Earth has Clouds of Water. Hot Exoplanets Have Clouds of Sand

A team of astronomers studied brown dwarfs to figure out how hot exoplanets form clouds of sand. They found that sand clouds can only exist in a narrow range of temperatures.

Water clouds are a common occurrence on Earth. And for our planet, making clouds is a relatively straightforward process. Water exists as either a solid or liquid on the surface. Sometimes it gets too hot and vaporizes, turning into a free-floating gas in the atmosphere. But then the water cools and gets trapped, forming clouds.

The Earth can play this trick with water because it has just the right temperatures. With different temperatures and chemistries, other planets can form other kinds of clouds. For example, Jupiter is famous for its ammonia-rich cloud layers.

Astronomers have long suspected that some exoplanets form clouds of silicates, the same minerals that make up the sand and rocks of the Earth’s crust. The process for making sand clouds is the same as for water clouds, just with the temperatures ramped up.

To examine this more carefully, a team of astronomers studied old data from the retired Spritzer Space Telescope. They looked at brown dwarfs, a class of astronomical object that sits between planets and stars. Most importantly for this research, brown dwarfs are naturally hot enough to support sand clouds.

Previous studies have shown hints of detections of sand clouds, but found nothing conclusive. In the new research, the astronomers grouped individual measurements of brown dwarfs together, combining the data.

With the combined data, the astronomers found conclusive evidence for sand clouds in brown dwarfs. But the brown dwarfs only supported sand clouds if they had a temperature cooler than 3,100 degrees Fahrenheit (about 1,700 degrees Celsius) and warmer than 1,900 F (1,000 Celcius).

If the brown dwarfs are too cool, the silicates can’t vaporize, and if they’re too warm, the silicates can condense into clouds.

Some exoplanets are in the same temperature range, and based on this research the astronomers believe that those worlds are likely to host sand clouds as well.

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