Two brown dwarfs orbit each other every 57 minutes — and one is draining the other dry

Astronomers using the Zwicky Transient Facility have uncovered a rare binary system in which two brown dwarfs orbit each other every 57 minutes — so close that one is actively stealing mass from the other. The system, ZTF J1239+8347, sits about 1,100 light-years away and represents one of the most extreme examples of stellar interaction ever documented at substellar scales.

Two brown dwarfs locked in a cosmic embrace

Brown dwarfs occupy the twilight zone between planets and stars — objects massive enough to fuse deuterium briefly but never quite igniting sustained hydrogen fusion like true stars. Finding them in tight binaries undergoing mass transfer is extraordinarily rare. ZTF J1239+8347 was identified by a team led by Samuel Whitebook at the California Institute of Technology (Caltech) through a targeted search for accreting periodic variable stars in the ZTF Variability survey (ZVAR). The discovery was published March 18 in The Astrophysical Journal Letters.

A 57-minute orbit and stable mass transfer

The two brown dwarfs in ZTF J1239+8347 have estimated masses between 13 and 80 Jupiter masses — placing them solidly in the brown dwarf regime. Their orbital period is approximately 57.41 minutes, which is itself remarkable: at this separation, the gravitational interaction between the two objects forces the less massive one (the donor) to transfer material onto its companion (the accretor) in a process known as stable mass transfer.

Extreme atmospheric conditions

The accretor has a radius of about 1.2 Jupiter radii and an atmospheric temperature near 1,500 K. The donor is estimated to have a radius between 0.9 and 1.4 Jupiter radii, with an atmospheric temperature likely below 1,200 K. Optical observations reveal that ZTF J1239+8347 shows extreme high-amplitude variability — a signature consistent with an orbiting hotspot partially buried within the accretor’s atmosphere, generated by the incoming stream of transferred material.

Why near-infrared tells a different story

Despite the dramatic variability in optical wavelengths, the thermal emission from the brown dwarf atmosphere contributes significantly to the near- and mid-infrared spectrum, making the system appear far less variable at those wavelengths. This adds complexity to the picture and underscores the importance of multi-wavelength follow-up observations for understanding the true physical nature of the accretion process.

A system worth watching with JWST

The authors propose continued monitoring of ZTF J1239+8347 to better constrain the system’s physical parameters. They specifically highlight the James Webb Space Telescope (JWST) as a powerful tool for measuring the accretor’s atmospheric temperature more precisely and for detecting the mass ratio of the system — something that cannot be achieved from ground-based near-infrared spectroscopy at this brightness level.

Source

Whitebook, S. et al. (2026). A mass transferring brown dwarf binary on a 57 minute orbit. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ae486e

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