Brown dwarfs have long been in a state of cosmic limbo. Though they have a few star-like qualities, these celestial bodies lack some of the fundamental characteristics that define a star; they’re cool, small, and emit very little light. This has led some scientists to argue that brown dwarfs are more akin to planets than they are to the luminous balls of plasma scattered throughout the Universe.
Now, a new study is strengthening the "planet" argument. For the first time, astronomers have detected powerful aurorae on a brown dwarf star located 20 light years from Earth. In a study published today in Nature, the researchers detail how they used a radio and an optical telescope to observe the aurorae, like Earth’s Aurora Borealis but much stronger, at the brown dwarf's magnetic poles. Since aurorae have only ever been observed on planets, this discovery points to the idea that brown dwarfs are more like giant planets than they are like stars.
"Is a brown dwarf more like a planet or more like a star?"
Brown dwarfs emit light like other stars do, but that’s where their similarities end. The light they illuminate is so dim, it can only be observed in the infrared. Brown dwarfs are also relatively cool compared to other types of stars — ranging from 1,880 degrees Fahrenheit to the temperature of a home oven. Yet what really confuses astronomers is how small these stars are — landing somewhere between 15 and 75 times the mass of Jupiter. That may seem big compared to our puny planet, but study author Gregg Hallinan says it's fairly tiny in the intergalactic scheme of things.
"What defines a star is if it's big enough to fuse hydrogen at its core," Hallinan, a professor of astronomy at the California Institute of Technology, tells The Verge. "Brown dwarves don’t quite have enough mass to burn hydrogen throughout their lifetime; they’re liked failed stars. So the question you might ask: is a brown dwarf more like a planet or more like a star?"
One way scientists have gone about answering this question is by studying the magnetic activity of brown dwarfs; magnetic fields manifest differently when surrounding a star versus a planet. Our Sun's magnetic field, for example, helps to heat up the star's outer atmosphere to a very high temperature, causing activity like sun spots, solar flares, and solar wind. Earth's magnetic field, however, acts something like a shield from this solar activity. When the charged particles of solar wind hit Earth's field, they get redirected to our planet's magnetic poles, where they get excited and emit the greens and blues of the aurora.
Jupiter has its own powerful aurora. (NASA, ESA)
All planets that have magnetic fields within our Solar System have aurorae, so Hallinan and other researchers believed that if brown dwarfs were more like planets, they too would exhibit aurorae.
To figure this out, Hallinan and his research team analyzed one brown dwarf in particular — LSRJ 1835+3259. Previously, Hallinan discovered that this dwarf pulses, sending out a blast of radio waves every few hours. That kind of activity is unlike anything ever observed from a large star, but similar pulsing has been seen from Jupiter. When Jupiter rotates every 10 hours, it blasts radio waves — and produces an aurora at the same time. They theorized the same thing was happening on this brown dwarf.
The researchers used the National Radio Astronomy Observatory's Very Large Array to observe LSRJ 1835+3259's radio emissions, along with the Palomar's Hale Telescope and the W. M. Keck Observatory's telescopes to gather optical data about the brown dwarf. Sure enough, each time LSRJ 1835+3259 rotated — every 2.84 hours — it pulsed a radio emission and an aurora appeared at a location on the brown dwarf's surface.
"The aurora is so much more powerful than anything we've seen in our Solar System."
"In the case of the brown dwarf, the aurora is so much more powerful than anything we've seen in our Solar System," says Hallinan. "Jupiter's aurorae are 1,000 times more powerful than the Earth's aurorae. Well this brown dwarf has aurorae that are at least 10,000 times more powerful than Jupiter's aurora." The aurorae's colors are also a bit more diverse, emitting red along with blue and green.
As for what is powering these aurorae, it could be the combination of the dwarf's rapid rotation, as well as an active planet that's orbiting the brown dwarf, spewing charged particles into the object's magnetic field.
"There's a possibility there’s something like an Io there," says Jonathan Nichols, an advanced fellow in planetary auroras at the University of Leiceste, who was not involved in the study. "Io is this very active, volcanic moon that's deeps inside Jupiter’s magnetic field. It's the planet's rapid rotation combined with this volcanic nature of Io that gives rise to Jupiter’s radio emissions and aurorae."
Overall, Nichols says the research is exciting because it strengthens the argument that brown dwarfs are more closely related to planets, kind of like "beefed-up Jupiters." "Typically, people tend to think of these things as failed stars," says Nichols. "My preferred explanation is they’re more like very successful planets, and this study demonstrates more planet behavior than stellar behavior."