Scientists Find Elusive Cloud-forming Chemical on a Brown Dwarf Called ‘The Accident’

New observations of a brown dwarf nicknamed The Accident, which is about 50 light-years from Earth, have found a cloud-forming molecule that hasn’t been seen on any other brown dwarf, exoplanet, or solar system object to date, helping scientists understand the hidden chemistry of Jupiter and Saturn and possibly other gas giant worlds. 

The discovery—detailed in the journal Nature and led by Associate Curator Jackie Faherty in the Museum’s Department of Astrophysics—shows evidence of a simple silicon molecule called silane. One of the most common elements in the universe, and thought to be key to the formation of clouds on gas giants, silane has gone largely undetected in the atmospheres of Jupiter and Saturn and in planets like them around other stars. 

The finding was made on a peculiar object that astronomers discovered by chance in 2020 by a collaboration with one of the volunteers in the NASA-funded Backyard Worlds: Planet 9 citizen science project, which the Museum co-founded. 

Brown dwarfs are celestial objects that are more massive than planets but lighter than stars. Called “The Accident,” this particularly unusual brown dwarf has a perplexing mix of physical features, some of which have been previously seen in only young brown dwarfs and others seen only in ancient ones. Earlier studies on The Accident found that it likely formed between 10 billion and 12 billion years ago, making it one of the oldest brown dwarfs ever discovered. 

The Accident is so faint and odd that researchers needed powerful ground- and space-based telescopes to further investigate it, first with the National Science Foundation-funded NOIRLab Gemini South telescope in Chile and then with NASA’s James Webb Space Telescope (JWST). Among several surprises, they found evidence of silane, which is silicon bonded with four hydrogen atoms. 

“The Accident was such a fascinating discovery, but it wasn’t until we saw the JWST data that we could convince ourselves of what we had found,” Faherty said. “We knew it would be special, but we didn’t realize it would be such an extraordinary chemical revelation about ancient planet-like objects.” 

Researchers have long predicted that silane exists in our solar system’s gas giants, Jupiter and Saturn, and by extension it should be present in the atmospheres of the thousands of brown dwarfs and extrasolar gas giant planets discovered in the Milky Way. But The Accident is the first such object where this molecule has been identified. 

“Sometimes it’s the extreme objects that help us understand what’s happening in the average ones,” Faherty said.

The universe is about 14 billion years old. Dated to 10 to 12 billion years old, The Accident formed at a time when hydrogen and helium were dominant, with trace amounts of other elements, including silicon. Over time, elements like carbon, nitrogen, and oxygen formed in the cores of stars, so planets and stars that formed more recently contain more of those elements.

The discovery of silane in The Accident’s atmosphere suggests that, in very old objects, silicon can bond with hydrogen to form a light molecule that rises to the upper layers of a gas giant’s atmosphere. But in younger objects, like Jupiter and Saturn, the silicon bonds with the more readily available oxygen, creating heavier molecules that sink deep below the surface of the atmosphere, where they are hidden from the view of telescopes.

The findings support astronomers’ theories about cloud formation on gas giants and demonstrate how a planet’s early history affects the composition of its atmosphere. 

“Learning about worlds beyond our solar system is difficult, but this discovery about a cloud precursor in the atmosphere of an ancient brown dwarf just gave us a new insight into a part of giant worlds that we’ve never had access to before,” said Genaro Suárez, a postdoctoral fellow in the Museum’s Department of Astrophysics and a coauthor on the study.