Most clouds on Earth are made of water, but beyond our planet there are many varieties of chemicals. The upper atmosphere of Jupiter, for example, is covered with yellow clouds made of ammonia and ammonium hydrosulfide. And in the worlds outside our solar system, there are clouds composed of silicates, the family of minerals that form rocks that make up more than 90% of the Earth’s crust. But researchers have been unable to observe the conditions under which these small clouds of dust grains form.
A new study appearing in the Royal Astronomical Society’s Monthly Notices provides insight: the research reveals the temperature range in which silicate clouds can form and are visible in the upper atmosphere of a distant planet. The finding was derived from observations of NASA’s retired Spitzer space telescope of brown dwarfs (celestial bodies that lie between planets and stars), but fits a more general 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 to Earth in size and temperature,” Stanimir Metchev said. , professor of exoplanet studies at Western. University of London, Ontario, and co-author of the study.
Cloudy chemistry
The steps to make any type of cloud are the same. First, heat the key ingredient until it becomes a steamer. Under the right conditions, this ingredient could be a variety of things, such as water, ammonia, salt or sulfur. Catch it, cool it enough for it to condense, and voila, clouds! Of course, the rock vaporizes at a much higher temperature than water, so silicate clouds are only visible in 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 initiate 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 a proxy for these planets.
Prior to this study, Spitzer data 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 this type of cloud in distant worlds.) This work was done during the first six years of the Spitzer mission (which was launched in 2003), when the telescope operated three refrigerated instruments. cryogenically. In many cases, however, the evidence of silicate clouds over brown dwarfs observed by Spitzer was too weak to stand alone.
For this latest research, astronomers collected more than 100 of these marginal detections and grouped them by the temperature of the brown dwarf. All of them were within the predicted temperature range where silicate clouds should form: between about 1,900 degrees Fahrenheit (about 1,000 degrees Celsius) and 3,100 F (1,700 C). While individual detections are marginal, together they reveal a definite feature of silicate clouds.