Ellen Phiddian
The James Webb Space Telescope (JWST) has helped astronomers to spot aurorae with no obvious source on a brown dwarf.
The researchers, who are presenting their findings at the 243rd meeting of the American Astronomical Society, believe that an aurora is the most likely explanation for the strange methane emissions they’ve seen on the brown dwarf.
The US-based team was using JWST to investigate 12 brown dwarfs (objects that are usually larger than Jupiter, but much smaller than stars).
One – called W1935, just over 40 light-years away – had an unusual methane signature. Instead of methane absorbing light, it was emitting light.
“We expected to see methane because methane is all over these brown dwarfs,” says research lead Dr Jackie Faherty, an astronomer at the American Museum of Natural History in New York.
“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?”
On Earth, aurorae are caused by particles from the Sun hitting Earth’s magnetic field. But W1935 doesn’t orbit a star – leading to even more puzzles. It’s also a very cold brown dwarf, with a surface temperature of around 200°C.
With the help of computer modelling, the researchers suggested that the light emissions might be caused by a temperature inversion, where temperatures rise the higher you get in an atmosphere.
“This temperature inversion is really puzzling,” says lead modeller Dr Ben Burningham, from the University of Hertfordshire, UK.
“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.”
The researchers believe the temperature inversions may be caused by similar processes to the inversions on Saturn and Jupiter.
“With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help in the explanation,” says Faherty.
“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.”
