Earlier this month, at the latest annual meeting of the American Astronomical Society (AAS)—the largest organization of professional astronomers in the country—many cosmic objects got their moment to shine.
In the case of brown dwarfs, though, that shine is pretty feeble.
Brown dwarfs exist in a kind of netherworld category between planets and stars. They’re massive enough that the pressure in their core is sufficient to fuse deuterium—an isotope of hydrogen—but not massive enough to fuse normal hydrogen, the self-sustaining process that defines a proper star. This mass range is from about 13 Jupiters up to about 75 times that gas giant’s heft (or about 0.075 times the mass of the sun).
They also generate heat through gravitational contraction, and some, near the tippy top of the mass scale, also fuse lithium. But all these processes are fleeting, leaving brown dwarfs relatively inert. Because of this, they are sometimes referred to as “failed stars,” a moniker that I think is blatantly unfair. Who are we to judge? Maybe brown dwarfs are really just overachieving planets.
But because the fusion process is ephemeral, shortly after brown dwarfs form, they simply cool and fade over time. This means that they’re extremely faint in visible light and can be completely invisible to optical telescopes, even when they’re quite close to Earth. The first brown dwarf that was discovered, Teide-1, located in the nearby Pleiades star cluster, wasn’t even confirmed until 1995. The good news is that these objects retain much of the leftover heat from their formation, so they emit an enduring infrared glow, making them far easier to spot in those wavelengths.
Still, nearly three decades after that initial discovery, there’s much that we don’t know about brown dwarfs. There’s still an aura of mystery about them—in one case that was announced at the AAS meeting, a quite literal aura.
CWISEP J193518.59-154620.3—let’s call it W1935 for short because that’s what astronomers do—is a brown dwarf located in the constellation Sagittarius. It’s very cold, as these things go: it’s somewhere around 200 degrees Celsius, making it incredibly faint. It wasn’t discovered until 2019 despite being only about 47 light-years from Earth. That’s extremely close on a galactic scale, practically on our doorstep.
Astronomers recently used the James Webb Space Telescope (JWST) to observe W1935 as part of a program to better understand the composition, structure and atmospheres of cold brown dwarfs. They separated the object’s light into individual colors in order to form its spectrum, which can be used to show the presence and abundance of different molecules such as water and carbon dioxide.
The spectrum revealed a surprise, though. Normally, gaseous atmospheric methane in a brown dwarf absorbs the infrared light that comes up from below, so there’s a dip in brightness at certain spectral wavelengths. What the astronomers saw was just the opposite: instead of absorbing infrared light, the methane was emitting light. That means there must be something pumping energy into the methane molecules in W1935’s atmosphere.
While no peer-reviewed paper has yet been published on the research, this spectral surprise raises some interesting questions. This brown dwarf is far too cold for its ambient temperature to be the energy source exciting the methane. While it’s possible that some internal processes are to blame, a far more likely explanation is that W1935 has an aurora, according to the astronomers who collected the data.
That’s a big surprise! On Earth, auroras are created when the sun’s solar wind of subatomic particles is swept up by our planet’s magnetic field. The particles are funneled down into our atmosphere, where they slam into its gaseous atoms and molecules, making them light up like a literal neon sign.
Brown dwarfs can have strong magnetic fields, so that certainly seems possible. The problem is W1935 is a cosmic loner; there are no stars near it that could feed it particles to make an aurora.
There’s another possibility, though, and it’s pretty intriguing. Jupiter has an aurora that’s fueled by the solar wind and also sparked by three of its moons: Io, Europa and Ganymede. In the case of the tectonically hyperactive Io, for example, sulfur that it volcanically spews out into space interacts with Jupiter’s magnetic field, creating an aurora.
Could something similar be occurring with W1935? If it has a moon or, more excitingly, even a planetary-mass body orbiting it, then volcanic activity on that companion could be driving the aurora. The influx of particles would be captured by the brown dwarf’s magnetic field and flow down into the atmosphere, exciting the methane molecules and causing them to glow. Even though it’s close to us, cosmically speaking, W1935 is still too far and faint for us to see any orbiting companions. But it’s possible that such a body could be indirectly detected. For example, just as we see with Jupiter’s moon-induced aurora, W1935’s aurora could cyclically wax and wane in sync with a companion’s orbital period. Although discerning that pattern would probably be extraordinarily difficult, in theory it could be possible.
Auroras have been detected around brown dwarfs before but never with one so cold as W1935. This discovery could lead to a better understanding of the behavior of brown dwarfs, especially ones with strong magnetic fields. And, who knows, maybe it could uncover a serendipitous planet or moon, too!
In general, nature tends to make few big objects and lots of little ones; for example, high-mass stars are rare, but lower-mass ones such as red dwarfs are common. If this rule extends to objects that are even more diminutive, brown dwarfs may be the most ubiquitous substellar objects in the universe. We’ve had nearly 30 years of observations showing just how interesting they are, and still they manage to surprise us. Clearly their time to shine is just beginning.