Hubble finds brown dwarfs can't maintain binary relationships over time

Brown dwarfs have a reputation problem. They're called "failed stars" because they lack the mass to ignite hydrogen fusion in their cores—the defining trait of a true star. But a new finding from the Hubble Space Telescope suggests their troubles run deeper than nomenclature. These objects, it turns out, can't hold onto companions. They start life in pairs, but somewhere along the way, they drift apart and end up alone.

The puzzle began with a simple observation: most stars in the universe travel with a partner. About 75 percent of massive stars exist in binary systems, as do roughly half of sun-sized stars. Brown dwarfs, by contrast, almost never show up with a companion when astronomers look for them in older populations. Yet younger brown dwarfs do have binary partners. Something happens over time that breaks these pairs apart.

Clémence Fontanive, a researcher at the Trottier Institute for Research on Exoplanets, led a Hubble survey to understand why. The team selected some of the coldest and lowest-mass brown dwarfs in the solar system's neighborhood—objects so cool they register only a few hundred degrees hotter than Jupiter. Because brown dwarfs can't sustain nuclear fusion, they radiate very little heat and remain cold for extraordinarily long periods. The researchers used Hubble's near-infrared instruments to detect water in the atmospheres of these frigid objects, a technique that allowed them to spot even the faintest companions separated by distances of roughly 298 million miles—about three times the Earth-sun distance.

The results confirmed what theory had long suggested: brown dwarf binaries don't survive. When these objects are young and still embedded in the molecular clouds where they formed, they stay paired. But as the cloud disperses and the brown dwarfs age, the gravitational bond holding them together weakens. Because brown dwarfs are so light, the pull between two of them is feeble compared to the gravitational influence of other stars passing nearby. A stray star moving through the neighborhood can easily wrench a brown dwarf binary apart, sending each object spinning off into isolation.

Fontanive explained the mechanism plainly: "Because brown dwarfs are so light, the gravitational hold tying wide binary pairs is very weak, and bypassing stars can easily tear these binaries apart." The team's earlier Hubble observations of extremely young brown dwarfs had confirmed that these objects could form in binaries at all—a crucial piece of evidence that the mechanisms creating low-mass pairs actually work. This new survey of older brown dwarfs provides the first direct observational evidence that those pairs don't last.

Beth Biller, a team member at the University of Edinburgh, put it plainly: most stars have companions, whether binary partners or orbiting planets. Brown dwarfs, by contrast, end up solitary. "After a brief period early in their lifespans, most brown dwarfs remain single for the rest of their very long existence," Biller said. The finding fills a gap in the literature on stellar multiplicity and offers insight into how gravitational dynamics change as objects lose mass. It also underscores what Hubble can do—detect the coldest, faintest objects in the sky and track their companions across vast distances. Without that capability, Fontanive noted, confirming these models would have been impossible. The research, published in Monthly Notices of the Royal Astronomical Society, suggests that the lower the mass of a stellar object, the harder it becomes to maintain a stable partnership over cosmic time.