Two Failed Stars in Our Cosmic Neighbourhood Seem to Have… Stripes?

Two Failed Stars in Our Cosmic Neighbourhood Seem to Have… Stripes?
Artist's impression of a brown dwarf. (Image: NASA/JPL-Caltech)

Never appreciated for what they are, brown dwarfs are often compared to what they are not. Indeed, they’re stuck in a celestial no man’s land, classified neither as stars nor gas giant planets. As new research shows, however, comparing brown dwarfs to gas giants like Jupiter is more appropriate than we realised.

A new paper in The Astrophysical Journal provides evidence of stripes and polar storms on brown dwarfs, similar to the ones seen on Jupiter. Using a space-based satellite, a research team led by planetary scientist Dániel Apai from the University of Arizona was able to construct basic maps showing the upper atmospheric layers of two nearby brown dwarfs. The paper is shedding new light on these enigmatic objects, while paving the way for future research.

Brown dwarfs are often referred to as failed stars, which kinda sucks if you’re a brown dwarf, of which there are many in the Milky Way. Indeed, our galaxy is absolutely bursting with these objects, with research from 2017 estimating a population around 100 billion. That’s a lot of failures, but hey, we can’t all be stars.

Brown dwarfs are typically as big as Jupiter but much more dense, packing in between 15 to 80 times more mass. At the same time, brown dwarfs are lightweights compared to stars, featuring insufficient pressure at their cores to trigger nuclear fusion, which is how stars are born. These objects are quite dim and difficult to see with telescopes, making it difficult for astronomers to study surface features.

“We wondered, do brown dwarfs look like Jupiter, with its regular belts and bands shaped by large, parallel, longitudinal jets, or will they be dominated by an ever-changing pattern of gigantic storms known as vortices like those found on Jupiter’s poles?” said Apai in a statement.

To find out, the team used NASA’s Transiting Exoplanet Survey Satellite (TESS), a space-based telescope normally used to detect exoplanets. Apai and his colleagues used TESS to observe two brown dwarfs, Luhman 16A and Luhman 16B, which are relatively close to Earth at approximately 6.5 light-years away. Both are roughly the same size as Jupiter, but they’re considerably more massive, 34 times and 28 times respectively, and about 1,500 degrees Fahrenheit hotter. The scientists managed to record over 100 rotations of both objects.

The data acquired by TESS, plus some fancy algorithmic work, allowed the team to measure changes in the brightness of the two brown dwarfs as they rotated. The objects exhibited fluctuations in brightness as they spun around, making it possible to create rudimentary maps of their upper atmosphere.

The scientists detected patterns consistent with high-speed winds running parallel to the equator. This was taken as evidence of atmospheric stripes similar to the ones seen on Jupiter. The associated winds, the authors speculate, are churning the atmosphere and delivering heat into the bowels of the brown dwarfs. The scientists also spotted signs of vortices in the polar regions, again similar to what’s seen on Jupiter.

“Wind patterns and large-scale atmospheric circulation often have profound effects on planetary atmospheres, from Earth’s climate to Jupiter’s appearance, and now we know that such large-scale atmospheric jets also shape brown dwarf atmospheres,” said Apai.

This discovery is a huge boost for scientists who study brown dwarfs. Should similar features appear on other brown dwarfs (this study presents a small sample size of two, so more work will be needed to validate these results), it’ll allow scientists to more meaningfully study the origin, climates, and temperatures of brown dwarfs. Looking ahead, the team is hoping to drill further down and study things like clouds, storms, and winds on brown dwarfs.

Excitingly, the technique used by this team could be used to map other distant objects, including Earth-like exoplanets.