I love clouds but have never really given much thought to exoclouds. These are clouds in the atmospheres of other planets and stars outside the solar system. Max Millar-Blanchaer of the California Institute of Technology and his team recently discovered that the closest known brown dwarf, Luhman 16A, shows signs of cloud bands similar to those seen on Jupiter and Saturn.
Brown dwarfs are objects heavier than planets but lighter than stars, and typically have 13 to 80 times the mass of Jupiter. Luhman 16A is part of a binary system containing a second brown dwarf, Luhman 16B. At a distance of 6.5 light-years, it’s the third closest system to our sun and located in the southern constellation of Vela the sails. Both brown dwarfs weigh about 30 times as much as Jupiter but they’re not much larger because the increased strength of gravity caused by their extra weight (mass) pulls them into compact balls.
Brown dwarfs are more massive than Jupiter but don’t possess enough material to fuse hydrogen into helium in their cores. Fusion creates the energy which makes stars shine. Brown dwarfs glow dimly from the modest heat they generate from gravitational contraction. That’s why they’re incredibly faint. Despite the fact that the Luhman system is one of the closest to Earth it was only discovered in 2013. Luhman 16A, the brighter of the pair, shines at 22nd magnitude — more than two billion times fainter than a typical bright star in the night sky.
Despite the fact that Luhman 16A and 16B have similar masses and temperatures of around 1,900° F (1,000° C), and presumably formed at the same time around 600 to 800 million years ago, each has its own weather. Luhman 16A displays stationary cloud bands that resemble those on Jupiter while Luhman 16B’s clouds are patchy. Patchy clouds on Earth alternately allow and block sunlight from reaching the ground; Luhman 16B’s clouds cover different amounts of its surface, causing its light to vary.
“Like Earth and Venus, these objects are twins with very different weather,” said Julien Girard of the Space Telescope Science Institute in Baltimore, a member of the discovery team. Water in its various forms is responsible for precipitation on Earth. On Venus it rains car battery acid (sulfuric acid). Yuck! On the Luhmann pair “it can rain things like silicates or ammonia. It’s pretty awful weather, actually,” said Girard.
The researchers used an instrument on the Very Large Telescope in Chile to study polarized light from the Luhman 16 system. Light from a lamp or the sun is unpolarized — light waves bounce (vibrate) in every direction like kids jumping and falling over each other in one of those bouncy houses. In contrast, polarized light vibrates in only one plane. You’re probably already familiar with polarized light without even knowing it. When light reflects from water, asphalt, or other nonmetallic surfaces it becomes polarized and produces a blinding glare. Polarized sunglasses have a special coating that blocks polarized light, greatly reducing the glare.
In their study of the Luhman system scientists used the technique of polarimetry to determine the properties of the brown dwarfs’ clouds. Instead of blocking the glare they measured it in great detail. When light is reflected off of particles such as cloud droplets, it can favor a certain angle of polarization. By measuring the preferred polarization of light from the objects, astronomers deduced the presence of clouds without directly “seeing” either brown dwarf’s cloud structure.
The polarimetry technique isn’t limited to brown dwarfs. It can also be applied to exoplanets orbiting distant stars. The atmospheres of hot, gas-giant exoplanets are similar to those of brown dwarfs. Measuring them is considerably more challenging because these objects are even fainter and very close to their host suns. But techniques honed studying brown dwarfs will ultimately take us there. You can read the team’s scientific paper for free here.
Thanks to the new discovery we can look up at night and imagine exoclouds bubbling up in the atmospheres of remote planets and stars. Do their inhabitants see faces in them like we do I wonder.