Aurora discovered over brown dwarf

The discovery of aurora on a low-mass star 18 lightyears from Earth is teaching scientists more about the outer atmospheres of exoplanets.

Artist's conception of aurora over a brown dwarf.
Credit: Chuck Carter and Gregg Hallinan, Caltech

Aurora is one of the most photographed and admired night sky occurrences on Earth, and was this year confirmed to be visible over Mars. Now, astronomers have discovered the first aurora ever seen in an object outside of our Solar System.

The aurora has been found glowing in the atmosphere of a brown dwarf and is 10,000 times more powerful than any seen before. The team that discovered the aurora, similar to the Northern or Southern Lights on Earth, say it indicates a huge difference between the magnetic activity of massive stars and that of brown dwarfs and planets.

"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,” says Gregg Hallinan of the California Institute of Technology (Caltech). “If you were able to stand on the surface of the brown dwarf we observed - something you could never do because of its extremely hot temperatures and crushing surface gravity - you would sometimes be treated to a fantastic light show courtesy of auroras hundreds of thousands of times more powerful than any detected in our solar system.”

The object, called LSR J1835+3259, is 18 lightyears from Earth and was observed using the Karl G. Jansky Very Large Array (VLA), along with the 5m Hale Telescope on Palomar Mountain and the 10m Keck Telescope in Hawaii.

Aurora occurs when charged particles enter a planet’s magnetosphere, where they accelerate along the planet’s magnetic field lines to the poles. Here, they collide with gas atoms in the atmosphere, producing the bright glow typical of auroras.

Using the VLA, the team detected a bright pulse of radio waves as the brown dwarf rotated. The Hale Telescope was then used to observe that the brown dwarf appeared to change optically in line with these radio pulses. Then, observing the hydrogen alpha emission line, they found that the object’s brightness varied periodically.

The Keck telescopes were used to measure the brightness of the brown dwarf and found that the hydrogen emission indicates auroras near its surface.

“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, albeit tens of thousands of times more intense,” says Hallinan. “We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs.”

As there is no stellar wind in the brown dwarf’s atmosphere, the team say the charged particles must be being dragged into the magnetosphere by another source, like an orbiting planet.

“For the coolest brown dwarfs we’ve discovered, their atmosphere is pretty similar to what we would expect for many exoplanets, 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,” says Hallinan.


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