The most powerful aurora ever seen in the universe has been found by a team of astronomers – and among its many important implications, it could help us to discover what role these peculiar “failed stars” play in the universe.
The remarkable discovery, published in the journal Nature, was made on a brown dwarf – an object between a planet and a star in size – 18 light-years from Earth. Called LSR J1835+3259, it was found to have an aurora 10,000 times more powerful than Jupiter’s, and one million times more powerful than Earth’s. It is the first time that an aurora has been spotted on a brown dwarf. Earth’s aurora appears green due to nitrogen and oxygen being present, but this aurora would be very red, owing to an abundance of hydrogen.
Using the Karl G. Jansky Very Large Array (VLA) in New Mexico, U.S., the team made the discovery by measuring radio emissions coming from the object. They also used optical data from the five-meter Hale Telescope on Palomar Mountain in San Diego and the 10-meter Keck Telescope in Hawaii.
Brown dwarfs are often referred to as failed stars because they did not gather enough mass to start nuclear fusion at their cores, but they are too large to be classed as planets, estimated to be between 13 and 80 Jupiter masses in size. Fully-fledged stars like the Sun do not have aurorae, mostly because the super-heated region of their atmosphere, known as the corona, stops them from forming. The discovery that brown dwarfs have aurorae therefore suggests that they are more planet-like than star-like.
“The question arises, where does stellar activity stop, and where does [auroral] activity start?” lead researcher Gregg Hallinan from the California Institute of Technology, and lead author on the paper, told IFLScience. Like aurorae on planets, this one is thought to cover a few percent of the brown dwarf. “It may also be confined to the polar regions, but we don’t know for certain yet,” he added.
Hallinan confirmed that it was the “most powerful” aurora that had ever been seen, and much more intense than anything observed in the Solar System. Indeed, he said that if you were hypothetically able to stand on the surface, the aurora would appear one million times brighter than Earth’s. But so far, the researchers are not entirely sure how this aurora is forming on the brown dwarf.
The VLA in New Mexico. Hajor/Wikimedia Commons.
In our Solar System, Earth’s aurora is driven by particles from the Sun, while Jupiter’s is a combination of particles emitted by its volcanic moon Io and its own rotation. “But for brown dwarfs, there is no star nearby, so we don’t know what’s causing it,” said Hallinan.
He suggested that it may be due to an Earth-sized planet in a tight orbit affecting the magnetic field. This planet would be so close that the brown dwarf “would fill the sky,” said Hallinan. Alternatively, it could be behaving in a similar way to Jupiter, powering its own aurorae with its rotation. The team plans to use the Hubble Space Telescope next to try to solve this mystery.
While this is the first brown dwarf aurora to be spotted, Hallinan said that he expected aurorae to be present on other brown dwarfs. And it could provide a vital clue in working out where exactly these objects fall in the cosmic scheme of things. “Certainly, in terms of magnetic activity, brown dwarfs are more like ‘super-Jupiters’ than ‘mini-suns’,” he said.
Interestingly, brown dwarfs could also be useful in studying exoplanets – worlds beyond the Solar System. "For the coolest brown dwarfs we've discovered, their atmosphere is pretty similar to what we would expect for many exoplanets,” Hallinan said in a statement, “and you can actually look at a brown dwarf and study its atmosphere without having a star nearby that's a factor of a million times brighter obscuring your observations.”