Astronomers have discovered the lowest-mass double neutron star system known. The pair is named PSR J1856-0039. The findings were published in Physical Review Letters.

The system was found with China’s Five-hundred-meter Aperture Spherical radio Telescope, known as FAST. Researchers led by the Chinese Academy of Sciences studied it from 2020 to 2025. The pulsar was first detected on May 4, 2020.

The two stars orbit each other every 2.36 hours. That is the second-shortest orbit among confirmed double neutron star systems. Their combined mass is 2.48841 solar masses. The pulsar weighs 1.304 solar masses. Its companion weighs 1.185 solar masses, one of the lightest neutron stars measured.

The orbit is shrinking as the pair emits gravitational waves. The measured decay matches Einstein’s general relativity predictions within 1.4 per cent. The stars will merge in about 82 million years. They may form a stable neutron star or collapse into a black hole.