According to Stanimir Metchev of the University of Western Ontario in London, Canada, this is the first full contextual understanding of any cloud outside the solar system. Metchev's colleague Genaro Suárez presented the new research on July 4 at the Cool Stars meeting in Toulouse, France.
There are many types of clouds in our solar system, from the puffs of water vapor on Earth to the bands of ammonia on Jupiter. Observations have also revealed the presence of "extrasolar clouds" on planets out of our solar system (SN: 9/11/19).
In contrast, only extrasolar clouds have been directly detected in the skies of brown dwarfs - dim, ruddy orbs that are too large to be planets but too small and cool to be stars. In 2004, NASA's Spitzer Space Telescope observed brown dwarfs and spotted spectral signatures of sand - more specifically, quartz and olivine grains. 2006 and 2008 also saw some tentative examples of sand clouds.
Mark Marley, a planetary scientist at the University of Arizona in Tucson, who was involved in one of those early discoveries, says floating in one of these clouds is like being in a sandstorm. A scoop of hot sand could be brought home if you took a scoop out of it."
At the time, astronomers found six examples of these silicate clouds. Marley says, "I kind of thought that was it." In theory, there should be a lot more brown dwarfs with sandy skies than six. A part of the Spitzer telescope ran out of coolant in 2009, so it was no longer able to measure the chemistry of similar clouds.
As Suárez explored archived Spitzer data for a different project, he discovered that there were unpublished or unanalyzed records for dozens of brown dwarfs. In the July Monthly Notices of the Royal Astronomical Society, the team reports that he analyzed all of Spitzer's low-mass stars and brown dwarfs, 113 objects in total, 68 of which had never been published before.
Marley finds it impressive that this was hidden in plain sight.
There were some brown dwarfs in the sample that did not show strong signs of silicate clouds. Together, the brown dwarfs followed a clear pattern. When a dwarf or low-mass star is hotter than about 1700 Celsius, silicates exist as a vapor, and there are no clouds to be seen. Under that temperature, clouds begin to form, becoming thickest around 1300 C. As the clouds sink deep into the atmospheres, the signal disappears for brown dwarfs cooler than about 1000 C.
It confirms previous suspicions that silicate clouds are widespread and reveals how they form. Since brown dwarfs are born hot and cool down over time, they should see all phases of sand cloud evolution. He notes that future research can extend the results to better understand the atmospheres of planets such as Jupiter.
In addition to studying atmospheric chemistry on exoplanets and brown dwarfs, the James Webb Space Telescope will also look for clouds (SN: 10/6/21).Marley hopes to combine the trends from this study with future results from JWST.
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