Cloudy Nights

Cloudy Nights

We don’t have a single picture that shows us the surface of an exoplanet — a planet in a star system other than our own. Yet astronomers already know what we’d see if we could get a good look at one class of exoplanet: clouds on the night side, and clear skies on the day side.

These planets are known as hot Jupiters. They’re as big as Jupiter, the giant of our own solar system, and many are much heavier than Jupiter. They’re hundreds of millions of miles closer to their stars than Jupiter is to the Sun, so they’re extremely hot.

Astronomers have learned about the planets in several ways. They’ve watched as a planet passes in front of its star, allowing starlight to filter through the planet’s atmosphere. And they’ve used infrared telescopes to measure changes in heat coming from a system as the planet orbits its star.

These observations reveal that nighttime temperatures on hot Jupiters hover around a thousand degrees. Daytime temperatures are much hotter. That suggests that clouds form on the night side of such a planet, trapping some of its heat. But it’s just too hot on the day side, so the clouds vaporize as they rotate into the light.

The clouds aren’t like anything on Earth, though. Instead of water, they’re made of silicates — like grains of sand. Some even contain the minerals that make up rubies and sapphires — hot jewels on some sizzling-hot planets.

More about exoplanets tomorrow.

Script by Damond Benningfield

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