Among the many exoplanets we’ve discovered in the Milky Way, there’s a special class called hot Jupiters, gas giant planets orbiting so close to their stars that their temperatures reach thousands of degrees.
These exoplanets are also tidally locked, having one face constantly facing the star while the other is in perpetual darkness and covered in clouds. A new study, published in the Astrophysical Journal, focuses on the twilight regions and their cloud coverages.
The American team looked at the phase of the exoplanets, and how their light changed as they rotate around their star. For the first time, they were able to match real observations of exoplanet light curves from NASA's Kepler telescope with models of how the clouds should be distributed around these hot Jupiters.
This was possible thanks to the stark difference in temperature between the relatively cool night-side and the scorching day-side, which can have temperatures between 700°C and 1,900°C (1,300 and 3,400 °F).
"The day-night radiation contrast is, in fact, easy to model," said lead author Vivien Parmentier, a postdoctoral researcher at the University of Arizona, Tucson, in a statement. “[The hot Jupiters] are much easier to model than Jupiter itself,”
The team confirmed that for some reason clouds tend to accumulate on the west side of the planets' dayside compared to their east side, and by studying all the exoplanets that have shown this behavior (and their temperatures), the team was able for the first time to find out what the clouds are made of.
"Cloud composition changes with planet temperature," Parmentier added.
"The offsetting light curves tell the tale of cloud composition. It’s super interesting because cloud composition is very hard to get otherwise."
The clouds are not made of water vapor but of exotic condensates like aluminum oxide, manganese sulfide, or even iron. Sulfide clouds might be dominant for cooler hot Jupiters, but the hottest planets will probably have oxides or metallic clouds around them.
Although we have discovered thousands of them, exoplanets remain very mysterious and in most cases, like this one, they are completely different from what we see in the Solar System.