Like planets, failed stars have beautiful auroras

"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. (Credit: Chuck Carter and Gregg Hallinan/Caltech)

Astronomers studying a dim star 20 light years away have found evidence of auroral displays, just like the northern lights that occur on Earth.

The findings suggest these types of stars, known as a brown dwarfs or failed stars, act more like supersized planets.

“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,'” says Stuart Littlefair from the University of Sheffield’s physics and astronomy department.

“We already know that brown dwarfs have cloudy atmospheres—like planets—although the clouds in brown dwarfs are made of minerals that form rocks on Earth; now we know brown dwarfs host powerful auroras too.

The findings appear in the journal Nature.

brown dwarf star
(Credit: Chuck Carter and Gregg Hallinan/Caltech)

“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, assistant professor of astronomy at the California Institute of Technology, who led the team.

“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.”

Powerful telescopes

Auroral displays result when charged particles manage to enter a planet’s magnetic field. Once within the magnetosphere, those particles get accelerated along the planet’s magnetic field lines to the planet’s poles where they collide with gas atoms in the atmosphere, producing the bright emissions associated with auroras.

brown dwarf seen from a radio telescope
brown dwarf, LSRJ 1835+3259, as seen with the National Radio Astronomy Observatory’s Very Large Array, pulsing as a result of the process that creates powerful auroras. (Credit: Stephen Bourke/Caltech)

During the study the research team conducted an extensive observation campaign of a brown dwarf called LSRJ1835+3259.

The team used the most powerful radio telescope in the world, the National Radio Astronomy Observatory’s Karl G. Jansky Very Large Array (JVLA) in New Mexico, as well as optical telescopes, including Palomar’s Hale Telescope and the W.M Keck Observatory’s telescopes to make their ground breaking observations.


Using the JVLA they detected a bright pulse of radio waves that appeared as the brown dwarf rotated around. The object rotates every 2.84 hours, so the team were able to watch nearly three full rotations over the course of a single night.

The astronomers worked with the Hale Telescope and observed the brown dwarf varied optically on the same period as the radio pulses. The team found that the object’s brightness varied periodically, indicating that there was a bright feature on the brown dwarf’s surface.

“It was incredibly exciting to track the optical light form the aurora during the night with the Hale Telescope in California, one of the most venerable telescopes in the world, while simultaneously tracking the radio emission with the JVLA, one the world’s newest radio telescopes,” says Garret Cotter from the University of Oxford, who also took part in the study.

Finally, the researchers used the Keck telescopes to precisely measure the brightness of the brown dwarf over time, which was no simple feat given that these objects are extremely faint, many thousands of times fainter than our own Sun.

The astronomers determined that the bright optical feature was likely caused by electrons hitting the hydrogen-dominated atmosphere of the brown dwarf to produce auroras.

Source: University of SheffieldCaltech