Auroras are among the most captivating phenomena on Earth, brightening polar skies with undulating ribbons of glowing charged particles. These eerie light shows have also been observed on many other planets in our solar system, leading astronomers to speculate that they might be as ubiquitous outside our stellar backyard as they are within it.
New research published today in Nature bolsters this theory with evidence of radiant auroral activity illuminating the skies of nearby brown dwarf LSR J1835+3259. Led by Caltech's Gregg Hallinan, the study demonstrates that brown dwarfs can produce auroras 10,000 times brighter than any seen so far.
"Over the last few years, there has been a lot of excitement for people who study brown dwarves," Hallinan told me over the phone, "because brown dwarves are somewhere in between stars and planets. There's always the question of whether they behave like stars or planets, because they are intermediate temperature and mass."
Bown dwarfs are typically about 15 to 75 times more massive than Jupiter—too small to undergo the nuclear fusion process necessary to produce starlight. But what these dim worlds lack in stellar cred, they make up for with dazzling auroral displays.
"We have a really powerful aurora on this brown dwarf, with evidence that a certain fraction of brown dwarfs have these [auroras]," Hallinan said regarding LSR J1835+3259, which is located about 20 light years away.
"There's a lot of energy being dumped on its atmosphere," he continued. "It may be that what's causing the aurora could also be causing some of the more extreme examples of weather variability observed on brown dwarfs. We're investigating that as we speak."
Studying auroral patterns on brown dwarfs is interesting on its own merits, because it could help clarify the mysterious internal dynamics of these substellar worlds. But with this work, Hallinan's team is also paving the way for the first detection of auroras on exoplanets. This will, in turn, narrow down the search for alien life, given that auroras can help measure the strength of a brown dwarf's magnetic field.
"The role of magnetic fields may be pretty important for planet habitability," Hallinan told me. "When our solar system was young, it was much more active and the planets were probably subjected to more intense solar winds and higher radiation content. Having a magnetic field may have acted as a shield against that activity."
"That may be the reason that we have water in our atmosphere whereas Venus does not, because we have a shield to protect us," he continued. "We want to learn more about magnetic fields—how pervasive they are in the universe, how important they may be in protecting planets capable of sustaining life, and what governs whether or not a planet has a magnetic field."
To that end, Hallinan has overseen the construction of a powerful new telescope called the Owens Valley Long Wavelength Array (OV-LWA), located near Bishop, California. The OV-LWA went online just a few months back, and is currently surveying the sky every few seconds looking for stars that are producing extreme solar wind and space weather. When it finds them, it searches for corresponding radio signatures from exoplanets lighting up with auroral glows generated by the flux in solar wind.
One hour of radio observation from OV-LWA. Credit: YouTube/Caltech
"What we've done is built a new kind of radio telescope that images the entire sky," said Hallinan, who is the principal investigator for the OV-LWA. "That way we can monitor every system continuously, looking for space weather."
"If we find [exoplanet auroras], I'll definitely be coming back to Nature," he added.
It's exciting to think that we may be on the cusp of witnessing the first exoplanetary auroras, not to mention that we have already detected brilliant auroras on brown dwarfs. Though these in-between worlds have developed a reputation for being "failed stars," this new research proves that when it comes to planetary phenomena like auroras, they are bonafide heavyweights.