Free Astronomy Magazine March-April 2024

24 MARCH-APRIL 2024 ASTRO PUBLISHING like Jupiter and Saturn. There is still ongoing work to understand the causes of their stratospheric heat- ing, but leading theories for the solar system involve external heat- ing by aurorae and internal energy transport from deeper in the atmos- phere (with the former a leading ex- planation). This is not the first time an aurora has been used to explain a brown dwarf observation. Astronomers have detected radio emission com- ing from several warmer brown dwarfs and invoked aurorae as the most likely explanation. Searches were conducted with ground-based telescopes like the Keck Observa- tory for infrared signatures from these radio-emitting brown dwarfs to further characterize the phenom- enon, but were inconclusive. W1935 is the first auroral candidate outside the solar system with the signature of methane emission. It’s also the coldest auroral candidate outside our solar system, with an ef- fective temperature of about 400 degrees Fahrenheit (200 degrees Celsius), about 600 degrees Fahren- heit warmer than Jupiter. In our solar system the solar wind is a primary contributor to auroral processes, with active moons like Io and Enceladus playing a role for planets like Jupiter and Saturn, re- spectively. W1935 lacks a compan- ion star entirely, so a stellar wind cannot contribute to the phenome- non. It is yet to be seen whether an active moon might play a role in the methane emission on W1935. “With W1935, we now have a spec- tacular extension of a solar system phenomenon without any stellar ir- radiation to help in the explana- tion,” Faherty noted. “With Webb, we can really ‘open the hood’ on the chemistry and unpack how sim- ilar or different the auroral process may be beyond our solar system,” she added. A stronomers used NASA’s James Webb Space Telescope to study 12 cold brown dwarfs. Two of them – W1935 and W2220 – appeared to be near twins of each other in composition, brightness, and temperature. However, W1935 showed emission from methane, as opposed to the anticipated absorption feature that was observed toward W2220. The team speculates that the meth-ane emission may be due to pro-cesses generating aurorae. [NASA, ESA, CSA, Leah Hustak (STScI)] Infrared Survey Explorer. Webb re- vealed in exquisite detail that W1935 and W2220 appeared to be near clones of each other in composition. They also shared similar brightness, temperatures, and spectral features of water, ammonia, carbon monox- ide and carbon dioxide. The striking exception was that W1935 showed emission from methane, as opposed to the anticipated absorption fea- ture that was observed toward W2220. This was seen at a distinct in- frared wavelength to which Webb is uniquely sensitive. “We expected to see methane be- cause methane is all over these brown dwarfs. But instead of absorbing light, we saw just the opposite: The methane was glowing. My first thought was, what the heck? Why is methane emission coming out of this object?” said Faherty. T he team used computer models to infer what might be behind the emis- sion. The modeling work showed that W2220 had an expected distribution of energy throughout the atmos- phere, getting cooler with increasing altitude. W1935, on the other hand, had a surprising result. The best model favored a temperature inver- sion, where the atmosphere got warmer with increasing altitude. “This temperature inversion is really puzzling,” said Ben Burningham, a co-author from the University of Hertfordshire in England and lead modeler on the work. “We have seen this kind of phenomenon in planets with a nearby star that can heat the stratosphere, but seeing it in an object with no obvious exter- nal heat source is wild.” For clues, the team looked in our own backyard, to the planets of our solar system. The gas giant planets can serve as proxies for what is seen going on more than 40 light-years away in the atmosphere of W1935. The team realized that temperature inversions are prominent in planets !