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.