Using the NASA/ESA/CSA James Webb Space Telescope, astronomers have detected a brown dwarf with infrared emission from methane, likely due to energy in its upper atmosphere. The upper-atmosphere heating that powers this emission is linked to aurorae. Named W1935, this brown dwarf resides at a distance of 47 light-years.
On Earth, aurorae 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 eerie, dancing curtains of light.
Jupiter and Saturn have similar auroral processes that involve interacting with the solar wind, but they also get auroral contributions from nearby active moons like Io (for Jupiter) and Enceladus (for Saturn).
“For isolated brown dwarfs like W1935, the absence of a stellar wind to contribute to the auroral process and explain the extra energy in the upper atmosphere required for the methane emission is a mystery,” said Dr. Jackie Faherty, an astronomer at the American Museum of Natural History, and colleagues.
Dr. Faherty and colleagues used Webb to observe a sample of 12 cold brown dwarfs.
Among those were W1935, an object that was discovered by citizen scientist Dan Caselden, who worked with the Backyard Worlds Zooniverse project, and W2220, an object that was discovered using NASA’s Wide Field Infrared Survey Explorer.
Webb revealed in exquisite detail that W1935 and W2220 appeared to be near clones of each other in composition.
They also shared similar brightness, temperatures, and spectral features of water, ammonia, carbon monoxide, and carbon dioxide.
The striking exception was that W1935 showed emission from methane, as opposed to the anticipated absorption feature that was observed toward W2220. This was seen at a distinct infrared wavelength to which Webb is uniquely sensitive.
“We expected to see methane because methane is all over these brown dwarfs,” Dr. Faherty said.
“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 astronomers used computer models to infer what might be behind the emission.
The modeling work showed that W2220 had an expected distribution of energy throughout the atmosphere, getting cooler with increasing altitude.
W1935, on the other hand, had a surprising result. The best model favored a temperature inversion, where the atmosphere got warmer with increasing altitude.
“This temperature inversion is really puzzling,” said Dr. Ben Burningham, an astronomer at the University of Hertfordshire.
“We have seen this kind of 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.”
For clues, the researchers looked in our own backyard, to the planets of our Solar System.
The gas giant planets can serve as proxies for what is seen going on 47 light-years away in the atmosphere of W1935.
The scientists realized that temperature inversions are prominent in planets like Jupiter and Saturn.
There is still ongoing work to understand the causes of their stratospheric heating, but leading theories for the Solar System involve external heating by aurorae and internal energy transport from deeper in the atmosphere (with the former a leading explanation).
According to the team, W1935 is the first auroral candidate outside the Solar System with the signature of methane emission.
It’s also the coldest auroral candidate outside our Solar System, with an effective temperature of about 200 degrees Celsius (400 degrees Fahrenheit).
In our Solar System, the solar wind is a primary contributor to auroral processes, with active moons like Io and Enceladus playing a role for planets like Jupiter and Saturn, respectively.
W1935 lacks a companion star entirely, so a stellar wind cannot contribute to the phenomenon. It is yet to be seen whether an active moon might play a role in the methane emission on W1935.
“With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help in the explanation,” Dr. Faherty said.
“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.”
The authors presented their findings this week at AAS243, the 243rd meeting of the American Astronomical Society in New Orleans, the United States.
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Jacqueline Faherty et al. 2024. JWST Indicates Auroral Signature in an Extremely Cold Brown Dwarf. AAS243, abstract #4359