[China Sky Eye FAST discovers extremely short period, lightest double neutron star system] September 16th. Recently, relying on the ultra-high detection sensitivity of China Sky Eye FAST, the FAST galactic plane pulsar survey team led by researcher Han Jinlin of the National Astronomical Observatory made an important discovery and captured a double neutron star system PSRJ1856-0039 in a tight orbit. This system is a binary neutron star system with an extremely short orbital period and the lightest total mass currently known. Multiple observation results have accurately verified the general theory of relativity, providing a key new goal for exploring extreme astrophysics, the nature of gravity, and the origin of elements in the universe. The relevant research results were published in Physical Review Letters (PRL) on September 15, 2026, and were selected by the journal editor as a key recommendation result.