On Earth and other planets, auroras occur when charged particles from the sun interact with the planet's magnetic field and collide with gas atoms in the atmosphere, exciting them and creating a visible glow. Any planet with a magnetic field is bound to host such a light show.
But to understand how a star can host a phenomenon associated with planets, you need to know that brown dwarfs are somewhere in between stellar and planetary status. They're bigger than gas giant planets like Jupiter, but smaller than other stars. Some are too small to maintain the fusion of hydrogen, which makes them pretty lousy by star standards.
"Brown dwarfs span the gap between stars and planets and these results are yet more evidence that we need to think of brown dwarfs as beefed-up planets, rather than 'failed stars,'" study author Stuart Littlefair of the University of Sheffield said in a statement.
Littlefair and his colleagues used a combination of radio and optical data from multiple telescopes to observe the brown dwarf called LSR J1835+3259, which is just 18 light years away. First they spotted jumps in radio waves as the star rotated, then they spotted a corresponding bright spot pulsing on its surface. They were then able to track the coming and going of that brightness and measure it, which led them to the conclusion that it was an aurora -- a periodic glow caused by particles hitting the cloudy, hydrogen-rich atmosphere of the not-quite-star.
It seems that the things we think of as tiny stars may have magnetic activity more like a planet's -- especially our solar system's large, distant planets, like Jupiter, the aurorae of which we don't yet understand all that well.
On Jupiter, aurorae aren't just caused by the sun's particles. They're also sparked by particles that come off of the planet's moons. High volcanic activity on the moon Io is known to cause a spike in aurora activity on Jupiter, for example. Jupiter's un-Earth-like auroras indicate that red dwarfs might also get their light-show-making particles from nearby planets or other sources. And the successful observation of the an aurora on this distant brown dwarf gives researchers hope of spotting them on distant exoplanets, too.
"What we see on this object appears to be the same phenomenon we've seen on Jupiter, for example, but thousands of times more powerful," study author Gregg Hallinan of the California Institute of Technology said in a statement. "This suggests that it may be possible to detect this type of activity from extrasolar planets, many of which are significantly more massive than Jupiter," he added.
But this discovery may open the door to a very complicated question: Can brown dwarfs really be called failed stars, or should we actually think of them as massive planets?
"In science, new knowledge often challenges our understanding," study author Garret Cotter of the University of Oxford said in a statement. "We know how controversial the situation was with Pluto, where astronomers had to look hard to try to decide if it is fundamentally one of the major planets of the solar system, or the first of the Kuiper Belt objects. Now, up at the other end of the size scale, we are challenged by seeing objects that traditionally would have been classified as stars, but seem to be showing more and more properties that make them look like super-sized planets."