Two faint embers circle each other in the dark so closely that one begins to tear glowing chunks off the other. That is the picture of the system ZTF J1239+8347, caught by the Zwicky Transient Facility telescope. It consists of two brown dwarfs — objects heavier than planets but too light to ignite the thermonuclear fire of a star like the Sun. One of them, like a hungry neighbor, pulls material from the other, and this stream crashes onto its surface at tremendous speed.
The pair completes an orbit around their common center of mass in just 57 minutes — faster than the hour hand of a clock makes a full circle. Because of this, the material doesn't have time to spread evenly; instead, it strikes a single point on the dwarf's surface, like a jet from a hose. That spot heats up to 8900 degrees — hotter than the surface of the Sun, even though the dwarf itself has a temperature of only 1500 degrees. Telescopes see the system's brightness change by a factor of two over each orbit: the hot spot alternately hides and emerges.
Previously, it was thought that brown dwarfs are too fluffy and light to lose angular momentum and spiral close enough for material to start flowing. But this pair proves that even such modest objects can come together within the lifetime of the Universe. The system lies just 300 light-years away — by cosmic standards, in our own backyard. And it is very faint, around 20th magnitude, so the upcoming Rubin Observatory survey will surely find many more such pairs.
as written by the authors arXiv:2603.17038
Mass transfer in stellar binaries has been well studied in most stellar mass ranges, with the notable exception of ultracool stars and substellar brown dwarfs. We report the discovery of ZTF J1239+8347 with the Zwicky Transient Facility (ZTF), a brown dwarf binary currently undergoing stable mass transfer with an orbital period of 57.41 minutes. Optical time-series photometry reveals an extremely high amplitude ($$> 2$$ magnitude peak-to-trough) variability at short wavelengths indicative of an orbiting hot spot slightly buried inside the atmosphere of the accretor. We use parallax measurements from \textit{Gaia} along with optical and near infrared spectra to infer an accretion temperature of $$T_\mathrm{eff} = 8904 \pm 54$$ K, an atmospheric temperature of the accretor of $$T_\mathrm{atmo} \approx 1500$$ K, and a slightly inflated accretor radius of $$R_{\rm acc} = 1.20^{+0.15}_{-0.11} \, \RJup$$. ZTF J1239+8347 is a direct impact accretor, typically only seen in double degenerate white dwarf binaries, which are approximately a million times denser than the components in ZTF J1239+8347. The existence of an accreting brown dwarf binary suggests that angular momentum loss can be strong enough to make ultracool binaries interact in a Hubble time. The observed faintness ($$\sim 20$$ mag) and relative proximity ($$\approx 300$$ pc) of ZTF J1239+8347 suggests that many similar systems are likely to be found by the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST).
Brown dwarfs are sometimes called "failed stars": they lack the mass to ignite fusion, yet they are still hotter than planets.