Astronomers have discovered a curious band of clouds swirling across the surface of a brown dwarf called Luhman 16A, similar to the stripes on the surface of Jupiter. The finding adds to the peculiarity of brown dwarfs – oddballs too big to be planets but too small to be stars. Although they are star-like in composition and formation, they lack the intense nuclear processes that happen inside stars.
Located just 6.5 light-years away, Luhman 16A is part of a pair with Luhman 16B, each weighing about 30 times the mass of Jupiter. They are the closest brown dwarf pair to Earth.
To identify the bands on the surface of Luhman 16A, the team used a technique called polarimetry. Polarimetry is the measurement of the polarization of light, a phenomenon that happens when light waves vibrate in a single plane. This is what’s used to create the 3D effect in cinemas (where we use special glasses to filter the light) and it has many applications in astronomy, from the study of planets to the research on the properties of the universe as a whole.
"I often think of polarimetric instruments as an astronomer's polarized sunglasses," said lead author Maxwell Millar-Blanchaer, a Robert A. Millikan Postdoctoral Scholar in Astronomy at Caltech, in a statement. "But instead of trying to block out that glare we're trying to measure it."
“Polarimetry is the only technique that is currently able to detect bands that don’t fluctuate in brightness over time,” added Millar-Blanchaer, whose study is published in The Astrophysical Journal. “This was key to finding the bands of clouds on Luhman 16A, on which the bands do not appear to be varying. This is the first time that it’s really been exploited to understand cloud properties outside of the solar system.”
The team cannot image the brown dwarf directly, but the technique is sensitive to cloud features. Combined with atmospheric modeling, the data from the polarimetry suggests that Luhman16A might rain silica. While not enticing weather, it is an expected one for an object with a temperature almost high enough to melt silver.
Next-generation telescopes such as the Extremely Large Telescope will be able to exploit this technique and push it far beyond the bounds of clouds to detect liquid water on the surface of a planet.