Researchers have spotted daily weather cycles on six exoplanets. It was possible thanks to sensitive observations from the Kepler Space Telescope.

How were researchers able to figure out the weather on these exoplanets? They measured changes in the planets as they orbited their host star and identified the day-night cycle, according to Lisa Esteves, lead author of the study published in The Astrophysical Journal.

“We traced each of them going through a cycle of phases in which different portions of the planet are illuminated by its star, from fully lit to completely dark,” said Esteves.

Clouds formed on the night side where temperatures were cooler.

“As the winds continue to transport the clouds to the day side, they heat up and dissipate, leaving the afternoon sky cloud-free,” said Esteves. “These winds also push the hot air eastward from the meridian, where it is the middle of the day, resulting in higher temperatures in the afternoon.”

Of the six exoplanets, researchers observed excess brightness on the morning side in four of them. For the other two, researchers saw excess brightness when the evening side was visible.

Translation? The excess brightness on the four exoplanets is most likely due to reflected sunlight. Esteves says thermal emission is ruled out because they are not hot enough to generate the excess light through that process.

The other two planets’ excess brightness can be explained by thermal emission. “A likely explanation is that on these two planets, the winds are moving heat towards the evening side, resulting in the excess brightness,” said Esteves.

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None of these planets are good candidates for life. Most of them have temperatures greater than 1,600 degrees Celsius. But, their size (comparable to Jupiter) and heat makes them good targets for these kind of observations.

Future telescopes will be able to spot and observe smaller exoplanets. “Someday soon we hope to be talking about weather reports for alien worlds not much bigger than Earth, and to be making comparisons with our home planet,” said co-author Ray Jayawardhana.

Image: An artist rendering showing an exoplanet with cloudy mornings and extremely hot afternons. Credit: Lisa Esteves

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