James Webb Space Telescope detects possible aurora on a cool brown dwarf

NASA’s James Webb Space Telescope has detected possible aurora on a cool brown dwarf, W1935, which is located about 47 light-years away from the Earth.

Astronomers have found methane emissions coming from the atmosphere of brown dwarf W1935, which cause a possible aurora.

NASA's James Webb Space Telescope has detected possible aurora on a brown dwarf W1935
NASA’s James Webb Space Telescope has detected possible aurora on a brown dwarf W1935. (Image credit: NASA, ESA, CSA, Leah Hustak (STScI))

Brown dwarfs are failed stars that fall in the category between planets and stars in terms of mass. They are more massive than gas giant planets (e.g., Jupiter) but less massive than the smallest stars. Generally, their mass is between 13 and 80 times the mass of Jupiter.

Aurora is created in Earth’s atmosphere when solar wind (energetic charged particles from the sun) are captured by Earth’s magnetic field. When these charged particles fall near Earth’s poles and interact with gas molecules, we see spooky dancing lights called auroras (Northern Lights or Southern Lights).

However, the possible detection of an aurora on the brown dwarf W1935 is an unexpected discovery because the brown dwarf is cold and lacks a host star (like our sun). Therefore, there is no obvious source of energy to heat its upper atmosphere and make the methane glow to create an aurora.

Aurora is created on Jupiter and Saturn due to the solar wind, along with the contributions from nearby active moons like Io (for Jupiter) and Enceladus (for Saturn).

It is yet to be seen whether a nearby active moon or interstellar plasma might play a role in the methane emission on brown dwarf W1935.

Detection story

While studying 12 cold brown dwarfs using NASA’s James Webb Space Telescope, a team of astronomers led by Jackie Faherty found two brown dwarfs, W1935 and W2220, which are identical in composition, brightness, and temperature. However, the only difference was that W1935 showed emission from methane and W2220 showed absorption spectra.

Brown dwarf W1935 shows methane emission lines in its infrared spectra, while W2220 does not
Brown dwarf W1935 shows methane emission lines in its infrared spectra, while W2220 does not. (Image credit: NASA, ESA, CSA, Leah Hustak (STScI))

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, over 300 degrees Celsius warmer than Jupiter.

Jackie Faherty said in a statement, “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 has used computer models to find the possible reason behind this emission. For the brown dwarf W2220, they found an expected distribution of energy throughout the atmosphere, getting cooler with increasing altitude. However, for W1935, they found a surprising result: temperature inversion, where the atmosphere got warmer with increasing altitude.

“This temperature inversion is really puzzling,” said Ben Burningham, a co-author from the University of Hertfordshire in England and lead modeler on the work. “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.”  

These results were presented at the 243rd meeting of the American Astronomical Society.

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Ashim

Ashim Chandra Sarkar founded Space & Telescope in 2022. He holds a M.Sc. in physics and has five years of research experience in optical astronomy. His passion for astronomy inspired him to open this website. He is responsible for the editorial vision of spaceandtelescope.com.

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