FAST Telescope Finds Lightest Double Neutron Star System Yet

China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world’s most sensitive single-dish radio telescope, has discovered the lightest double neutron star system ever recorded, opening a rare natural laboratory for testing Einstein’s theory of general relativity and probing the origins of the universe’s heaviest elements.

The system, designated PSR J1856-0039, completes one orbit every 2.36 hours and has a combined mass of just 2.488 times that of the Sun, according to findings published on 15 September in the journal Physical Review Letters, which selected the study as an editors’ highlight. The discovery was announced by researchers at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) and reported by Xinhua, China’s state news agency.

A Rare Cosmic Pair

Neutron stars are the extremely dense remnants left behind when massive stars explode as supernovae. Though only about 20 kilometers across, they can hold more mass than the Sun and spin rapidly, emitting regular radio pulses that function as extraordinarily precise cosmic clocks. When two such stars orbit each other, they form a double neutron star (DNS) system — an arrangement so rare that only about 30 have been firmly identified to date.

The newly found system is unusual on two counts. Its orbital period of 2.36 hours is the second-shortest ever measured for a DNS system, placing the two stars extraordinarily close together. And its total mass sets a new lower bound for the class. According to the peer-reviewed study, the visible pulsar weighs 1.304 ± 0.022 solar masses, while its companion weighs 1.185 ± 0.022 solar masses — one of the lightest neutron stars known, approaching the theoretical minimum.

“In this system, the visible pulsar weighs about 1.30 solar masses, while its companion is about 1.19 solar masses — making both among the lightest neutron stars ever detected and close to the theoretical minimum,” said Han Jinlin, a professor at NAOC who led the study, as quoted by China Daily.

Han, who heads the FAST Galactic Plane Pulsar Snapshot (GPPS) survey team, said the companion’s near-minimum mass provides important constraints on the poorly understood physics of supernova explosions.

A Test of General Relativity

The system’s extremely tight orbit produces an intense gravitational environment that allows physicists to test Einstein’s theory under extreme conditions. Using follow-up FAST observations, the team measured several relativistic effects, including the gradual rotation of the system’s elliptical orbit, a gravitational redshift that stretches light toward longer wavelengths as it escapes the strong field, and a slight slowing of time known as the Einstein delay.

The researchers also detected the system losing energy through gravitational-wave emission, causing the orbit to shrink over time. The measured rate of this decay matches the prediction of general relativity to within a ratio of 1.009 ± 0.014 at 68 percent confidence — a strong confirmation of the theory, according to the paper published in Physical Review Letters (a preprint is available on arXiv).

Based on these measurements, the team estimates the two neutron stars will merge in roughly 82 million years, most likely forming a heavier neutron star rather than collapsing directly into a black hole. Such mergers are of deep cosmological interest: they emit gravitational waves and forge heavy elements such as gold and platinum through the rapid capture of neutrons, making DNS systems a key source of the universe’s heaviest matter.

A Rare Chance to Probe Frame Dragging

Beyond confirming known relativistic effects, the system offers a rare opportunity for a future scientific first. Its small orbital inclination and ultrashort period make it an excellent candidate for detecting frame dragging — the Lense-Thirring effect, in which a spinning body drags the surrounding fabric of spacetime along with its rotation.

“In simple terms, a spinning neutron star stirs the spacetime around it. Among known binary neutron star systems, only one or two such systems are promising for such a measurement,” Han told China Daily.

Detecting frame dragging would allow scientists to measure a neutron star’s moment of inertia, offering a rare window into how matter is distributed inside these ultradense objects — a problem that remains one of the central puzzles of nuclear astrophysics. Han said long-term, high-precision monitoring with FAST could help determine how the pulsar’s mass is distributed as it spins, providing clues about its internal structure and how gravity behaves under extreme conditions.

A Telescope Built for Discovery

The discovery was made possible by FAST’s extraordinary sensitivity. Known as “China Sky Eye,” the telescope is equipped with an L-band 19-beam cryogenic receiver that can observe 19 areas of the sky simultaneously, allowing it to detect extremely faint radio pulses. Using a snapshot survey mode developed by the research team, scientists efficiently scanned the Milky Way’s stellar disk — the broad, densely populated region of the galaxy where most pulsars are found.

The GPPS survey has now discovered nearly 900 new pulsars. That steady output has made FAST a leading engine of global pulsar science since it began operations. Just days earlier, on 12 September, ITHome reported that the same telescope had built the world’s largest neutral-hydrogen galaxy sample, detecting some 156,000 sources — another marker of its growing scientific reach. The DNS discovery was supported by key research and development programs of China’s Ministry of Science and Technology and institutionalized projects of the Chinese Academy of Sciences.

What to Watch For

The finding adds a significant new data point to astrophysicists’ understanding of neutron star formation and mass distribution, and it sharpens the case for continued high-precision monitoring. If FAST can detect frame dragging in PSR J1856-0039, it would deliver a measurement that has eluded astronomers across nearly the entire known DNS population.

The two stars will not actually collide for another 82 million years. But the data they yield in the coming years, astronomers say, could reshape understanding of what happens inside a neutron star — and where the planet’s gold and platinum ultimately came from.