Luhman 16B's surface as it rotates about its axis. Image: ESO/I. Crossfield
If you're not quite a star, and not quite a planet, what do you look like? An international team of scientists has managed, using the European Southern Observatory in Chile, to create the first weather map of a brown dwarf, which are weakly-glowing objects that sit between gas giants and dim low mass stars in the taxonomy of celestial bodies.
Mapping the weather of a brown dwarf may seem like an obscure thing to do, but they’re similar enough to some exoplanets we’ve found that understanding the weather, heat, and movement happening on a brown dwarf might help us read the weather on far off worlds.
The brown dwarf in question is called WISE J104915.57-531906.1B, more simply known as Luhman 16B. Luhman 16B is half of a pair of cold brown dwarfs (called, as you might have guessed, Luhman 16AB) that sit about six light years from the Sun in the southern constellation of Vela. That’s fairly close; they’re the third-closest known brown dwarfs after Alpha Centauri and Barnard's Star, but they weren’t discovered until 2013.
As for the weather, it’s rather warm on Luhman 16B. Astronomers estimate the average temperature to be upwards of 2,000 degrees Fahrenheit, possibly with molten iron rain.
Mapping Luhman B's surface weather is a significant accomplishment. Ian Crossfield from the Max Planck Institute for Astronomy in Heidelberg, Germany, the lead author on the Nature paper that describes the results, puts his team’s work into perspective. “Previous observations suggested that brown dwarfs might have mottled surfaces, but now we can actually map them. Soon, we will be able to watch cloud patterns form, evolve, and dissipate on this brown dwarf," he said in an ESO release.
A wide-field view of the sky around Luhman AB. Image: ESO/Digitized Sky Survey 2
Brown dwarfs form just like stars do, through accretion of gas and dust in the interstellar medium, and are similarly composed largely of hydrogen gas. They aren’t quite massive enough to start the process of nuclear fusion in their core, the process that would make them shine and turn them into stars.
Because brown dwarfs don’t emit visible light like stars but rather glow dimly in the infrared end of the electromagnetic spectrum, these objects are incredibly hard to find. The first brown dwarf was confirmed in 1995, and astronomers have only found a few hundred since.
Luhman 16B is the fainter of the two stars in its system, and early observations showed that it changed in brightness every few hours as it rotated, a clue to astronomers that the dwarf star might have surface features. And so the more detailed observations began. Using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) instrument on the ESO's Very Large Telescope, astronomers managed to map the light and dark features on Luhman 16B’s surface. Luhman 16A, meanwhile, appears to have a fairly featureless atmosphere.
Luhman 16B's surface, as imaged by the Very Large Telescope in 16 equally-spaced segments. Image: ESO/I. Crossfield
Where stars are concerned, CRIRES is tuned to help astronomers gather data that helps determine and explain stellar evolution, stellar mass, stellar winds, atmospheric structures and oscillations in cool stars, as well as stellar magnetic fields.
In observing Luhman 16B, CRIRES helped astronomers measure not just the star’s changing brightness but determine whether the dark and light features were moving away from or towards the Earth. This allowed astronomers to use Doppler imaging techniques, which can yield information on the brown dwarf’s inclination relative to our Earthly perspective. Taking all this data together, they were able to recreate a map of the movement across the brown dwarf’s surface.
The ability to map the surfaces of brown dwarf extends beyond our learning more about this failed stars. Brown dwarfs have atmospheres that are quite similar to the hot gas giant exoplanets we’ve found. By studying brown dwarfs, which are comparatively easier to see and measure, astronomers just might learn some new things about the atmospheres of these young distant worlds in the process.
It’s an exciting prospect, says Crossfield. “Our brown dwarf map helps bring us one step closer to the goal of understanding weather patterns in other solar systems… It’s exciting that we're starting to map objects out beyond the Solar System!” he said.