Brown dwarfs - celestial objects that fall between stars and planets - are shown in this illustration with a range of temperatures, from hottest (left) to coldest (right). The two in the middle represent those in the right temperature range for clouds made of silicates to form. (Illustration by NASA/JPL-Caltech)
A new study led by researchers at Western University provides critical information on sand clouds observed in distant planets and helps affirm a larger theory of how planetary atmospheres work.
Most clouds on Earth are made of water, while the top of Jupiter’s atmosphere is blanketed in yellow-hued clouds made of ammonia and ammonium hydrosulfide. On worlds far beyond Earth, scientists know of clouds composed of silicates, the family of rock-forming minerals that includes the most common component of sand. But researchers haven’t been able to observe the conditions under which these clouds of small dust grains form, until now.
Published in the journal Monthly Notices of the Royal Astronomical Society , the research reveals the temperature range at which silicate clouds can form and are visible by telescopes at the top of a distant planet’s atmosphere. Although the finding was derived from observations by NASA’s retired Spitzer Space Telescope of ’brown dwarfs’ - celestial bodies that fall in between planets and stars - it offers a more general understanding of how planetary atmospheres work.
"Understanding the atmospheres of brown dwarfs or hot planets where silicate clouds can form can also help us understand what we would see in the atmosphere of a planet that’s closer in size and temperature to Earth," said Stanimir Metchev , Canada Research Chair in Extrasolar Planets at Western University and co-author on the study.
The steps to make any type of cloud are the same. First, heat the key ingredient until it becomes a vapour - with the right conditions, it could be a variety of things including water, ammonia, salt, or sulphur. Trap it, cool it just enough for it to condense, and clouds are formed. Of course, rock vaporizes at a much higher temperature than water, so silicate clouds form only on hot worlds, which include planets and the brown dwarfs used for this study.
Though brown dwarfs form like stars, they aren’t massive enough to kickstart fusion, the process that causes stars to shine. Many brown dwarfs have atmospheres almost indistinguishable from those on gas-dominated planets, such as Jupiter, so they can be used as an analogue for the other.
Before this study, Spitzer data had already found evidence suggesting the presence of silicate clouds in a handful of brown dwarf atmospheres. (Soon, 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, when the telescope was operating three cryogenically cooled instruments. In many cases, though, the evidence of silicate clouds on brown dwarfs observed by Spitzer was too weak to stand on its own.
For this latest research, Metchev and Western Science post-doctoral researcher Genaro Suárez gathered more than 100 of those marginal detections and grouped them by the temperature of the brown dwarf. All of them fell within the predicted temperature range for when silicate clouds should form (see image below): between about 1,900 Fahrenheit (about 1,000 C) and 3,100 F (1,700 C). So, while the individual detections are marginal, together they reveal a definitive trait of silicate clouds.
Silicate clouds may be visible in brown dwarf atmospheres, 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 vaporize; too cold, and they turn into rain or sink lower in the atmosphere. (Illustration by NASA/JPL-Caltech)
"We had to dig through the Spitzer data to find these brown dwarfs where there was some indication of silicate clouds, and we really didn’t know what we would find," said Suárez, 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 charting hotter than the top end of the range identified in the study, silicates remain a vapour. Below the bottom end, the clouds will turn into rain or sink lower in the atmosphere, where the temperature is higher.
In fact, Metchev and Suárez believe silicate clouds exist deep in Jupiter’s atmosphere, where the temperature is much higher than it is at the top, owing to atmospheric pressure. The silicate clouds can’t rise higher up, because at lower temperatures the silicates will solidify, and won’t remain in cloud form.
"If the top of the atmosphere were thousands of degrees hotter, the planet’s ammonia and ammonium hydrosulfide clouds would vaporize and the silicate clouds could potentially rise to the top," said Metchev, also a Western Institute for Earth & Space Exploration faculty member.
It is important to study brown dwarfs and their atmospheres because they are very similar to planets like Jupiter. Findings, like the recent discovery by Metchev and Suárez, provide significant data about the composition of gaseous planet atmospheres, which is a step forward in the understanding of the conditions for Earth-like planets around other stars.
"Our understanding of atmospheres in brown dwarf worlds could let us know how particular the Earth atmosphere is and, therefore, whether or not other worlds have conditions to host life as we know it on Earth," said Suárez.