The “ex” in “exoplanet” does not stand for extreme. But for many planets orbiting other stars, it might as well—especially when it comes to weather.
Setting aside the inherent wonder and wildness of alien weather, understanding clouds on exoplanets is vital if scientists want to puzzle out the histories and inner lives of these worlds, said astrophysicist Christiane Helling of the Space Research Institute of the Austrian Academy of Sciences.
“What we eventually want to understand is how planets form and how they evolve,” explained Helling. “And in order to understand the evolution of planets in the stellar context, and even in the galactic context, we need to understand cloud formation.”
Clouds obscure everything beneath them and can “hide” chemicals that would otherwise float free in a planet’s atmosphere, potentially leading scientists to the wrong conclusions about what planets are made of. And because scientists use the chemical compositions of exoplanets to infer their evolution, clouds can meddle with inferences about how planets form and change over time, too.
Here are four planets with wild, rocky weather that scientists are actively working to understand.
1. A Warm Neptune with Silicate Showers
A typical weather forecast for the warm, Neptune-like exoplanet WASP-107b might be something like “cloudy, with a chance of sandstorms.”
A typical weather forecast for the warm, Neptune-like exoplanet WASP-107b might be something like “cloudy, with a chance of sandstorms.”
That’s because researchers recently detected clouds made of tiny, solid particles of silicate minerals—not unlike sand on Earth—drifting high in the planet’s atmosphere.
These clouds probably rain out in lower layers of the atmosphere where temperatures are high enough that their particles start to liquefy. “At some point, they will evaporate when they go too deep,” said astronomer Michiel Min of the Netherlands Institute for Space Research, who was part of the team that discovered the clouds. “And then this vapor is mixed up again, and the whole cycle starts again.”
Researchers who model exoplanet atmospheres have known there should be silicate clouds on hot Jupiters for decades. But it’s only now, thanks to JWST, that it’s possible to directly detect them.
Researchers who model exoplanet atmospheres have long known there should be silicate clouds on exoplanets like WASP-107b, said Helling. But only now, thanks to the James Webb Space Telescope (JWST), is it possible to directly detect such clouds using transit spectroscopy—a technique that can reveal the chemical composition of exoplanet atmospheres. WASP-107b is an especially good target for this kind of interplanetary weather watching thanks to its unusually “puffy” atmosphere, which makes transit spectroscopy easier.
“We have the ability to sound the atmospheres of exoplanets for real now, thanks to James Webb,” said astronomer Achrène Dyrek of the French Alternative Energies and Atomic Energy Commission and one of Min’s colleagues.
“This mineral cloud layer,” added Min, “has been predicted. And now we just see it. And I think that’s cool.”
2. A Shiny Planet with Mirrorlike Metal Clouds
The ultrahot Neptune LTT 9779b is the shiniest exoplanet found to date. And it probably owes its unusual gleam to a deck of glittering metal clouds.
The ultrahot Neptune LTT 9779b is the shiniest exoplanet found to date. And it probably owes its unusual gleam to a deck of glittering metal clouds.
Earlier this year, scientists watched as LTT 9779b slipped behind its host star. The eclipse briefly dimmed the star’s light and allowed scientists to measure the planet’s albedo and other properties for the first time. This technique works well only for very bright planets, and given LTT 9779b’s temperature and size, scientists weren’t sure the method would work at all. They had no idea the planet would be so reflective.
“We were very surprised,” said astronomer Sergio Hoyer, who studied LTT 9779b while at the Marseille Astrophysics Laboratory in France. “It took us a while to go through the data and to believe what we were seeing was true.”
LTT 9779b is not only the brightest known planet but also one of just a tiny handful of Neptune-like worlds with extremely tight orbits. Indeed, ultrahot Neptunes are so rare that the band of space immediately around a star is sometimes called the “Neptunian desert.”
Scientists still aren’t sure why the radiation from LTT 9779b’s uncomfortably close star didn’t strip away the planet’s atmosphere. But it’s possible, said Hoyer, that the planet’s metal clouds, whose presence was inferred on the basis of the planet’s albedo and other properties, could have protected it—a bit like a mirrored shield.
3. An Extreme Hot Jupiter with Metal and Gemstone Rain
WASP-121b is extreme, even as hot Jupiters go. The planet is so close to its star that gravity has pulled it into a shape more squashed than spherical, and its water vapor atmosphere is so hot it glows. Now, scientists have evidence that suggests this wild world has fittingly wild weather: rain showers of molten metal and liquid gems.
Unlike the sandy clouds of WASP-107b, the metal and gemstone rain clouds on WASP-121b weren’t directly detected. Instead, scientists concluded the phenomena must exist on the basis of simulations of the planet informed by their telescope observations.
For the new study, scientists used the Hubble Space Telescope to detect the infrared light cast by both the dayside and nightside of the planet. WASP-121b is tidally locked, meaning that one side of the planet always faces toward its star while the nightside never sees daylight.
Temperatures at the top of WASP-121b’s atmosphere on the dayside climb above 3,200°C. That’s more than hot enough to vaporize many metals and stones. But the nightside is far milder, around 1,500°C. There, it’d be cool enough to form clouds made of iron and minerals, including perovskite, forsterite, enstatite, and corundum—a mineral more familiar as ruby or sapphire, depending on its color.
Any metal or mineral clouds on WASP-121b’s nightside probably wouldn’t last long. Previous work has suggested that strong equatorial winds whip around the planet. Any clouds swept to the dayside would rain out and vaporize, only to condense into clouds again after being returned to the cooler nightside by the wind.
4. A Hot Jupiter with Quartz Crystal Clouds
WASP-17b is a tidally locked, blazing-hot gas giant that swings around its star once every 3.7 days. Its dayside temperatures are high enough to melt steel. And research published in October suggests that its twilight and dawn skies glitter with wispy clouds of fine, crystalline quartz.
“They form very high up in the atmosphere. So these are at very low pressures, and they’re forming really, really small particles,” said astrophysicist David Grant of the University of Bristol, who was part of the team that discovered the quartz clouds. “So you can imagine this really fine distribution of tiny silicon dioxide crystals.”
WASP-17b is an ideal place to go looking for alien clouds. It’s hot, it’s nearby, and its atmosphere is unusually inflated or “puffy”—although this scorched world weighs in at just about half the mass of Jupiter, it’s about twice as wide. All those characteristics make it a perfect target for transit spectroscopy. But although previous studies revealed telltale signs of clouds in WASP-17b’s atmosphere, it wasn’t until now, with new data from JWST, that researchers were able to directly measure its clouds’ composition.
“Clouds made of silicon dioxide—that’s kind of wild in itself,” said Grant. And if the quartz-dominated spectrum can be confirmed, said Min, the clouds on WASP-17b would be quite distinct from the sandy silicate clouds of WASP-107b, which seem to be dominated by other minerals.
—Elise Cutts (@elisecutts), Science Writer
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