Two-for-One, Three Times: New Candidate Brown Dwarf Binaries

In astronomy, seeing isn’t always believing, and extra checks are required to make sure we don’t fool ourselves into oversimplifying nature. With the help of citizen scientists, a team of astronomers recently illustrated the value of such thoroughness and demonstrated that three previously observed fuzzy blobs may actually be rare, close-in pairs of brown dwarfs.

Point Sources Confusion

Computer renderings of planets, brown dwarfs, and a small star side by side. All are very similarly sized, though their radii increase in the order given.

A comparison of the size of planets, brown dwarfs, and stars. Click to enlarge. [NASA/JPL-Caltech]

Images of the night sky are undeniably filled with wonders, and features like diffuse nebula and swirling galaxies never fail to impress. However, the most common components of any astronomical image are more mundane: any given image taken with a telescope is filled with small, fuzzy blobs. Astronomers and the public alike most commonly assume these blobs are individual stars, but it’s important to remember that we can’t usually tell by eye alone. Stars are so far away that they are effectively “point sources,” meaning we don’t actually resolve their shapes. They look round and fuzzy not because we actually see the outlines of stars, but instead because our telescope optics and Earth’s atmosphere place caps on the smallest object we can resolve.

This means that what looks like one small, fuzzy blob can actually be two stars packed tightly together, both so far away from Earth that their images are blurred together into one. In rare but exciting cases, these blobs don’t correspond to stars at all, and instead are actually images of one or more brown dwarfs. These objects have masses that fall between stars and planets, and a more complete understanding of their formation would enable better models of both their bigger and smaller cousins. Binaries, or pairs of these objects orbiting each other, are especially useful since long-term radial velocity studies can reveal the masses of each constituent. Astronomers know of only a handful of brown dwarf binaries, so each discovery of a new pair, or even evidence that a given blob might be a pair, is cause for excitement.

WISE Citizens

A photograph of a large silver cylinder sitting vertically within a clean room. It is surrounded by four technicians in protective clothing. They are roughly half the height of the telescope.

The WISE telescope prior to its launch in 2009 from Vandenberg Air Force Base, California. WISE was placed in hibernation in 2011 but reawakened in 2013 to hunt for near-earth objects. [NASA/JPL-Caltech]

For several years now, nearly 80,000 citizen scientists participating in the Backyard Worlds: Planet 9 initiative have been poring over archival images from NASA’s Wide-field Infrared Survey Explorer (WISE) mission. This telescope, which launched in 2009, hibernated in 2011, then reanimated as a near-Earth object hunter in 2013, produced reams of infrared images that could contain brown dwarfs that were missed by the automated processing routines. After volunteers flagged three potential new brown dwarfs, a team of astronomers led by Alexia Bravo (United States Naval Observatory) decided to investigate further.

Bravo and collaborators used the Southern Astrophysical Research (SOAR) Telescope in Chile to collect the spectrum of each target, then compared these to spectra of previously confirmed brown dwarfs. In a happy surprise, none of the “templates” fit their new data very well. These objects were strange: at some wavelengths, they looked like a certain type of large brown dwarf, while at others, they looked like a much smaller and cooler variant.

Two wavelength vs. flux plots, where the data and model agree much better in the bottom panel.

Data and best-fitting models for one of the three candidates. The top panel shows the data (black) and best-fitting model (red) assuming the spectrum is composed of only one brown dwarf. The bottom panel shows how the same data are better matched by a model that allows for a pair of objects. Click to enlarge. [Adapted from Bravo et al. 2023]

The team realized that each of their spectra could be better fit by assuming that they weren’t looking at just one brown dwarf, but a close-together pair. Each of these candidates will require confirmation, and one of them in particular may turn out to be just a highly variable but single brown dwarf. Still, each of these objects could reveal rich information about brown dwarfs with future observations. These detections are also manifestations of the power of questioning the obvious and demonstrate the benefits of enlisting an engaged public to better know our galaxy.


“An Investigation of New Brown Dwarf Spectral Binary Candidates From the Backyard Worlds: Planet 9 Citizen Science Initiative,” Alexia Bravo et al 2023 AJ 166 226. doi:10.3847/1538-3881/acffc1