Imagine a world where the clouds are made of metal and liquid rubies and sapphires rain down from the sky. A new study shows that, on the hot Jupiter exoplanet WASP-121 b, this could be the reality.
In 2015, scientists discovered WASP-121 b, a gas giant exoplanet 880 light-years from Earth. The alien world is what is known as a "hot Jupiter," a class of gas giants that have physical similarities to Jupiter but orbit their stars much closer (hence, their "hot" nature). Since its discovery, researchers have further explored this world and its strange atmosphere.
In a new study, scientists using the Hubble Space Telescope have made the first detailed measurement of the atmosphere on the planet's cooler nightside. And this nighttime atmosphere seems to have a number of strange and remarkable qualities including metal clouds and rain made of what could be liquid gems.
"It's exciting to study planets like WASP-121 b that are very different to those in our solar system, because they allow us to see how atmospheres behave under extreme conditions," co-author Joanna Barstow, a researcher at the Open University in the U.K., said in a statement (opens in new tab).
This hot Jupiter is tidally locked, meaning it has one side that is always facing its star and another side that faces away. On WASP-121 b's star-facing dayside, metals and minerals evaporate. The dayside's upper atmosphere can get as hot as about 5,400 degrees Fahrenheit (3,000 degrees Celsius), so hot that water in the atmosphere glows and molecules break down, according to a statement.
But on the planet's nightside, the team found that the atmospheric temperature is essentially cut in half. This difference in temperature causes strong winds to blow from west to east around the planet, pulling water through the atmosphere from the dayside to the nightside. As water molecules are pulled apart into hydrogen and oxygen atoms by the heat on the dayside, the cool temperatures on the nightside then recombine the atoms into water vapor. That water is pulled back to the dayside by the winds and pulled apart in a continuous cycle.
The temperatures on the nightside are never low enough for water clouds to form during this cycle, but that doesn't mean that clouds don't form at all. While water clouds don't form, metal clouds do.
Previous Hubble data showed signs of metals including iron, magnesium, chromium and vanadium existing as gasses on the planet's dayside. But in this study, the researchers have found that on the planet's nightside, it gets cold enough for these metals to condense into clouds.
And, just as the strong winds pull water vapor and atoms around the planet to break apart and recombine, metal clouds will blow to the planet's dayside and evaporate, condense back on the nightside and so on.
But metal clouds aren't the only strange phenomenon these researchers spotted on this hot Jupiter. They also found evidence of possible rain in the form of liquid gems.
Scientists were surprised that, among the metals they detected in the planet's atmosphere, they did not find aluminum or titanium. They think that this surprising finding could be explained by the metals condensing and raining down into lower levels of the planet's atmosphere outside of the scope of these observations, according to the statement.
This metal vapor condensed into metallic rain would see aluminum condense together with oxygen, forming corundum. This is a metallic compound that, when tainted by other metals suspected in the planet's atmosphere, would form what we know on Earth as rubies or sapphires, according to the statement.
This work shed new light on the hot Jupiter's nightside, but the team also studied the planet's dayside.
"To probe the entire surface of WASP-121 b, we took spectra with Hubble during two complete planet revolutions," co-author David Sing, a researcher at Johns Hopkins University in Maryland, said in the statement.
By combining this information about WASP-121 b's nightside with data about the exoplanet's dayside, the team is working to reveal new insights into how the world's atmosphere works as a whole. The team was able to observe the complete water cycle of the planet and, in doing so, became the first to study the full water cycle of an exoplanet, according to the statement.
Still, there is much to learn. The team plans to further explore the exoplanet with NASA's James Webb Space Telescope, a next-gen observatory that recently arrived in space to study the farthest reaches of the universe in infrared.
"To better understand this planet, we're going to observe it with the James Webb Space Telescope within the first year of its operation," lead author Thomas Mikal-Evans, a researcher with the Max Planck Institute for Astronomy, said in the statement.
This work was published today (Feb. 21) in the journal Nature Astronomy.