Astronomers for the first time have spotted an aurora outside of our solar system.
Gregg Hallinan, assistant professor of astronomy at the California Institute of Technology, and a team of astronomers were observing a brown dwarf 20 light-years away using some of the world’s most sophisticated radio and optical telescopes when they came upon the discovery, according to a new study published in the journal Nature on Thursday.
Brown dwarfs, which are sometimes called “failed stars,” are too big to be planets, but they are too small to sustain hydrogen fusion reactions at their core in order to make them into stars. The dim objects are also hard to see or classify.
“We’re finding that brown dwarfs are not like small stars in terms of their magnetic activity; they’re like giant planets with hugely powerful auroras,” Hallinan, the lead author of the study said.
He explained that a person standing on a brown dwarf’s surface (impossible, of course, because of its extreme heat and crushing gravity) would be greeted by a brilliant light show a million times more powerful than the auroras that happen on Earth.
“As the electrons spiral down toward the atmosphere, they produce radio emissions, and then when they hit the atmosphere, they excite hydrogen in a process that occurs at Earth and other planets,” Hallinan said. “We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs.”
Earth’s auroras are formed when charged particles from the solar winds enter the planet’s magnetic field, which sends them to the planet’s poles. Once there, the particles collide with atoms of gas in the atmosphere, creating the vibrant light displays that are occasionally seen in the sky.
On Earth, auroras are typically green in color, but on brown dwarfs red-colored auroras are more prominent because of the higher levels of hydrogen in their atmospheres.
However, astronomers are also unsure exactly how auroras are being created on the brown dwarf, because these celestial bodies do not have stellar winds. One theory is that an orbiting planet moving through the brown dwarf’s magnetosphere is creating the phenomenon, but researchers will not fully know the origin without mapping the auroras first.
In the early 2000s, astronomers found that brown dwarfs were emitting radio waves. This piqued interest in the substellar objects because they do not generate large flares and charged-particle emissions, meaning they do not emit radio waves the same way stars do.
Instead, brown dwarfs pulse radio frequencies, Hallinan discovered in 2006. This pulsation is similar to what planets in our solar system that have auroras do, giving these “failed stars” some qualities similar to planets, according to NASA.
The discovery may help researchers learn more about how brown dwarfs generate magnetic fields. It is also a stepping-stone to studying exoplanets, because the atmosphere on brown dwarfs is similar to what astronomers believe could be found on planets in solar systems other than ours.
“That could be particularly interesting, because whether or not a planet has a magnetic field may be an important factor in habitability,” Hallinan said.
