Clouds of molten iron droplets and silicate crystals may be curling across the surface of a newly met neighbor, say astronomers, in a report that opens a new door in celestial meteorology.
After last year's announcement that a pair of brown dwarfs had been discovered just six light years from our own sun, astronomers went bounding toward Chile's Very Large Telescope for a glimpse. On one of the two globes, they saw something that the brown dwarf community could only have dreamed of until now: patterns of light that both shifted and rotated, indicating weather.
A brown dwarf – an orb with a rapidly boiling center – falls somewhere between a planet and a star. Sometimes disparaged as "substellar objects," they are hotter and denser than planets, but not dense enough to sustain the nuclear fusion that powers stars. Brown dwarfs don't emit much visible light, and until this study, they had appeared to astronomers as little more than dim points of infrared.
But the Jupiter-sized twin globes of Luhman 16 are now our solar system's third-closest known neighbors (after the Alpha Centauri system and Barnard's star), and their proximity has allowed astronomers to form a picture of of their surfaces, using Doppler imaging.
Isn't that the same Doppler who proposed that the pitch of a passing engine seems to drop as it speeds away, because its sound waves stretch out relative to an observer? It is: in the very same way, light waves appear to stretch out as they emanate from a rotating globe's disappearing edge. And this stretching makes it easier to observe irregularities in the lines gathered by a spectrograph, which separates incoming light waves into different frequencies.
In the case of Luhman 16A, no irregularities showed up, suggesting a homogenous atmosphere. But Luhman 16B, observed over the course of one five-hour rotation, gave off clear indications of big dark spot in its northern hemisphere, and a light mark at its northern pole. Like Earthly weather systems, the spots changed shape but persisted over the course of the five-hour day.
The report, published in the current issue of the science journal Nature, includes a call for further study to determine exactly what is happening in those spots: "Future mapping efforts should reveal whether we are mapping variations in temperature, cloud properties or atmospheric abundances" like methane gas.
Beth Biller, a University of Edinburgh astronomer who co-authored the study, says she expects all three of those weather elements will emerge as factors on Luhman 16B.
The specific frequencies picked up by the spectrograph, she says, indicated the presence of clouds containing both "iron droplets and very hot silicates." Yet the irregularities they detected in the infrared spectrum did not entirely overlap with those they found in the optical light spectrum.
"What is clear," she says, "is that different things are happening at different depths of the atmosphere."
Luhman 16B's dusty clouds of iron may swirl at temperatures near 1880 degrees Fahrenheit, so Earthly settlers are not likely to come calling anytime soon. Yet our global weather patterns may have one thing in common: a polar vortex, suggested by the bright spot on the brown dwarf's north pole.
"Successive full nights of Doppler imaging could observe the formation, evolution and breakup of global weather patterns—the first opportunity for such a study outside the Solar System," reports the study.