WASP-121b is a hot Jupiter, a gas giant orbiting so close to its star that it is not round, and it has a temperature in the thousands of degrees. It is so hot that its clouds are made of molten metal and now, for the first time, astronomers have worked out what is happening to its night side, including a rain of liquid gems.
As reported in Nature Astronomy, an international group has tracked the emission of water across the day side and night side, as well as other elements. The planet is tidally locked, which means one side always faces its star and the other is in perpetual darkness.
On the dayside, water molecules break apart. The temperature there is about 3000 °C (5,400 °F), which is hotter than some stars. It is hot enough to separate the oxygen from the hydrogen, but not for long.
Powerful winds moving at 5 kilometers per second (11,000 miles per hour) take those elements over to the night side where it’s only half as hot and where hydrogen and oxygen recombine to form water vapor. They used the Hubble Telescope to track a specific light feature of excited water molecules. By tracking the water cycle on this distant planet, the work has provided a truly global view of WASP-121b’s atmosphere.
"We saw this water feature and mapped how it changed at different parts of the planet's orbit. That encodes information about what the temperature of the planet's atmosphere is doing as a function of altitude," lead author Thomas Mikal-Evans from the Max Planck Institute for Astronomy, said in a statement. "We're now moving beyond taking isolated snapshots of specific regions of exoplanet atmospheres, to study them as the 3D systems they truly are."
The temperature across the planet never becomes low enough for water clouds to form. The clouds on the planet are made of metal such as iron, magnesium, and vanadium. The Hubble data supports the idea that it's cool enough on the night side for metal clouds to form. Once the powerful winds blow them back to the day side, they evaporate.
The researchers were surprised that they did not detect aluminum and titanium. They propose a fascinating explanation. When aluminum condenses with oxygen, it forms a compound known as corundum. You may not be familiar with corundum but you’ll be familiar with it when it has impurities made of chromium, iron, titanium, or vanadium. That’s how one makes sapphires or rubies.
The team has reserved time on the JWST to continue their study of the planet. They hope to detect carbon monoxide, which they believe exists in the planet’s atmosphere.
"That would be the first time we could measure a carbon-bearing molecule in this planet's atmosphere," Mikal-Evans continued. "The amount of carbon and oxygen in the atmosphere provides clues on where these kinds of planet form."