The surprising discovery by the James Webb Space Telescope (JWST) regarding methane emissions and the potential for auroras on a distant brown dwarf may indicate that this “failed star” is orbiting an active moon.
Using the James Webb Telescope, astronomers have made an astonishing discovery regarding methane emissions from a brown dwarf, also known as a “failed star.” They report that this finding suggests the brown dwarf has auroras and may even be orbited by an unexplored exomoon.
This discovery about the brown dwarf is particularly surprising because these cold and isolated worlds are thought to be too cool for methane to emit infrared light.
Brown Dwarfs and Auroras
Illustration of a brown dwarf and its infrared emissions as observed by the James Webb Telescope. (Image: NASA, ESA, CSA, LEAH HUSTAK, Space Telescope Science Institute)
These findings are the result of JWST’s investigation of 12 brown dwarfs. They suggest that these failed stars could produce auroras similar to the northern and southern lights on Earth, as well as the lights seen on Jupiter and Saturn.
The research team focused on the cold brown dwarf CWISEP J193518.59–154620.3 (W1935), located 47 light-years from Earth. Despite W1935’s limited mass, estimated to be between 6 to 35 times that of Jupiter, it is known to have a surface temperature of about 204 degrees Celsius, hot enough to bake.
Jackie Faherty, the lead researcher and senior education manager at the American Museum of Natural History, stated: “Methane is predicted to be present on giant planets and brown dwarfs, but we usually see it absorbing light rather than emitting it. Initially, we were quite surprised by what we saw, but we found this discovery very intriguing.”
Why Are Some Stars Considered “Failed”?
Brown dwarfs have earned the nickname “failed stars” because, although they form directly from a collapsing cloud of gas and dust like a star, they do not have enough mass to ignite nuclear fusion of hydrogen into helium in their cores.
Composite image of Jupiter captured by JWST’s NIRCam, showing the planet’s rings and two of its moons, Amalthea and Adrastea. The blue glow around Jupiter’s poles represents auroras. (Image: NASA, ESA, CSA, ERS Jupiter Team).
This process is what classifies a star as a main sequence star; thus, brown dwarfs are more massive than the largest planets but smaller than even the smallest stars.
While observing several brown dwarfs with JWST, Faherty and her colleagues noticed that W1935 was similar but had one intriguing difference: it was emitting methane, which has never been observed around a failed star before.
W1935’s model revealed that this particular brown dwarf also exhibits what is called “temperature inversion.” This phenomenon occurs when a planet’s atmosphere becomes colder at deeper levels. It is commonly seen in planets orbiting stars that heat their atmospheres from above, but this was not expected for W1935, as this brown dwarf is isolated and lacks an external heat source.
To solve this mystery, the research team closely examined the gas giants of our solar system, Jupiter and Saturn. Both of these gas giants exhibit methane emissions and have atmospheres that show temperature inversions.
The big question is: what is causing the auroras on W1935?
This is a dilemma because solar wind—a stream of charged particles from the sun—is the primary driver of auroras for Jupiter, Saturn, and Earth. These charged particles interact with the magnetic fields of the planets and funnel down field lines, interacting with particles in the atmosphere. This heats the upper layers of the atmosphere, causing light emissions near the planet’s poles. However, since W1935 lacks a host star generating stellar wind, this process cannot be the main driver of auroras for this solitary brown dwarf.
The auroras of W1935 exist without a star or wind. This suggests that the brown dwarf may be orbited by an active moon.
