China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) has identified PSR J1856-0039, a double neutron star system located about 18,500 light-years from Earth, with an orbital period of only 2.36 hours — shorter than a working afternoon. A paper led by researcher Han Jinlin of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) was published in *Physical Review Letters* on September 15, 2026, and selected by the journal's editors as a highlighted contribution.
Key points
- Rarity: Only about 30 double neutron star systems are known; PSR J1856-0039 is a newly added example, found through FAST's Galactic plane pulsar survey.
- Record-lightest system: Total mass is 2.488 solar masses — the lightest known double neutron star system. The visible pulsar weighs ~1.304 solar masses; the companion ~1.185 solar masses, among the lightest neutron stars ever measured. Per Han Jinlin: the companion is "almost at the theoretical minimum allowed mass, providing key clues for studying supernova explosion mechanisms."
- Relativity tests: Four relativistic effects appear simultaneously — periastron precession, gravitational redshift, time dilation, and orbital decay from gravitational-wave emission. The measured orbital decay matches the general relativity prediction with a ratio of 1.009 ± 0.014, consistent within ~1.4%. Taiwan's *Commercial Times* also reported the detection of the "Einstein delay."
- Future prospects: Because of its small orbital inclination and extremely short period, this system is one of only one or two known candidates where frame-dragging (Lense-Thirring precession) may be detectable — which would allow inference of the neutron star's moment of inertia and constrain its internal equation of state. Both remain unmeasured and require years of high-precision timing.
- Fate: Models predict merger in roughly 82 million years, most likely producing a heavier neutron star, or possibly collapsing into a black hole after spin-down.
- How it was found: FAST, the world's most sensitive single-dish radio telescope, uses a self-developed snapshot mode with a 19-beam L-band cryogenic receiver. It has discovered about 900 new pulsars to date; PSR J1856-0039 was singled out from this sample through long-term timing follow-up.
Why double neutron stars matter
These systems form through two successive supernova explosions of massive stars, leaving two compact remnants that survive as a bound pair. They steadily lose orbital energy by radiating gravitational waves, shrinking until they merge. Such mergers are a major production channel for heavy elements like gold and platinum in the universe.
Broader context
In the same week, Tsinghua University's Di Li team, using FAST jointly with the Karl G. Jansky Very Large Array, reported 118 neutral-hydrogen supershells in the Andromeda galaxy in *Nature Astronomy* (September 17, 2026), confirming that successive supernova explosions power galactic turbulence. Meanwhile, NAOC's StarWhisper Telescope, which integrates an AI agent into telescope control, has found 8 early supernova candidates as part of the Sitian Project's pilot array, and was cited by Stanford HAI's *AI Index Report 2026* as a representative AI-agent case in physics and astronomy.
The common thread: discovery bandwidth is expanding (FAST finds nearly a thousand pulsars a year), but extracting physics from them — identifying the one system in 900 worth years of tracking, then the one or two effects worth measuring — still depends on human judgment rather than discovery speed.
References
1. NAOC, "FAST discovers the lightest double neutron star system with the shortest orbital period," published in *Physical Review Letters* (2026-09-15, Editors' Suggestion), 2026-09-16 2. Science and Technology Daily (reporter Lu Chengkuan), front page, 2026-09-17. https://www.cas.cn/cm/202609/t20260917_5120488.shtml 3. CCTV News, 2026-09-16. https://ysxw.cctv.cn/article.html?item_id=16806516892485029486 4. Commercial Times (Taiwan), 2026-09-16. https://www.ctee.com.tw/news/20260916701195-430801 5. Xinhua (reporter Wei Mengjia), on the *Nature Astronomy* supershell result, 2026-09-18. https://www.k618.cn/takx/202609/t20260918_20059610.html