Baffling the world of astrophysics is a recent discovery made by astronomers from NASA's James Webb Space Telescope. They observed an unusual phenomenon in a celestial body known as a brown dwarf, particularly one named W1935. Brown dwarfs, often considered celestial bodies situated between the planets and the stars, are more enormous than Jupiter but smaller than an average-sized star. What stirred the scientific community is the detection of infrared emissions from methane in the upper atmosphere of W1935. This discovery was unexpected, primarily because W1935 is cold and doesn't have an associated star, leaving no apparent energy source for the upper atmosphere. Thus, the team proposed that the methane emission could be linked to auroral processes, akin to those seen in our solar system.
Artist's Concept of Brown Dwarf W1935 Credit: NASA, ESA, CSA e L. Hustak (STScI)
METHANE EMISSION AND AURORAE
In our solar system, gas giants like Jupiter and Saturn commonly display methane emission. The upper-atmosphere heating that fuels this emission is associated with aurorae, a phenomenon known for its stunning visual spectacle. Aurorae, such as our Earth's Northern and Southern Lights, are created when energetic particles from the Sun are captured by the planet's magnetic field. These particles cascade into our atmosphere along magnetic field lines, especially near the poles, colliding with gas molecules to create vibrant, dancing curtains of light. Jupiter and Saturn, too, have similar auroral processes that interact with the solar wind. Moreover, they receive auroral contributions from their respective active moons, Io and Enceladus.
For solitary brown dwarfs like W1935, the lack of a stellar wind to contribute to the auroral processes and explain the extra energy in the upper atmosphere needed for the methane emission becomes a mystery. The team postulates that either unaccounted internal processes similar to the atmospheric phenomena of Jupiter and Saturn or external interactions with either interstellar plasma or a nearby active moon could help account for the emission.
A COSMIC DETECTIVE STORY
The discovery of aurorae unfolded like a gripping detective story. A team led by renowned astronomer Jackie Faherty from the American Museum of Natural History in New York was awarded time with the Webb telescope to scrutinize 12 cold brown dwarfs. Among those were W1935, an object discovered by citizen scientist Dan Caselden in collaboration with the Backyard Worlds Zooniverse project, and W2220, an object found using NASA’s Wide Field Infrared Survey Explorer. Webb revealed in intricate detail that W1935 and W2220 appeared to be near clones of each other in composition, sharing similar brightness, temperatures, and spectral features of water, ammonia, carbon monoxide, and carbon dioxide. The notable exception was that W1935 showed emission from methane, in contrast to the anticipated absorption feature observed towards W2220. This was seen at a distinct infrared wavelength to which Webb is uniquely sensitive.
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| Spectra W1935 vs W2220 Credit: NASA, ESA, CSA e L. Hustak (STScI) |
THE PUZZLE OF TEMPERATURE INVERSION
Faherty's team resorted to computer models to infer the cause behind the emission. The modeling effort revealed that W2220 had an expected energy distribution throughout the atmosphere, becoming cooler with increasing altitude. Conversely, W1935 had a surprising outcome. The best model favored a temperature inversion where the atmosphere got warmer with increasing altitude. "This temperature inversion is really puzzling," admitted Ben Burningham, a co-author from the University of Hertfordshire in England and lead modeler on the work. The team turned towards our solar system for clues, realizing that temperature inversions are prominent in gas giants like Jupiter and Saturn. Despite ongoing work to decipher the causes of their stratospheric heating, leading theories involve external heating by aurorae and internal energy transport from deeper within the atmosphere.
CONTEXTUALIZING BROWN DWARF AURORA CANDIDATES
This isn't the first instance where an aurora has been used to explain a brown dwarf observation. Astronomers have previously detected radio emission from several warmer brown dwarfs and proposed aurorae as the most likely explanation. Efforts were made to seek infrared signatures from these radio-emitting brown dwarfs using ground-based telescopes like the Keck Observatory to further characterize the phenomenon, but the results were inconclusive. W1935 becomes the first auroral candidate outside our solar system with the signature of methane emission. It’s also the coldest auroral candidate outside our solar system, with an effective temperature of about 400 degrees Fahrenheit (200 degrees Celsius), about 600 degrees Fahrenheit warmer than Jupiter.
THE MYSTERY CONTINUES
In our solar system, the solar wind is a key contributor to auroral processes, with active moons like Io and Enceladus playing a role for planets like Jupiter and Saturn, respectively. However, W1935 lacks a companion star entirely, so a stellar wind cannot contribute to the phenomenon. It remains to be seen whether an active moon might play a role in the methane emission on W1935. "With W1935, we now have a spectacular extension of a solar system phenomenon without any stellar irradiation to help in the explanation,” Faherty noted. “With Webb, we can really ‘open the hood’ on the chemistry and unpack how similar or different the auroral process may be beyond our solar system,” she added.
CONCLUSION
The James Webb Space Telescope, the world's leading space science observatory, is committed to unraveling mysteries of our solar system, peering beyond to distant worlds around other stars, and probing the enigmatic structures and origins of our universe and our place in it. Webb is an international program led by NASA in partnership with ESA (European Space Agency) and the Canadian Space Agency. For those eager to partake in the adventure of discovery, consider joining the Backyard Worlds: Planet 9 citizen science project and search the realm beyond Neptune for new brown dwarfs and planets, or try NASA’s new Burst Chaser citizen science project.
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