Predicting Alien Weather

Apologies for the misleading headline. Nobody is predicting weather on planets outside of our Solar System. We're just monitoring and recording it (as previously discussed here), and have been for some time. But recent observations of a very near pair of brown dwarf objects has led to something new: We're watching the weather on stars themselves!

Sorry, that was also misleading. Brown Dwarf's aren't technically stars since they sit on the hazy boundary between the largest gas-giant planets and the smallest stars. They are big and heavy enough to generate a lot of heat in their cores, causing them to glow dimly in infra-red light, but they aren't quite hot enough to begin nuclear fusion reactions. They're like ambitious planets that failed to become stars.

Because they aren't as bright as stars, they are really hard to see. That means that, out of all the hundreds of billions of stars in the galaxy, we've only found a few hundred brown dwarfs and they were all discovered within the past twenty years. The nearest of these, Luhman 16AB, is a binary pair, orbiting each other closely about six light years away from Earth. This makes them the 3rd closest object to our Solar System that we know of, after the famous Alpha Centauri triple star, and Barnard's Star (a red dwarf, and also the star which moves the fastest across our sky). In cosmic terms, six light years is practically in our back yard, which means astronomers trying to understand brown dwarfs can study Luhman 16AB quite closely.

The first thing astronomers learn about a Binary star is the period of their orbit, how widely they are separated, and what their masses are. This isn't particularly interesting though: It's just collecting numbers for a catalogue. Astronomer Ian Crossfield (Max Planck Institute for Astronomy, Heidelberg, Germany) wanted to learn enough about this particular pair of stars to see how well they matched up to theoretical predictions, and so either confirm or disprove what we think we know about brown dwarfs. He began by looking at them individually with the CRIRES spectrograph on the European Southern Observatory's Very Large Telescope (VLT) observatory in Chile. The glowing hot gases in the outer atmosphere of stars emit lines of very precise colours, and a spectrograph lets astronomers identify those lines, and observe how far they deviate from where they are supposed to be. This both identifies the gases which are present, and reveals how fast it is moving, how hot it is, and more.

Crossfield quickly learned that one of the two companion stars, Luhman B, rotates extremely rapidly: it makes one full turn about its axis in only 4.9 hours, enough to stretch its shape out into a squashed sphere. But as observations continued over time, he noticed that the brightness fluctuated. Since some astronomers had already predicted that Brown Dwarfs might have mottled, cloudy surfaces, he suspected that this might be the cause of the fluctuations. So he commissioned time on the telescope to observe the star for five hours - slightly more than one full rotation. He then carefully studied the spectra, comparing the brightness of the approaching edge to that of the receding edge. Sure enough, whenever the approaching edge dimmed or brightened, the receding edge would do the same thing a few hours later. This was exactly what would have been expected if the fluctuations were actual features on the surface and if we were watching them move as the star rotated.

With careful processing, Crossfield's team were even able to start mapping out those features, and simulate an image of how the star would look in infra-red if viewed from close up. These results were summarised in a paper, published in Nature earlier this year. In Crossfield's own words, 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 - eventually, exometeorologists may be able to predict whether a visitor to Luhman 16B could expect clear or cloudy skies."

He later added, "Our brown dwarf map helps bring us one step closer to the goal of understanding weather patterns in other solar systems. 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!"

Crossfield's paper can be read here.

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Source: mysteries-and-strangeness.blogspot.com