The European Southern Observatory's Very Large Telescope has been used to create history's first-ever map of weather on the surface of a brown dwarf. These images provide us with an unprecedented look at the atmospheric features of these poorly understood "failed stars."
Top Animation adapted from this video, via ESO/I. Crossfield.
The brown dwarf in question is WISE J104915.57-531906.1B. Known informally as "Luhman 16B," the object is one of two failed stars discovered last year by astronomer Kevin Luhman in infrared survey data acquired by NASA's sky-mapping WISE telescope. At just 6.5 light-years from Earth, Luhman 16B (and its sibling, Luhman 16A) are not only the closest brown dwarfs to our Solar System, they're the third-nearest system to Earth – the only closer-systems being Alpha Centauri and Barnard's Star.
An artist's conception of Luhman 16B, based on the first ever map of the weather on the surface of the nearest brown dwarf to Earth. | Credit: ESO/I. Crossfield/N. Risinger.
Astronomers have suspected for years that brown dwarfs have cloudy atmospheres. The so-called "failed stars" are born hot, slowly cooling as they age. As they drop below 2300K (~2023°C), the objects reach a temperature that is hypothesized to be favorable to the condensation of silicate minerals and molten droplets of iron into patchy cloud systems. But these clouds are thought to exist only in passing, disappearing as a brown dwarf cools to temperatures below 1300K (~1027°C).
Unfortunately, their faintness and relatively small size has made it notoriously difficult to image brown dwarfs in close detail. But the proximity of Luhman 16 A&B to our solar system puts them within reach of VLT's gaze. At so near a distance, the ESO's powerful ground-based telescope could do more than image these objects – it could actually map out the light and dark features on the surface of Luhman 16B. While Luhman 16B is the fainter than its sibling, it had previously been observed changing in brightness every few hours as it rotated. It was a sign, astronomers believed, that the brown dwarf might have observable surface features. These images, which appear in the January 30th issue of Nature, appear to confirm this hypothesis; together they comprise the first global, 2D map of large-scale brightness variations of a brown dwarf – brightness variations researchers interpret as indicative of thicker (dark) and thinner (bright) cloud coverage:
Luhman 16B, imaged by ESO's VLT at six equally spaced time intervals, as it rotates once about its axis | Credit: ESO/I. Crossfield.
"Previous observations suggested that brown dwarfs might have mottled surfaces, but now we can actually map them," the study's lead author, Ian Crossfield of the Max Planck Institute for Astronomy, said in a statement. "Soon, we will be able to watch cloud patterns form, evolve, and dissipate on this brown dwarf — eventually, exometeorologists may be able to predict whether a visitor to Luhman 16B could expect clear or cloudy skies."
A surface map of Luhman 16B recreated from VLT observations | Credit: ESO/I. Crossfield.
Too big to qualify as planets, but too small to ignite the internal nuclear reactions required to become full-blown stars, brown dwarfs are of particular interest to astronomers because of what they can teach us about planetary and star formation, and, in the case of Luhman 16B, weather in other parts of the Universe. The atmospheres of brown dwarfs, for example, are thought to be similar to those of hot gas giant exoplanets, which occupy systems too distant to be imaged in any great detail by instruments like the VLT. A brown dwarf like Luhman 16B gives us a unique chance to study the weather systems of distant worlds on a similar, closer cosmic object.
"Our brown dwarf map helps bring us one step closer to the goal of understanding weather patterns in other solar systems," said Crossfield. "From an early age I was brought up to appreciate the beauty and utility of maps. It's exciting that we're starting to map objects out beyond the Solar System!"
The researchers' findings are published in the latest issue of Nature.
Feeling crafty? Crossfield's team has created a couple of foldable, origami versions of their Luhman 16B map. Download the blueprints here.
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