Cloudy exoplanets are fairly common, with many worlds around other stars featuring continuous cloud banks and belts. Now, for the first time, researchers have spotted clouds actually forming and dissipating, changing on timescales of only a few hours. The salt-rich clouds of potassium chloride and sodium chloride on Kepler-434 b condense and rain down on their planet only to evaporate back into gases, rising up in the atmosphere to begin the cycle again.
Researchers have spotted similar cloud changes on other planets that orbit their stars in ellipses, and they suspect the process is widespread. With their relatively high eccentricities, these exoplanets may provide insights into how clouds wax and wane on other worlds.
“These eccentric planets are just these natural laboratories where you’re not changing the planet, you’re just slowly cranking up the heat and seeing what happens,” said Jason Dittmann, an exoplanet researcher at the Massachusetts Institute of Technology. “This is almost as close as a laboratory experiment as you can get.”
Dittmann presented the research at the winter meeting of the American Astronomical Society in Honolulu, Hawaii.
Clouds of Kepler-434 b
Dittmann scoured data from NASA’s planet-hunting Kepler mission to investigate Kepler-434 b, approximately 3 times the mass of Jupiter and the only known planet in its system. Dittmann noted that Kepler-434 b had an unusual light fingerprint, and follow-up observations from the ground revealed that the exoplanet’s path is slightly stretched into an ellipse, changing the amount of radiation it receives over its nearly 13-day orbit.
Kepler-434 b’s strange light signals shift in relation to its orbit. When the planet is close to the star, it glows brightly, then fades as it moves away. Dittmann determined that the changing brightness is caused by clouds forming and dissipating with the planet’s shifting temperatures.
When Kepler-434 b is far from its star, temperatures are cool, and gas in the atmosphere collects around solid particles to form fairly stable clouds. As the planet draws closer and temperatures increase, the clouds dissipate into gas in the atmosphere, allowing researchers to observe the bright temperatures of its glowing surface.
“It’s like the sun comes up and the fog goes away,” Dittmann said.
When Kepler-434 b is orbiting in close proximity to its star, its clouds grow too massive to maintain their integrity. At this point, they begin to rain. Because Kepler-434 b is a gas giant, the solid salts don’t fall to solid ground. Instead, they rain into the planet’s interior, where rising temperatures can cause them to evaporate. The rising gas returns to the atmosphere and begins the process again.
“Very Different Types of Clouds”
The new observations are intriguing, according to Laura Kreidberg, a researcher who studies exoplanet atmospheres at Harvard University. Kreidberg is not a part of the Kepler-434 b research. “Characterizing the atmosphere of planets beyond the solar system is difficult in the best of times,” she said. “Detecting the signal of a cloud at all is a major achievement. Detecting variability goes a step further beyond that.”
Dittmann plans to continue to scour the Kepler data in search of other planets with changing cloud cover. “We think this could be a general thing,” he said.
Observing multiple worlds with changing clouds can help researchers to draw better conclusions about the widespread process of cloud formation for distant planets.
“The physics and chemistry in these atmospheres are so complicated that it’s hard to make a statement about just one,” Kreidberg said.
In addition, Kreidberg hopes that theorists will begin to construct models that can explain in greater detail what is happening in the atmosphere of Kepler-434 b. “None of the solar system planets have changes in temperatures like what this planet is experiencing,” she said. “These are very different types of clouds from the Earth and other planets in the solar system.”
—Nola Taylor Redd (@NolaTRedd), Science Writer