Most clouds on Earth are made of water, but beyond our planet they come in many chemical varieties. The upper atmosphere of Jupiter, for example, is covered in yellow clouds made of ammonia and ammonium hydrosulfide. And on worlds outside our solar system there are clouds made up of silicates, the family of rock-forming minerals that make up over 90% of Earth’s crust. But researchers have not been able to observe the conditions under which these clouds of tiny dust grains form.
A new study published in the Monthly Notices of the Royal Astronomical Society provides some insight: The study reveals the temperature range where silicate clouds can form and be seen in the upper atmosphere of a distant planet. The discovery came from observations by NASA’s retired Spitzer Space Telescope of brown dwarfs—celestial bodies that fall between planets and stars—but it fits into a broader understanding of how planetary atmospheres work.
“Understanding the atmospheres of brown dwarfs and planets where silicate clouds can form can also help us understand what we would see in the atmosphere of a planet that is closer in size and temperature to Earth,” said Stanimir Mechev, professor of exoplanet research at Western University in London, Ontario and co-author of the study.
Cloud chemistry
The steps to create each type of cloud are the same. First, heat the key ingredient until steaming. Under the right conditions, this ingredient can be a variety of things, including water, ammonia, salt, or sulfur. Trap it, cool it enough to condense, and voilà – clouds! Of course, rock evaporates at a much higher temperature than water, so silicate clouds are only seen on hot worlds, like the brown dwarfs used for this study and some planets outside our solar system.
Although they form like stars, brown dwarfs are not massive enough to start fusion, the process that makes stars shine. Many brown dwarfs have atmospheres almost indistinguishable from those of gas-dominated planets such as Jupiter, so they can be used as proxies for these planets.
Prior to this study, Spitzer data had already suggested the presence of silicate clouds in a handful of brown dwarf atmospheres. (NASA’s James Webb Space Telescope will be able to confirm these types of clouds on distant worlds.) This work was done during the first six years of the Spitzer mission (which launched in 2003), when the telescope operated with three cryogenically cooled instruments. In many cases, however, the evidence for silicate clouds on brown dwarfs observed by Spitzer was too weak to stand on its own.
For this latest study, the astronomers collected more than 100 of these marginal findings and grouped them by brown dwarf temperature. They all fall within the predicted temperature range for where silicate clouds should form: between about 1,900 degrees Fahrenheit (about 1,000 degrees Celsius) and 3,100 F (1,700 C). Although the individual detections are minor, together they reveal a definitive feature of silicate clouds.
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