by the planet and its host star over a full planetary orbit. Planetary phase curves can tell us something about how the temperature of the planet’s atmosphere varies from day to night. A phase curve previously measured for WASP-121b suggested that the planet’s night side might be cool enough for clouds to form — what can new data from JWST tell us about this hot, metallic world?

WASP-121b light curve

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

observed and modeled planet-to-star emission

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]

Mikal-Evans and coauthors modeled the light curve, finding that the warmest point in WASP-121b’s atmosphere is a few degrees east of the point at which its star is closest, called the substellar point. Models of hot Jupiters like WASP-121b predict offsets that tend to be much larger, closer to 10 degrees. The team suggested two possible reasons for this difference: 1) the atmosphere is so hot that the gas is ionized, and the interaction between ionized gas and the planet’s magnetic field slows the eastward transfer of heat, or 2) the hottest point is actually located farther east, but clouds on the planet’s night side affect our interpretation of the light curve.

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.


“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