It’s a well known fun fact of astronomy that if you use the right kind of telescope, Jupiter looks every bit as bright as the Sun. In this case, the right kind means not a visible-light telescope but a radio telescope. Our Solar System’s biggest planet fairly sizzles with radio waves. Those emissions of electromagnetic energy turn out to be accompanied by powerful auroras, very much like the northern and southern lights we see on Earth, but 100 times brighter.
Saturn has auroras too, a fact that has long suggested that they might be common throughout not just the solar system, but the entire Milky Way. Now there’s hard evidence to back up that speculation. Radio emissions that have been picked up from brown dwarfs—objects bigger than planets but smaller than stars—can best be explained by the same process that gives Jupiter its crackle, says Jonathan Nichols, of the U.K.’s University of Leicester, in a paper in The Astrophysical Journal. And were there’s crackle, he argues, the brilliant light shows of the auroras are likely to follow.
But it’s that “likely” part that’s the catch. Not any radio waves are sufficient to indicate auroras: they have to be circularly polarized, which, as Nichols helpfully explains, is just radio-astronomer talk for “twisted.” Jupiter illustrates why this matters. The planet’s nearest large moon, Io, is the most volcanically active body in the solar system, and all those eruptions pour a stream of charged particles into space. These are grabbed by Jupiter’s magnetic field and whirled around at high speed. The particles then slam into the planet’s atmosphere, causing them to glow and producing the aurora. The high-speed motion also causes the particles to emit powerful radio waves—twisted waves, given how rapidly they’re spinning—and those we can detect. Things work slightly differently on Earth: our auroras are caused by charged particles from the Sun, not from a volcanic Moon, but the effect is the same.
Those telltale waves are just what Nichols has spotted around the brown dwarfs he has observed with the Very Large Array radio telescope in New Mexico. Since most of the brown dwarfs he is looking at are wandering alone through space, the particles that trigger their radio blasts—and presumably their auroras as well, though those can’t be seen directly yet—don’t come from a nearby star. “There could be an orbiting body like Io,” says Nichols, “but the brown dwarf could simply be plowing through the interstellar medium,” the thin gas that lies between the stars. That, by itself, could stir up a sufficient number of particles.
Given that brown dwarfs are far more massive than Jupiter, it’s no surprise that their radio emissions are 100,000 times stronger (and their auroras, presuming they’re really there, would be far brighter too). That’s the only reason the emissions can be seen from tens of light-years away. While we could never spot emissions from a twin of Jupiter at that distance, many exoplanets—that is, worlds orbiting stars beyond the Sun—are much closer to their parent suns than Jupiter, and may have much more powerful magnetic fields.
If so, then their aurora-linked radio emissions might be detectable just as those of the brown dwarfs are, and Nichols and several colleagues have been given some time on the European Low Frequency Array (LOFAR), based in the Netherlands, to try and find them. Our galaxy, they may learn, is not just a more complex place than we knew, but a more colorful one too.