Brilliant auroras spotted on brown dwarfs suggest these objects behave more like planets than stars.
Dim brown dwarfs are too small to sustain hydrogen fusion reactions at their cores, which is a defining factor of stars, but are too massive to be planets, Caltech reported.
To gain insight into these mysterious "failed stars," a team of researchers observing a brown dwarf 20 light-years away using both radio and optical telescopes. The findings revealed powerful auroras near the brown dwarf's magnetic poles, suggesting these types of objects are even more planet-like than we thought.
"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," said Gregg Hallinan, assistant professor of astronomy at 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."
In the early 2000s researchers noticed that brown dwarfs emit radio waves, but assumed they were caused by the action of their extremely hot atmosphere heated by magnetic activity, as they are in stars. The problem with that idea is the fact that brown dwarfs do not generate large flares and charged-particle emissions like the Sun and other stars. In 2006, Hallinan discovered that brown dwarfs can pulse at radio frequencies.
"We see a similar pulsing phenomenon from planets in our solar system," Hallinan said, "and that radio emission is actually due to auroras."
Auroras are known to occur when charged particles enter a planet's magnetosphere and are accelerated along magnetic field lines to the poles, where they collide with gas atoms and create bright emissions. To determine if this phenomenon occurs on brown dwarfs, the researchers observed brown dwarf LSRJ 1835+3259 using the National Radio Astronomy Observatory's Very Large Array (VLA), which is the world's most powerful radio telescope. They observed a bright pulse of radio waves appearing as the object rotated. They then used the Hale Telescope to watch how the brown dwarf varied optically on the same pulses as the radio pulses. Finally, they used the Keck telescopes to measure the brightness of the brown dwarf over time. This allowed them to establish the hydrogen emission was a signature of auroras on the brown dwarf.
"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," Hallinan said. "We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs."
Since there is no stellar wind on brown dwarfs to drive the magnetosphere, the researchers know there must something else generating a current to produce the auroras. Future mapping could potentially reveal what this mysterious factor is.
The findings were published in a recent edition of the journal Nature.
