Astronomers using NASA‘s James Webb Space Telescope have unexpectedly discovered a brown dwarf, W1935, with infrared emission from methane. This is unusual because the brown dwarf is cold, lacks a host star, and has no apparent source for the upper atmosphere energy. The team speculates that the methane emission may be due to processes generating aurorae.
Methane emission in gas giants like Jupiter is linked to aurorae, and the team suggests a similar mechanism may be at play with the brown dwarf W1935.
The discovery of infrared emission from methane in the cold brown dwarf W1935, lacking a host star, poses a mystery. Typically, auroral processes are linked to a stellar wind power methane emission in gas giants like Jupiter. However, for isolated brown dwarfs without a stellar wind, the energy source in the upper atmosphere is unclear.
The team speculates that internal processes similar to those on Jupiter and Saturn or external interactions with interstellar plasma or a nearby active moon may explain the observed methane emission. Further studies are needed to unravel this intriguing phenomenon.
The discovery of auroral processes, like those seen in Jupiter and Saturn, in two brown dwarfs—W1935 and W2220—was an intriguing detective story. The objects were found to be near clones in composition, brightness, temperature, and spectral features, except for the unexpected methane emission in W1935.
While W2220 exhibited the anticipated absorption feature in methane, W1935 displayed emission. Aurorae-like processes in brown dwarfs lacking a stellar wind raise questions about the energy source in their upper atmospheres, hinting at internal or external mechanisms that differ from those in our solar system’s gas giants.
Jackie Faherty, an astronomer at the American Museum of Natural History in New York, said, “We expected to see methane because methane is all over these brown dwarfs. But instead of absorbing light, we saw just the opposite: The methane was glowing. My first thought was, what the heck? Why is methane emission coming out of this object?”
The team employed computer models to understand the cause of the methane emission. In the case of W2220, the expected distribution of energy throughout the atmosphere showed a cooling trend with increasing altitude. However, the model for W1935 indicated a temperature inversion, where the atmosphere becomes warmer with increasing altitude.
This unexpected inversion suggests the presence of unique processes or interactions that differ from conventional expectations, highlighting the complex and intriguing nature of these distant, isolated brown dwarfs.
Ben Burningham, a co-author from the University of Hertfordshire in England and lead modeler, said, “This temperature inversion is really puzzling. We have seen this phenomenon in planets with a nearby star that can heat the stratosphere, but seeing it in an object with no obvious external heat source is wild.”
The team turned to our solar system for analogs to unravel the mystery of the unexpected temperature inversion seen in the atmosphere of W1935. Planets like Jupiter and Saturn, which also exhibit temperature inversions, were considered proxies to gain insights into the distant brown dwarf’s atmospheric processes.
While the specific mechanisms behind stratospheric heating in our solar system’s gas giants are not fully understood, ongoing studies suggest potential contributions from external heating by aurorae and internal energy transport from deeper atmospheric layers, with auroral processes being a leading explanation.
W1935 is the first auroral candidate beyond our solar system to exhibit methane emission. Notably, it is also the coldest auroral candidate observed outside our solar system, boasting an effective temperature of approximately 400 degrees Fahrenheit (200 degrees Celsius).
This makes W1935 about 600 degrees Fahrenheit warmer than Jupiter, a significant distinction given its status as a cold, isolated brown dwarf without a host star. The unexpected methane emission detected in its upper atmosphere adds to this celestial object’s intrigue.
Faherty said, “With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help explain. With Webb, we can really ‘open the hood’ on the chemistry and unpack how similar or different the auroral process may be beyond our solar system.”
These findings are being presented at the 243rd meeting of the American Astronomical Society in New Orleans.