A team of astronomers found signs of cloud bands resembling those of Jupiter or Saturn swirling around the closest-known brown dwarf, Luhman 16A — the first exoclouds discovered using polarimetry — according to a study published in The Astrophysical Journal
Signs of first 'exoclouds' seen swirling around nearby brown dwarf
These exoclouds — or clouds beyond the bounds of our home solar system — are the first witnessed via the polarimetry technique. Brown dwarfs occupy the categorical space between stars and planets — heavier than the latter, yet lighter than the former — and are usually 13 to 80 times Jupiter's mass. Luhman 16A is part of a binary system, where a second brown dwarf, called Luhman 16B, resides.
At roughly 6.5 light-years, this system is the third-closest to our sun, after Alpha Centauri and Barnard's Star. Both of the system's brown dwarfs weight roughly 30 times Jupiter's weight.
While Luhman 16A and 16B have comparable masses and temperatures (roughly 1,000 degrees C, or 1,900 degrees F), and may have formed at the same time, they display notably different weather patterns. Luhman 16B shows evidence of irregular, patchy clouds — unlike its "twin," which shows the more familiar and stationary cloud bands. Luhman 16B's cloudy features create notable variations in brightness, unlike Luhman 16A.
"Like Earth and Venus, these objects are twins with very different weather," said member of the discovery team Julien Girard of Space Telescope Science Institute in Baltimore, Maryland. "It can rain things like silicates or ammonia. It's pretty awful weather, actually."
Polarimetry, a new method used to find exoclouds
Researchers from the study used an instrument equipped on the Very Large Telescope in Chile to witness polarized light from the Luhman 16 system. A property of life called polarization represents the direction in which light oscillates. Most people are familiar with polarized sunglasses' ability to block out one direction of polarization — reducing glare and improving contrast.
"Instead of trying to block out that glare, we're trying to measure it," said lead author Max-Blanchaer of the California Institute of Technology (Caltech), in Pasadena, California, according to phys.org.
When light strikes particles like cloud droplets, it reflects — often at a specific angle of polarization. Astronomers measure this "preferred polarization" of light from distant systems to determining the presence of clouds without even knowing either brown dwarf's cloud structure.
"Even from light-years away, we can use polarization to determine what the light encountered along its path," Girard added.
"To determine what the light encountered on its way we compared observations against models with different properties: brown dwarf atmospheres with solid cloud decks, striped cloud bands, and even brown dwarfs that are oblate due to their fast rotation. We found that only models of atmospheres with cloud bands could match our observations of Luhman 16A," said another discovery team member, named Theodora Karalidi, of the University of Central Florida in Orlando, Florida.
The future of exoplanet astronomy
It's interesting to note that the polarimetry technique is also used for cases far beyond nearby brown dwarfs — in cases of exoplanets orbiting distant stars. Of course, measuring polarization signals from exoplanets presents astronomers with a new challenge — since they're relatively faint and immersed in the greater glare of their host star, information gained from the study of brown dwarfs like the ones above might enhance future exoplanet studies, reports phys.org.
This foreshadows NASA's forthcoming James Webb Space Telescope, which is expected to enhance our study of systems like Luhman 16 and look for signs of varying brightness in infrared light — an indication of clouds. NASA's Wide Field Infrared Survey Telescope (WFIRST) will launch with a coronagraph (referring to the starlight, not the virus) capable of conducting polarimetry, and could even detect giant exoplanets via reflected light — leading to signs of clouds in their atmospheres.