s in five different
photometric filters (top) and
radial velocity measurements (bottom) of ZTF J1239+8347 over two orbital periods (or orbital phases). The ZTF J1239+8347 system consists of two brown dwarfs orbiting extremely tightly around each other, revolving once every 57.41 minutes. The distance between the two brown dwarfs is smaller than the
distance between Earth and its moon. Near-infrared spectroscopy suggests that the more massive brown dwarf has an
effective temperature of about , whereas the less massive brown dwarf has an effective temperature of up to . Based on their near-infrared spectra, both brown dwarfs in the ZTF J1239+8347 system are inferred to be
L-type brown dwarfs, each with masses between 60 and 80
Jupiter masses and ages between 0.1 and 2 billion years. Both brown dwarfs are inferred to have slightly inflated radii due to their high temperatures; the hotter and more massive component is estimated to have a radius of
Jupiter radii, while the cooler and less massive component is estimated to have a radius of 0.9–1.4 Jupiter radii.
Hydrogen and trace amounts of
sodium and
potassium in the brown dwarfs' atmospheres have been spectroscopically detected in the ZTF J1239+8347 system, in the form of
absorption lines. In the
visible and
ultraviolet spectrum, the ZTF J1239+8347 system exhibits a high
luminosity that could be explained if it also had a high temperature of . This high temperature indicates that one of the brown dwarfs in the system is being heated, most likely as a result of mass transfer. The extremely short orbital period of the ZTF J1239+8347 system supports the conclusion that its brown dwarfs are undergoing mass transfer. In the process of mass transfer (also known as
Roche lobe overflow), the more massive component (the accretor) actively pulls material directly from the less massive component (the donor). The
accreted material is concentrated into a narrow stream that directly impacts the accretor's atmosphere and heats it up, creating an hotspot that glows in blue and
ultraviolet light. Due to the
Coriolis effect and possibly strong
magnetic fields of the brown dwarfs, the stream of material may be deflected above or below the accretor's equator. As the brown dwarfs orbit each other, the accretor's hotspot rotates in and out of view from Earth, which leads to periodic changes in the system's observed brightness.
Light curve analysis shows that the system's peak-to-trough brightness
amplitude is very large (>2
magnitudes) in ultraviolet, but decreases over longer wavelengths. Based on the light curve of ZTF J1239+8347, the hotspot of the accretor is estimated to span a radius of Jupiter radii over its atmosphere. Slight asymmetries in the system's light curve suggest that the hotspot is extended longitudinally over the accretor's atmosphere, possibly as a result strong winds in the accretor. At maximum brightness, the visible spectrum of ZTF J1239+8347 shows prominent hydrogen absorption features, which implies that the accretor's hotspot glow is originating from inside the accretor's hydrogen-rich atmosphere or envelope. However, the ZTF J1239+8347 system also shows visible hydrogen
emission lines during minimum brightness, when the accretor's hotspot is out of view; this may be caused by either reprocessing in the accretor's atmosphere, or the donor reflecting light from the accretor's hotspot. The origin of the ZTF J1239+8347 binary system is unclear. Whitebook and colleagues suggested in 2026 that a third star might have gravitationally pushed the brown dwarfs closer together, leading to their currently tight orbit. The brown dwarfs' orbits are expected to
decay over time due to
gravitational radiation and
magnetic braking, but the timescale for this process is uncertain. The brown dwarfs will eventually merge to form a new star, with their combined mass large enough to initiate
nuclear fusion. == References ==