Most clouds on Earth are made up of water, but beyond our planet they come in many chemical varieties. The top of Jupiter’s atmosphere, for example, is covered in yellow clouds composed of ammonia and ammonium hydrosulfide. And on worlds outside our solar system, there are clouds composed of silicates, the family of rock-forming minerals that make up more than 90% of the earth’s crust. But the researchers were unable to observe the conditions under which these clouds of small dust grains form.
A new study published in the Monthly Notices of the Royal Astronomical Society provides insight: Research reveals the temperature range at which silicate clouds can form and are visible at the top of a distant planet’s atmosphere. The finding was derived from observations by NASA’s retired Spitzer Space Telescope of brown dwarfs – celestial bodies that sit between planets and stars – but it fits into a more general understanding of how atmospheres work. planetary.
“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 closer in size and temperature to Earth. “said Stanimir Metchev, professor of exoplanet studies at Western University in London, Ontario, and co-author of the study.
The steps to create any type of cloud are the same. First, heat the key ingredient until it becomes a vapor. Under the right conditions, this ingredient can be a variety of things, including water, ammonia, salt, or sulfur. Trap it, cool it just enough for it to condense, and voila – clouds! Of course, rock vaporizes at a much higher temperature than water, so silicate clouds are only visible on hot worlds, like the brown dwarfs used for this study and some planets outside of our solar system.
Although they form like stars, brown dwarfs aren’t massive enough to trigger 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, data from Spitzer 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 (launched in 2003), when the telescope operated three cryogenically cooled instruments. In many cases, however, the evidence for silicate clouds over brown dwarfs observed by Spitzer was too weak to stand on its own.
For this latest search, the astronomers gathered more than 100 of these edge detections and grouped them by the temperature of the brown dwarf. All were within the predicted temperature range for where the silicate clouds are expected to form: between about 1,900 degrees Fahrenheit (about 1,000 degrees Celsius) and 3,100 F (1,700 C). While the individual detections are marginal, together they reveal a definitive feature of silicate clouds.