Webb finds signs of possible aurora on isolated brown dwarf

Astronomers using NASA’s James Webb Space Telescope have discovered a brown dwarf (an object more massive than Jupiter but smaller than a star) with infrared emission from methane, likely due to energy from its upper atmosphere. This is an unexpected discovery because the brown dwarf, W1935, is cold and has no host star; therefore, there is no obvious source of energy in the upper atmosphere. The team speculates that the methane emission could be due to aurora-generating processes.

These results are presented at the 243rd meeting of the American Astronomical Society in New Orleans.

To help explain the mystery of methane’s infrared emission, the team looked to our solar system. Methane emissions are a common feature of gas giants like Jupiter and Saturn. The warming of the upper atmosphere which fuels this emission is linked to the aurora.

On Earth, auroras are created when energetic particles blown into space from the sun are captured by Earth’s magnetic field. They cascade down into our atmosphere along magnetic field lines near Earth’s poles, colliding with gas molecules and creating strange dancing curtains of light. Jupiter and Saturn have similar auroral processes that involve interaction with the solar wind, but they also receive auroral contributions from nearby active moons like Io (for Jupiter) and Enceladus (for Saturn).

For isolated brown dwarfs like W1935, the absence of stellar wind to contribute to the auroral process and explain the additional energy in the upper atmosphere required for methane emission is a mystery. The team speculates that unaccounted for internal processes, such as the atmospheric phenomena of Jupiter and Saturn, or external interactions with interstellar plasma or a nearby active moon could help explain this emission.

A detective story

The discovery of the aurora unfolded like a detective novel. A team led by Jackie Faherty, an astronomer at the American Museum of Natural History in New York, was granted time with the Webb telescope to study 12 cool brown dwarfs. Among these were W1935, an object discovered by citizen scientist Dan Caselden, who worked with the Backyard Worlds Zooniverse project, and W2220, an object discovered using NASA’s Wide Field Infrared Survey Explorer.

Webb revealed in exquisite detail that W1935 and W2220 appeared to be close clones of each other in composition. They also shared similar brightness, temperatures, and spectral characteristics of water, ammonia, carbon monoxide, and carbon dioxide. The striking exception was that W1935 showed methane emission, as opposed to the expected absorption feature seen around W2220. This was observed at a distinct infrared wavelength to which Webb is particularly sensitive.

“We expected to see methane because methane is everywhere on these brown dwarfs. But instead of absorbing light, we saw the exact opposite: methane was glowing. My first thought was: what what is happening? Why is the methane emission coming out of this object??” Faherty said.

The team used computer models to deduce what might be causing the emission. Modeling work showed that W2220 had an expected distribution of energy throughout the atmosphere, cooling with increasing altitude. W1935, on the other hand, had a surprising result. The best model favored a temperature inversion, where the atmosphere warmed with increasing altitude.

“This temperature inversion is really puzzling,” said co-author Ben Burningham of the University of Hertfordshire in England and lead modeler of the work. “We’ve observed this kind of phenomenon on planets with a nearby star that can heat the stratosphere, but seeing it in an object with no obvious external heat source is crazy.”

Clues from our solar system

To find clues, the team searched our own backyard for planets in our solar system. The gas giant planets may serve as a proxy for what is happening more than 40 light years away in the atmosphere of W1935.

The team realized that temperature inversions are important on planets like Jupiter and Saturn. Work is still underway to understand the causes of their stratospheric warming, but the main theories about the solar system involve external heating by auroras and internal energy transport from deep in the atmosphere (the former being the main explanation).

Brown dwarf aurora candidates in context

This is not the first time an aurora has been used to explain an observation of a brown dwarf. Astronomers have detected radio emissions from several hotter brown dwarfs and cited auroras as the most likely explanation. Research has been conducted with ground-based telescopes like the Keck Observatory to look for infrared signatures of these radio-emitting brown dwarfs to further characterize the phenomenon, but has not been conclusive.

W1935 is the first auroral candidate outside the solar system with signature methane emission. It is also the coldest auroral candidate outside our solar system, with an effective temperature of about 400 degrees Fahrenheit (200 degrees Celsius), or about 600 degrees Fahrenheit warmer than Jupiter.

In our solar system, the solar wind is one of the main contributors to auroral processes, with active moons like Io and Enceladus playing a role for planets like Jupiter and Saturn, respectively. W1935 has absolutely no companion star, so a stellar wind cannot contribute to the phenomenon. It remains to be seen whether an active moon could play a role in W1935’s methane emission.

“With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help explain it,” Faherty noted. “With Webb, we can really ‘open the hood’ of chemistry and discover how similar or different the auroral process may be beyond our solar system,” she added.