New Findings on Brown Dwarf Atmospheres

I often think of brown dwarfs in terms of the planets that might form around them, and the question of whether even these small ‘failed stars’ may be capable of sustaining life. Have a look, for example, at Luhman 16AB, two brown dwarfs in the Sun’s immediate neighborhood. There are some indications of a planet here which, if it were ever confirmed, would make it the second closest known exoplanet to the Earth, at least for now. We can rule out planets of Neptune mass or greater with a period of between one and two years, but future Hubble observations, already approved for August of next year, may tell us more.

Image: Luhman 16AB, two brown dwarfs in the Sun’s neighborhood. Credit: NASA / JPL / Gemini Observatory / AURA / NSF.

But brown dwarfs, incapable of fusing chemical elements, have their own planetary characteristics. It’s this intriguing aspect of this population that gives us a kind of bridge to exoplanet systems, because brown dwarfs are often found alone, without a bright nearby star to hinder observations.

Thus we get a new paper from Daniel Apai (University of Arizona) and colleagues that looks at the weather on brown dwarfs, finding it to be similar in some ways to what we have seen on gas giants both in our own Solar System and elsewhere. And in some ways not.

Apai’s team used the Spitzer space telescope to monitor six brown dwarfs over the space of a year, observing each of them through 32 full rotations. Brightness changes are apparent as a brown dwarf rotates, due to its clouds varying throughout the atmosphere. The study of these brightness variations allows us to get a sense of how these hot clouds — largely thought to be made up of iron droplets and silicate dust — are distributed.

We already knew that brown dwarfs tend to have atmospheric storms, and it seemed reasonable to relate these to what we see on Jupiter in the form of storms like the Great Red Spot. For that matter, we can also find analogs on the other outer planets, from Saturn through Neptune, even if the kinds of clouds we find on Neptune, for example, are made of ice. And yet the brightness variations the researchers found were far more rapid than expected, with changes evident over the course of a single Earth day. That meant the model needed adjustment, at least when we move beyond Jupiter into the realm of more massive objects.

Image: This artist’s concept shows a brown dwarf with bands of clouds, thought to resemble those seen at Neptune and the other outer planets. Credit: NASA/JPL-Caltech.

The paper in Science describes the team’s work with a supercomputer and a new computer algorithm that creates maps of how clouds travel on brown dwarfs. Emerging from this is a model that involves variations in large waves moving through the atmosphere at different rates. We get, in other words, a different pattern than we see in elliptical storms like the Great Red Spot, which has persisted for centuries and changes little. On brown dwarfs, we move to the rapid propagation of waves in short time periods.

Theodora Karalidi (University of Arizona), who performed the supercomputer work on this model, says that it can explain how clouds travel on these objects:

“When the peaks of the two waves are offset, over the course of the day there are two points of maximum brightness,” Karalidi said. “When the waves are in sync, you get one large peak, making the brown dwarf twice as bright as with a single wave.”

What intriguing objects these are. Free floating brown dwarfs can be roughly the same diameter as Jupiter but far more massive, with atmospheres made up mostly of hydrogen and helium. Their cloud distribution in atmospheric bands and waves has similarities to our gas giant planets, but now we see just how changeable their cloud patterns can be in short time-frames. Thickening and thinning in a matter of hours, these hot clouds can change quickly while remaining confined to bands in different latitudes in which they move at different speeds.

Thus the brown dwarf lives up to its reputation as being something of a cross between a star and a giant planet, giving us its own unique atmospheric signature. Unlike the frenzied activity of a stellar atmosphere, brown dwarf atmospheric winds fall into regular belts and zones. But unlike the gas giants we are familiar with, they are in a state of rapid agitation and change.

Much work remains to be done, as we’re a long way from understanding the drivers of these waves. The paper is Apai et al., “Zones, spots, and planetary-scale waves beating in brown dwarf atmospheres,” Science Vol. 357, Issue 6352 (18 August 2017), pp. 683-687 (abstract).