Metal clouds found on the hot Jupiter WASP-121 b using the Hubble Space Telescope

Artistic representation of the exoplanet WASP-121 b. This planet always presents the same face to its star. This day side is thus continuously illuminated, and very hot, while the other, the night side, is constantly exposed to the coldness of space. Credit: Engine House VFX / MPIA.

Astronomers explore the unusual atmospheric conditions of a hot exoplanet

An international group of astronomers which includes Jake Taylor, a Postdoctoral Researcher at the Institute for Research on Exoplanets, has made the first detailed measurement of atmospheric nightside conditions of a tidally locked hot Jupiter. By including measurements from the dayside hemisphere, they determined how water changes physical states when moving between the hemispheres of the exoplanet WASP-121 b. While airborne metals and minerals evaporate on the hot dayside, the cooler night side features metal clouds and rain made of liquid gems. This study, published on February 21st in Nature Astronomy, is a big step in deciphering the global cycles of matter and energy in the atmospheres of exoplanets.

The first discovery of an exoplanet orbiting a Sun-like star more than 25 years ago introduced a new and exotic class of planets, hot Jupiters: Jupiter-like giant gas planets on close orbits around their parent stars, separated by only a few stellar diameters. Due to their proximity, the irradiation from the star heats the planet to several hundred to a few thousand degrees Celsius. Of the almost 5000 known exoplanets, more than 300 are such hot Jupiters.

Using the Hubble Space Telescope, the international team led by Thomas Mikal-Evans from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, investigated the atmospheric properties of the hot Jupiter WASP-121 b. Astronomers discovered this exoplanet in 2015. It is located in the constellation Puppis at a distance of 855 light-years from Earth. WASP-121 b’s mass is about 20% greater than that of Jupiter, while it has a diameter that is nearly twice as large.

Despite the discovery of thousands of exoplanets, we’ve only been able to study the atmospheres of a small fraction due to the challenging nature of the observations,” Mikal-Evans points out. “So far, most of these measurements have provided limited information, such as basic details on the chemical composition or average temperature in specific subregions of the atmosphere.”

The first exploration of an exoplanet’s nightside environment

The new observations allowed the astronomers to obtain the most detailed insight yet into the conditions of an exoplanet nightside hemisphere. Like all hot Jupiters, WASP-121 b’s rotation is tidally locked to its orbit around its parent star. Hence, one 30-hour orbit around the star requires the same amount of time as the planet needs to rotate once on its axis. As a result, the hemisphere pointing towards the star – the dayside – always suffers the roasting hot stellar surface. Likewise, the cooler night side constantly faces the cold and dark space. By merging the data from the dayside and nightside hemispheres, the team’s analysis leads to the first elaborate view of how an exoplanet atmosphere functions as a global system.

Metal clouds and rain made of liquid gems

Liquid ruby and sapphire could be raining into the atmosphere of the gas giant exoplanet WAPS-121 b. Credit: Wikimedia Commons (ruby, sapphire).

Instead of water clouds such as those on Earth, clouds on WASP-121 b mainly consist of metals such as iron, magnesium, chromium and vanadium. Previous observations have revealed the spectral signals of these metals as gases on the hot dayside. The new Hubble data indicate that temperatures drop low enough for the metals to condense into clouds on the nightside. Eastward flowing winds that carry the water vapour across the nightside would also blow these metal clouds back around to the dayside, where they again evaporate.

Strangely, aluminium and titanium were not among the gases detected in the atmosphere of WASP-121 b. A likely explanation for this is that these metals have condensed and rained down into deeper layers of the atmosphere, not accessible to observations. This rain would be unlike any known in the Solar System. For instance, aluminium condenses with oxygen to form the compound corundum. With impurities of chromium, iron, titanium or vanadium, we know it as ruby or sapphire. Liquid gems could therefore be raining on the nightside hemisphere of WASP-121 b.


Prospects with the James Webb Space Telescope

Jake Taylor, NEAT Postdoctoral Fellow at the Institute for Research on Exoplanets (iREx) and co-author of the study, specialises in analysing space telescope data that reveal the atmosphere of exoplanets. He contributed to establishing the composition and structure of WASP-121 b’s atmosphere using Hubble Wide Field Camera 3 data. 

Jake Taylor, NEAT Postdoctoral Fellow at the Institute for Research on Exoplanets (iREx), is a co-author of the study.

“WASP-121 b will be studied with the James Webb Space Telescope soon,” explains Dr.Taylor. “These Hubble observations give us a first insight into what the NEAT GTO observation for WASP-121 b will tell us about the extreme weather conditions on this planet.” 

Jake joined iREx at the Université de Montréal in the Summer of 2021 specifically to work on NEAT, the James Webb Space Telescope observing program that uses Canadian Guaranteed Time Observations (GTO) to study a variety of exoplanets’ atmospheres, including that of WASP-121 b. 

By covering wavelengths beyond Hubble’s range, the Webb Telescope’s observations will allow the team to determine the amount of carbon in the atmosphere, which could hold clues about how and where WASP-121 b formed in the protoplanetary disk. The measurements will even be precise enough to learn about the wind speeds at different altitudes inside the atmosphere.

Everyone at iREx and in the international team is eager to learn more about WASP-121 b with the Webb Telescope!

About this study

Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b ” by Mikal-Evans et al. was published on February 21th, 2022, in Nature Astronomy. In addition to Thomas Mikal-Evans (MPIA, Germany; MIT Kavli Institute, USA) and Jake Taylor (iREx, UdeM, Canada; University of Oxford, UK), the team includes 10 co-authors from the USA, UK and India. 


Adapted from a MPIA Press Release by

Dr. Markus Nielbock
Max-Planck-Institut für Astronomie Presse- und Öffentlichkeitsarbeit
MPIA-Campus Königstuhl 17 D-69117 Heidelberg
Tel. +49(0)6221 528-134 Mobil +49(0)15678 747326, 

Media Contact
Marie-Eve Naud
EPO Coordinator, Institute for Research on Exoplanets
Université de Montréal, Montréal, Canada

Nathalie Ouellette
Coordinator, Institute for Research on Exoplanets
Université de Montréal, Montréal, Canada

Scientific Contacts
Jake Taylor
NEAT Postdoctoral Researcher
Université de Montréal, Montréal, Canada 

Additional links
Scientific paper (Nature)
Scientific paper (open source version)
MPIA Press Release