The Physics of Raining Sand

A new paper presents an analysis of 113 red and brown dwarf stars that discovered silicate clouds in the cool brown dwarfs

Beth Johnson
3 min readAug 19, 2022


IMAGE: Brown dwarfs — celestial objects that fall between stars and planets — are shown in this illustration with a range of temperatures, from hottest (left) to coldest (right). The two in the middle represent those in the right temperature range for clouds made of silicates to form. CREDIT: NASA/JPL-Caltech

As we find more and more worlds across our galaxy, researchers are focusing more and more on what kinds of environments can support life and what kinds of weather can make existence harder or easier. And it turns out that there are conditions out there that create weather that not even sci-fi writers had led me to expect.

On objects ranging from gas giants to brown dwarfs, there are atmospheric layers that are so warm that stuff that sand is made of — silicates — can form atmospheric layers. And, just like water vapor in the atmosphere can turn into drops that fall as rain, it is possible for the small dust grains to glom together and fall from the sky as pieces of sand.

While it’s believed that Jupiter has this kind of a lawyer deep in its atmosphere, we don’t have the ability to see that layer, so we can’t study that layer directly. To see what is going on, we need warmer objects, and to find those objects, researchers went hunting through the Spitzer Space Telescope archive looking for brown dwarfs. These objects are tens of times larger than Jupiter and may temporarily burn heavy hydrogen — deuterium and tritium — in their cores, but for most of their lives, they just hang around as this weird transition object that isn’t really a planet and kind of failed at being a star.

In a new paper in Monthly Notices of the Royal Astronomical Society, researchers Genaro Suarez and Stanimir Metchev describe their analysis of 113 red and brown dwarfs. They found that it is in the cool brown dwarfs where silicate clouds can be seen. In these 1000–1700 degrees Celsius stars, these spectra show clear evidence of vaporized rock clouds.

This is one of those delightful research projects where a young researcher went searching through a data archive to see if something that was expected to exist could actually be found. According to Suarez: We had to dig through the Spitzer data to find these brown dwarfs where there was some indication of silicate clouds, and we really didn’t know what we would find. We were very surprised at how strong the conclusion was once we had the right data to analyze.

Moral? Great science — and raining silicate — can be found in data archives, just waiting for a researcher to search for the right parameters.

More Information

NASA press release

Ultracool dwarfs observed with the Spitzer infrared spectrograph — II. Emergence and sedimentation of silicate clouds in L dwarfs, and analysis of the full M5–T9 field dwarf spectroscopic sample,” Genaro Suárez and Stanimir Metchev, 2022 May 2, Monthly Notices of the Royal Astronomical Society

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Beth Johnson

Planetary scientist, podcast host. Communication specialist for SETI Institute and Planetary Science Institute. Buy me a coffee: