FAST Telescope Finds Lightest Double Neutron Star System
China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) has discovered a double neutron star system with the lowest total mass ever measured, a result that researchers say provides fresh support for Albert Einstein’s general theory of relativity and opens a rare window into the physics of extreme matter.
The system, designated PSR J1856–0039, was identified by a team led by Han Jinlin (韩金林) of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC). The findings were published on September 15, 2026, in Physical Review Letters (PRL) and selected by the journal’s editors as an Editors’ Suggestion — a distinction reserved for a small fraction of papers judged especially important or interesting.
An Exceedingly Rare System
Double neutron star (DNS) systems — pairs of ultra-dense stellar remnants orbiting one another — are among the rarest known objects in the cosmos. According to Xinhua News, only about 30 such systems have been discovered to date.
The Chinese-language report described neutron stars as “extremely dense celestial remnants formed after the collapse of massive stars,” whose remarkably regular rotation has earned them the nickname of “cosmic precision clocks.” Such systems form through two successive supernova explosions and eventually merge via gravitational-wave radiation, making them key sources of heavy elements such as gold and platinum.
“This class of celestial system is an important birthplace of heavy elements such as gold and platinum in the universe, and also a naturally ideal laboratory for studying extreme high-density states of matter, testing general relativity, and analyzing supernova explosion mechanisms,” the Xinhua report stated.
Record-Setting Orbit and Mass
The newly discovered system is extraordinary for both its tight orbit and its low mass. The two neutron stars complete one orbit every 2.36 hours, an exceptionally compact configuration that produces pronounced relativistic effects.
The total mass of the system is just 2.48841 ± 0.00015 solar masses — the lowest ever measured for any known double neutron star system, and the only confirmed example with a total mass below 2.5 solar masses. The visible pulsar weighs about 1.304 solar masses, while its companion is approximately 1.185 solar masses — one of the lightest neutron stars ever measured, approaching the theoretical lower mass limit predicted by supernova simulations.
According to the arXiv preprint accompanying the PRL publication, the pulsar is “mildly recycled” with a spin period of 23.4 milliseconds, orbiting in a “compact eccentric orbit (e = 0.106).”
The Xinhua report noted that the 2.36-hour orbit ranks as the second-shortest among known double neutron star systems, placing PSR J1856–0039 among the most relativistically extreme double neutron star systems known. The record here is the system’s mass, not its orbital period.
Precision Test of General Relativity
FAST’s sensitivity allowed researchers to measure several post-Keplerian parameters — subtle effects arising from the strongly curved spacetime around the two compact objects. These included the orbital period derivative, periastron advance, and the Einstein delay.
The results matched the predictions of general relativity to high precision. The ratio of the observed orbital decay due to gravitational-wave emission to the value predicted by general relativity was 1.009 ± 0.014, a near-perfect agreement.
“Data calculations show that the ratio of the measured orbital period decay value to the general relativity predicted value is 1.009 ± 0.014, confirming the correctness of general relativity,” the Xinhua report said.
This extends a long tradition of binary-pulsar gravity tests, following landmark systems such as PSR B1913+16 — the discovery that earned the 1993 Nobel Prize in Physics — and the double pulsar PSR J0737–3039A/B.
A Merger 82 Million Years Away
The team’s modeling predicts that the two neutron stars will spiral together and merge in approximately 82 million years, most likely forming a more massive neutron star, though the PRL abstract allows for the possibility that the remnant could collapse into a black hole after spin-down.
“According to the team’s model projections, this system will merge in about 82 million years, most likely eventually forming a more massive neutron star,” Xinhua reported.
That eventual merger places the system squarely within the science of r-process nucleosynthesis — the mechanism thought to forge the heaviest elements in the universe, including gold and platinum.
A Future Probe of Frame-Dragging
The discovery may also enable a measurement that has proven elusive in astronomy. The system’s very small orbital inclination and short period make it a prime candidate for detecting the Lense–Thirring effect — the dragging of spacetime around a rotating body, a prediction of general relativity first described by Josef Lense and Hans Thirring in 1918.
Only one or two other double neutron star systems are considered plausible targets for probing this effect. Because frame-dragging is directly tied to a neutron star’s moment of inertia, long-term monitoring with FAST could allow researchers to measure that quantity precisely, providing rare constraints on the internal composition of these extreme objects.
“Long-term monitoring could potentially probe the Lense–Thirring precession,” the PRL abstract notes.
A Growing FAST Legacy
The discovery also underscores FAST’s emergence as the world’s leading instrument for pulsar science. The telescope, located in Guizhou Province, is the largest and most sensitive single-dish radio telescope in operation.
Equipped with a high-performance L-band 19-beam cryogenic receiver, FAST has now discovered roughly 900 new pulsars through the FAST Galactic Plane Pulsar Snapshot (GPPS) survey. PSR J1856–0039 is the fifth double neutron star system found by the survey. The pulsar was first detected on May 4, 2020, and its orbit was confirmed through a 2.5-hour tracking observation on December 1, 2022, following 17 observing sessions spanning 2020 to 2025.
The research was supported by the Ministry of Science and Technology’s key R&D program and institutionalized projects of the Chinese Academy of Sciences.
What to Watch
The result, published in one of physics’ most selective journals, represents a significant milestone for Chinese astrophysics and for pulsar astronomy more broadly. As researchers continue to monitor PSR J1856–0039 with FAST, the system’s record-low mass and extreme orbital geometry may yield further insights into the equation of state governing neutron star interiors — and possibly the first clean detection of frame-dragging in a double neutron star system. For now, the discovery stands as a striking demonstration of how a single, sensitive instrument can transform a rare cosmic object into a laboratory for testing the fundamental laws of physics.