Three giant antennas — radio astronomy telescopes LOFAR, uGMRT, and VLA — listened to the binary system of cold methane dwarfs J1019+65 for over 70 hours. And suddenly, they picked up silence. At frequencies from 200 to 300 megahertz, the signal cut off abruptly — as if an interferometry interferometer had registered a radio beacon switching off.

Electrons in the brown dwarf’s plasma whirl around magnetic field lines and amplify emission at one key frequency. That frequency depends only on field strength: the frequency in megahertz equals roughly 2.8 times the field in gauss. When the field is too weak, the emission shuts off. The cutoff at 200–300 MHz indicated a field of 126 gauss — nearly nine times Jupiter’s, but far weaker than thermal models predicted.

This lighthouse reveals that such objects have two radio faces: a global field acting like a steady, predictable beacon, while localized spots generate chaotic flares. Most importantly, a simple equilibrium law holds: magnetic force balances the planet’s spin (Coriolis force). Stellar evolution theories and numerical simulation models that relied on heat were off by factors of tens to hundreds. Now, by catching similar cutoffs from other cold dwarfs — objects straddling the boundary of star formation — astronomers will be able to map their invisible magnetic fields; and, in time, those of exoplanet exoplanets as well.