Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected methane emission from CWISEP J193518.59-154620.3 (W1935 for short), an isolated brown dwarf with a temperature of approximately 482 K. Their findings also suggest that W1935 might generate aurorae similar to those seen on our own planet as well as on Jupiter and Saturn.
W1935 is located around 47 light-years away in the constellation of Sagittarius.
The brown dwarf was co-discovered by Backyard Worlds: Planet 9 citizen science volunteer Dan Caselden and NASA’s CatWISE team.
The mass for W1935 isn’t well known but it likely ranges between 6 and 35 times the mass of Jupiter.
After looking at a number of brown dwarfs observed with Webb, Dr. Jackie Faherty from the American Museum of Natural History and colleagues noticed that W1935 looked similar but with one striking exception: it was emitting methane, something that’s never been seen before on a brown dwarf.
“Methane gas is expected in giant planets and brown dwarfs but we usually see it absorbing light, not glowing,” Dr. Faherty said.
“We were confused about what we were seeing at first but ultimately that transformed into pure excitement at the discovery.”
Computer modeling yielded another surprise: W1935 likely has a temperature inversion, a phenomenon in which the atmosphere gets warmer with increasing altitude.
Temperature inversions can easily happen to planets orbiting stars, but the brown dwarf is isolated, with no obvious external heat source.
“We were pleasantly shocked when the model clearly predicted a temperature inversion,” said Dr. Ben Burningham, an astronomer at the University of Hertfordshire.
“But we also had to figure out where that extra upper atmosphere heat was coming from.”
To investigate, the astronomers turned to our Solar System. In particular, they looked at studies of Jupiter and Saturn, which both show methane emission and have temperature inversions.
The likely cause for this feature on solar system giants is aurorae, therefore, the researchers surmised that they had uncovered that same phenomenon on W1935.
Planetary scientists know that one of the major drivers of aurorae on Jupiter and Saturn are high-energy particles from the Sun that interact with the planets’ magnetic fields and atmospheres, heating the upper layers.
This is also the reason for the aurorae that we see on Earth, commonly referred to as the northern or southern lights since they are most extraordinary near the poles.
But with no host star for W1935, a solar wind cannot contribute to the explanation.
There is an enticing additional reason for the aurora in our Solar System.
Both Jupiter and Saturn have active moons that occasionally eject material into space, interact with the planets, and enhance the auroral footprint on those worlds.
Jupiter’s moon Io is the most volcanically active world in the Solar System, spewing lava fountains dozens of miles high, and Saturn’s moon Enceleadus ejects water vapor from its geysers that simultaneously freezes and boils when it hits space.
More observations are needed, but the researchers speculate that one explanation for the aurora on W1935 might be an active, yet-to-be discovered moon.
“Every time an astronomer points Webb at an object, there’s a chance of a new mind-blowing discovery,” Dr. Faherty said.
“Methane emission was not on my radar when we started this project but now that we know it can be there and the explanation for it so enticing I am constantly on the look-out for it. That’s part of how science moves forward.”
A paper on the findings was published in the journal Nature.
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J.K. Faherty et al. 2024. Methane emission from a cool brown dwarf. Nature 628, 511-514; doi: 10.1038/s41586-024-07190-w
