Artist’s impression of the exoplanet WASP-121 b.
Patricia Klein and MPIA
Literally alien weather on a relatively close exoplanet created a nighttime environment with metal clouds and a rain of liquid gems.
This is the conclusion of a study for the exoplanet WASP-121 b, which examined how water changes physical states when moving between the hemispheres. One side is permanently in daylight, quite close to its parent star, while the other side cools in the nighttime, creating this most unusual environment.
The work has larger implications as we move into a new generation of observatories better able to probe the atmospheres of larger planets. So far, despite the thousands of exoplanets found, only a handful of atmospheres have been so well-studied.
“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,” lead author Thomas Mikal-Evans, from the Max Planck Institute for Astronomy in Germany, said in a statement.
Fortunately, the planet is a relatively close one to Earth, at 855 light-years away (for comparison, the closest known planetary system is roughly four light-years away.) WASP-121 b was found in 2015 and has a mass roughly 20% greater than Jupiter, with a diameter roughly twice as large as our solar system’s most giant planet.
The planet takes just 30 hours to orbit its parent star, and astronomers used the Hubble Space Telescope to examine two revolutions in detail. Along with some modeling, this approach allowed the team to examine the upper atmosphere of WASP-121 b and to get the complete water cycle of an exoplanet, a unique feature of the study.
Among many other findings of the study, the researchers showed that the upper atmosphere reaches temperatures of more than 5,400 Fahrenheit, which causes water to glow and to break down into atomic components. On the night side, the temperature is roughly half of that you see in the daytime.
“This extreme temperature difference between the two hemispheres gives rise to strong winds that sweep around the entire planet from west to east, dragging the disrupted water molecules along. Eventually, they reach the nightside,” the statement said.
“The lower temperatures allow the hydrogen and oxygen atoms to recombine, forming water vapor again before being blown back around to the dayside and the cycle repeats,” the statement continued. “Temperatures never drop low enough for water clouds to form throughout the cycle, let alone rain.”
Previous work showed that metals are present as gas on the dayside, while Hubble revealed that the temperatures on the night side allow the metals to condense into clouds and to rain out the aforementioned gems.
Researchers plan to probe this world again using the newly launched James Webb Space Telescope, which is more optimized for heat-seeking (infrared) wavelengths and has a sharper field of view.
“These 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,” the statement added.
A study based on the research was published in Nature Astronomy.
