Brown dwarf W0855 was already special. A few times the size of Jupiter and super-cold, it’s halfway between a star and free-floating planet. Now ice clouds have been tentatively found in its atmosphere—which would mark the first time they’ve ever been seen on an extrasolar world.
The solar system’s fourth-nearest companion doesn’t make it easy. It’s so faint that “I wanted to put on Rocky, do a Braveheart speech to the telescope operators,” said study author Jacqueline Faherty, who used the Las Campanas Observatory in Chile. She is the first astronomer to observe W0855 from the ground since it was found in data from NASA’s space-going Wide-Field Infrared Explorer (WISE) in April.
Faherty’s work, which will be published in the Astrophysical Journal Letters, measured W0855’s brightness in different color bands. When compared with simulations of likely brown dwarf atmospheres, these data suggest W0855 boasts clouds of water ice and sulfide.
On Earth, high-altitude cirrus clouds offer a point of comparison. Unlike cumulus clouds, which can contain both water vapor droplets and ice, cirrus clouds are composed of just ice crystals. Brown dwarf atmospheres are so cold and low-pressure that clouds there would form in much the same way, said astronomer Caroline Morley, whose published models were used by Faherty.
Yet Morley and other astronomers unaffiliated with the study warn that this discovery is preliminary. “This tentative detection is made just with a few [brightness] points,” Morley wrote in an email. And Edward Wright, who studied W0855 with WISE, is skeptical that drawing conclusions from Morley’s models is the right idea. “The clouds depend on interpreting models which aren’t necessarily very good,” he said.
It’s not that the presence of ice clouds would be shocking—just that they might not have been found yet. Kevin Luhman, who first discovered W0855, is also unconvinced. He wrote via email that, “there’s another set of cloudless models that she did not consider, and they actually agree well with her data.”
According to these objectors, Faherty’s assumptions aren’t unreasonable. But her results depend on the brown dwarf having the same chemical blend as the Sun and on it being in chemical equilibrium—dependencies her paper also acknowledges.
Regarding the cloud-free alternatives Luhman mentions, said Faherty, no “valid” models currently exist for comparison. Not only is the physics behind those other models unpublished, but the modelers themselves have lost confidence in their work, she said.
All agree that NASA’s forthcoming James Webb Space Telescope will settle the question. The Webb’s coveted infrared sensitivity will let astronomers measure W0855’s whole spectrum, not just a few colors.
For now, at least, Faherty is grateful even to find W0855 at new wavelengths and push the discussion forward. “It’s so faint that it’s at the limits, at the very hairy edge of what you can do from the ground,” she said. Her struggles with half-star, half-planet W0855 tease an even harder next step: understanding the atmospheres of planets orbiting faraway stars.