The James Webb Space Telescope has peered into a strange binary world. Here, a dead star, a white dwarf, slowly roasts its companion, a brown dwarf. The object, named ZTFJ0038+2030, completes a full orbit in just a few hours. One side of the brown dwarf always faces the white dwarf, much like the Moon faces Earth.

Astronomers have for the first time built a complete light curve of such a pair. The NIRSpec spectrograph on JWST tracked the system for an entire orbit. During an eclipse, the white dwarf hid behind the brown dwarf, and the telescope saw only the nightside of the cool companion. This allowed the team to separate the radiation from the two bodies. It turned out that heat hardly reaches the dark side. The efficiency of heat transfer is less than 10%. The nightside looks like a typical cold brown dwarf, with no signs of heating.

The spectrum revealed an unexpected detail: on the nightside, a sharp absorption feature of carbon dioxide stands out at a wavelength of 4.2 microns. Yet the gas itself is evenly distributed throughout the atmosphere. This means the contrast is not due to chemistry but to physics — temperature and atmospheric dynamics. The measured luminosity of the brown dwarf indicates the system's age: about 8 billion years. That is nearly twice as old as the Sun.

Such pairs are living laboratories. They bridge the physics of exoplanets and substellar objects. In the future, similar observations will help us understand what happens to planetary systems after their stars die.