There is a common trait between worlds with clouds

July 8. () –

A new study using archival observations from the now-retired Spitzer Space Telescope found a common trait among distant worlds where exotic clouds form.

Most clouds on Earth are made of water, but beyond our planet they come in many chemical varieties. The upper part of Jupiter’s atmosphere, for example, it is covered with 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 more than 90% of the Earth’s crust. But researchers have not been able to observe the conditions under which these clouds of tiny dust grains form.

The new research, which appears in Monthly Notices of the Royal Astronomical Society, provides insight: it reveals the temperature range in which silicate clouds can form and are visible in the upper atmosphere of a distant planet.

The finding stemmed from NASA’s retired Spitzer Space Telescope observations of brown dwarfscelestial bodies that fall between planets and stars, but it fits into a more general understanding of how planetary atmospheres work.

“Understanding the atmospheres of brown dwarfs and planets where silicate clouds can form it can also help us understand what we would see in the atmosphere of a planet closer in size and temperature to Earth“, said it’s a statement Stanimir Metchev, a professor of exoplanet studies at Western University in London, Ontario, and a co-author of the study.

The steps to make any type of cloud are the same. First, heat the key ingredient until it turns to steam. Under the right conditions, that ingredient could be a variety of things, like water, ammonia, salt, or sulfur. Catch it, cool it enough 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 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 a representation of those planets.

Before 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 these kinds 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 three cryogenically cooled instruments. However, in many cases, the evidence for silicate clouds in brown dwarfs observed by Spitzer was too weak to stand on its own.

For this latest investigation, astronomers pooled more than 100 of those fringe detections and grouped them by brown dwarf temperature. All of them fell within the predicted temperature range where silicate clouds should form: between about 1,000 and 1,700 degrees Celsius. While individual detections are marginal, together they reveal a definite feature of silicate clouds.

“We had to dig into the Spitzer data to find these brown dwarfs where there was any hint of silicate clouds, and we really didn’t know what we’d find,” said Genaro Suarez, a postdoctoral researcher at Western University and lead author of the new study. “We were very surprised at 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, silicates remain in vapor form. Below the lower end, the clouds will turn to rain or sink into the atmosphere, where the temperature is higher.

In fact, the researchers believe that silicate clouds exist deep in Jupiter’s atmosphere, where the temperature is much higher than at the top, due to atmospheric pressure. Silicate clouds cannot rise higher, because at lower temperatures the silicates will solidify and will not remain in cloud form. If the upper atmosphere were thousands of degrees warmer, the planet’s ammonia and ammonium hydrosulfide clouds would vaporize and silicate clouds could potentially rise to the top.

Scientists are finding an increasingly varied collection of planetary environments in our galaxy. For example, they have found planets with one side permanently facing their star and the other permanently in shadow, a planet where clouds of different compositions may be visible, depending on the side observed. To understand those worlds, astronomers will first need to understand the common mechanisms that shape them.