A bizarre water cycle, metal clouds, gem rain and gigantic storms are emerging: astronomers have gained insights into the strange weather phenomena in the atmosphere of hot Jupiter WASP-121 b. They are caused by the extreme differences between the eternally hot day side and the cooler night side of the exoplanet – because it orbits its star in a locked rotation. The new insights represent a major step towards understanding global matter and energy cycles in the atmospheres of exoplanets, say the researchers.
Infernal giant worlds with bizarre features: Of the almost 5000 known exoplanets, around 300 are assigned to the category of hot Jupiters. These are particularly large gas planets that orbit their central stars in very narrow orbits. Due to the proximity, the radiation from the star sometimes heats these planets up to several thousand degrees Celsius. However, only one side is particularly affected: In the case of hot Jupiters, the rotation is tied to the orbit around their stars by tidal forces: one orbit takes the same time as the planet needs to rotate once on its axis. Consequently, there is always a hot day on one side and a comparatively cool eternal night on the other. One can imagine that this imbalance is literally causing a lot of commotion in the atmospheres of hot Jupiters.
Researchers led by Thomas Mikal-Evans from the Max Planck Institute for Astronomy in Heidelberg are investigating these exotic worlds. Hot Jupiter WASP-121 b, located about 855 light-years from us, was in their sights. It is almost twice the diameter of our Jupiter and orbits its host star extremely closely – whizzing around it once every 30 hours. Its day side is almost roasted. On the night side, on the other hand, the planet is constantly aligned with cold and dark space.
Day and night side in view
The scientists have now gained information about the characteristics of both sides through spectral analyzes based on data from NASA’s Hubble Space Telescope. “To study the entire surface of WASP-121 b, we used Hubble to record spectra during two complete planetary orbits,” explains co-author David Sing of Johns Hopkins University in Baltimore. Using this technique, and aided by modeling the data, astronomers have been able to study WASP-121 b’s upper atmosphere across the planet. This provided a comprehensive picture of how the exoplanet’s atmosphere works as a global system.
As the researchers report, the results made it possible to draw conclusions about the water cycle of WASP-121 b. So it differs drastically from what we know. On Earth, water frequently changes its physical state: solid ice melts into liquid water, which evaporates into gas and then condenses into droplets that form clouds. The cycle then closes when there are raindrops that rain down on the surface. On WASP-121 b, on the other hand, the water cycle is characterized by extreme heat: on the perpetual day side, the upper atmosphere heats up to 3000 degrees Celsius, according to the spectral data. The water begins to glow and many of the molecules even break down into their atomic components, the researchers explain.
On the night side, on the other hand, there are significantly lower temperatures, which creates a strong “need for compensation” that manifests itself in the form of storms. The Hubble data shows that they whip around the planet from west to east at over 17,000 kilometers per hour, carrying the broken-up water molecules with them. Eventually they reach the night side. The lower temperatures allow the hydrogen and oxygen atoms there to recombine and form water vapor. Later they are then blown back to the day side and the cycle repeats itself. However, as the researchers emphasize, the temperatures never drop so low that water clouds can form, let alone rain.
Metallic clouds and “noble” rain
Instead, WASP-121 b has clouds of a bizarre kind: According to the team’s models, they are made up mostly of metals like iron, magnesium, chromium and vanadium. The new Hubble data suggests temperatures on the nightside are dropping low enough for those metals to condense into clouds there. They are then blown again onto the day side, where they vaporize again. “The winds can probably move these clouds across the entire planet in about 20 hours,” says co-author Tansu Daylan of the Massachusetts Institute of Technology in Cambridge. The researchers suspect that in the deeper layers of the atmosphere, the elements aluminum and titanium condense to such an extent that they rain down. Apparently, this could also result in special compounds: when condensing aluminum combines with oxygen, corundum is formed. With impurities of chromium, iron, titanium or vanadium, we know this substance as ruby or sapphire. So, on the night side of WASP-121 b, it could rain liquid gems, the scientists say.
“Studying planets like WASP-121 b, which are very different from those in our solar system, is exciting because they give us insights into how atmospheres behave under extreme conditions,” says co-author Joanna Barstow from the Open University in Milton Keynes (UK). Mikal-Evans adds: “To understand this planet even better, we plan to observe it with the James Webb Space Telescope within the first year of its operation.” Among other things, the astronomers hope that the planned investigations will provide clues as to how and where WASP-121 b once formed in its star’s protoplanetary disk. They also want to keep an eye on the rapid activity in the atmosphere: the measurements could record the wind speeds at different heights within the planet’s atmosphere.
Source: Max Planck Institute for Astronomy, Massachusetts Institute of Technology, Article: Nature Astronomy, doi: 10.1038/s41550-021-01592-w