Brown dwarfs—celestial objects that fall between stars and planets—are shown in this illustration with a range of temperatures, from the hottest (left) to the coldest (right). The two in the middle represent those in the correct temperature range to form clouds made of silicates. Credit: NASA/JPL-Caltech
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 appears in 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 fits into 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 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.
Murky 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 voila – 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.
Silicate clouds can be visible in a brown dwarf’s atmosphere, but only when the brown dwarf is cooler than about 3,100 degrees Fahrenheit (about 1,700 degrees Celsius) and warmer than 1,900 F (1,000 C). Too hot and the clouds evaporate; too cold and they turn to rain or sink lower in the atmosphere. Credit: NASA/JPL-Caltech
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.
“We had to dig into the Spitzer data to find these brown dwarfs where there was some indication of silicate clouds, and we really didn’t know what we were going to find,” said Genaro Suarez, a postdoctoral researcher at Western University and lead author of the new study. “We were very surprised how strong the conclusion was once we had the right data to analyze.”
In atmospheres hotter than the upper end of the range identified in the study, the silicates remain vapor. Below the low end, the clouds will turn to rain or sink lower in the atmosphere where the temperature is higher.
In fact, researchers believe that silicate clouds exist deep in Jupiter’s atmosphere, where the temperature is much higher than it is at the top due to atmospheric pressure. Silicate clouds cannot rise higher because at lower temperatures the silicates will solidify and not remain in cloud form. If the upper atmosphere were thousands of degrees hotter, the planet’s ammonia and ammonium hydrosulfide clouds would evaporate and silicate clouds could potentially rise to the top.
Scientists are discovering an increasingly diverse menagerie of planetary environments in our galaxy. For example, they have discovered planets with one side permanently facing their star and the other permanently in shadow – a planet where clouds of different composition can be seen depending on the side viewed. To understand these worlds, astronomers will first need to understand the general mechanisms that shape them.
Scientists improve weather predictions for brown dwarfs More information: Genaro Suárez et al, Ultracool dwarfs observed with the Spitzer infrared spectrograph. II. Occurrence and Precipitation of Silicate Clouds in L Dwarfs and Analysis of the Complete M5–T9 Field Spectroscopic Sample of Dwarfs, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1205 Courtesy of Jet Propulsion Laboratory
Citation: NASA helps decipher how some distant planets have sand clouds (2022, July 7) Retrieved July 7, 2022, from
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