Unexplained Auroras Found on a Lonely Brown Dwarf

By: Monica Young January 10, 2024 0

James Webb Space Telescope observations of a faint, giant world have revealed the signature of aurorae — even though the world has no star.

The ethereal glow of Northern Lights — a sought-after sight on Earth — has now been found on a giant world many times Jupiter’s mass, astronomers announced at this week’s meeting of the American Astronomical Society in New Orleans.

a blue orb with a red ring at its top on a black background — This artist concept portrays the brown dwarf W1935, which is located 47 light-years from Earth. Astronomers using NASA’s James Webb Space Telescope found infrared emission from methane coming from W1935. This is an unexpected discovery because the brown dwarf is cold and lacks a host star; therefore, there is no obvious source of energy to heat its upper atmosphere and make the methane glow. The team speculates that the methane emission may be due to processes generating aurorae, shown here in red.
*NASA, ESA, CSA, and L. Hustak (STScI)*

This finding in itself isn’t so unusual, as charged particles coming from the Sun, as well as from Jupiter’s volcanic moon Io, create auroras for many solar system planets. So it might come as no surprise that this world, called W1935, might exhibit the same phenomenon. Only, in this case, the world is far from any star.

Jackie Faherty (American Museum of Natural History) came across this world when she and her team received time on the James Webb Space Telescope to examine the atmospheres of 12 of the coldest brown dwarfs. Brown dwarfs are worlds more massive than Jupiter but not quite massive enough to sustain nuclear fusion in their cores. So as they age, they cool. The coldest of the brown dwarfs are thus the oldest, the smallest, or both. But the coolest brown dwarfs are also the most difficult to see, out of reach of ground-based telescopes and only just within JWST’s grasp. (For context, these brown dwarfs are still fairly warm at 400ºF, about the right temperature to bake cookies.)

Of the 12 brown dwarfs that Faherty’s team investigated, one jumped out. Citizen scientist and participant in the Backyard Worlds project Dan Caselden discovered the one dubbed W1935 as a faint fuzz moving against background stars. With JWST’s sensitive instruments, that faint fuzz became an intricate infrared spectrum, revealing a whole world of chemistry — and one unexpected feature.

a graph with an area of high wavelengths highlighted in red — Astronomers used NASA’s James Webb Space Telescope to study 12 cold brown dwarfs. Two of them – W1935 and W2220 – appeared to be near twins of each other in composition, brightness, and temperature. However, W1935 showed emission from methane, as opposed to the anticipated absorption feature that was observed toward W2220. The team speculates that the methane emission may be due to processes generating aurorae.
*NASA, ESA, CSA, and L. Hustak (STScI)*

When unveiling the spectrum, Faherty pointed out, “Every wiggle that you see in here is real, a real molecule that's absorbing light.” But amid those wiggles, one defied explanation. “It was like a pebble in a shoe I couldn't get rid of,” Faherty says.

At wavelengths shorter than about 4 microns, the methane molecule absorbs most of the light. But amid the methane absorption was a bump, which Faherty’s team eventually realized was methane in emission. To be detected in emission, the gas responsible would have to be hotter than the 400ºF brown dwarf — in other words, auroras.

Auroras require two things: a strong magnetic field, which a brown dwarf can generate if it rotates quickly enough. But they also require hot, ionized gas, or plasma. W1935 is an isolated brown dwarf, far from any host star, so where does the plasma come from?

Faherty doesn’t know either, but she has ideas. One of them, and perhaps the most enticing, is a geologically active moon, like Jupiter’s Io. Io’s volcanoes generate charged particles that then escape the moon’s gravity, captured instead in Jupiter’s magnetic field. Likewise on Saturn, the water-ice geysers of Enceladus generate charged particles that contribute to the ringed planet’s lights.

Other possibilities are no less intriguing. The planet might have encountered a “burp” of plasma in the space between stars, left behind by some forming object. In that case future JWST observations might show the auroras on W1935 disappear as the particle source dissipates.

The auroras may even shed some light on what’s happening in our own solar system. While the giant planets do have the Sun and active moons as a source of plasma, the auroras they generate are nevertheless brighter than expected.

“Frankly, I feel like I walked into a drama into drama that's happening in the solar system,” Faherty says. “Jupiter, Saturn, Uranus, Neptune, they also have too much energy in their upper atmospheres.” However, she adds that one of the leading explanations for this — that gravity waves within the planets are pumping energy to the outer layers — doesn’t pan out for W1935, because the world shows only methane in emission, not any of the other gases.

While W1935 is too faint for any kind of ground-based follow-up, the team intends to ask for additional Webb observations of the brown dwarf. Perhaps future data will show the auroral emission varying in strength or disappearing — or maybe a moon may even reveal itself by its gravitational tug.