China's FAST telescope finds first still-evolving primordial pulsar triple system

Researchers led by Han Jinlin (韩金林) of the National Astronomical Observatories of the Chinese Academy of Sciences have shown that pulsar J0435+3233, discovered by the FAST radio telescope known as "China Sky Eye", is the first primordial triple system still in an evolving stage, according to the observatories. It is the second confirmed pulsar triple system. The team combined FAST radio data with multi-wavelength data from international optical and gamma-ray telescopes, and the results were published on October 9 in The Astrophysical Journal Letters. [ 2 , 3 , 4 ]

The pulsar was first discovered with FAST on June 8, 2020, by the observatories' drift-scan survey team, and has a spin period of only 3.2 milliseconds. A Xinjiang Astronomical Observatory team then monitored it with FAST for five years, obtaining 192 measurements. They found that the pulsar and a white dwarf orbit their common centre of mass every 8 days on a circular orbit, and that the pulsar's period increases by 1 nanosecond a year, two orders of magnitude more than similar pulsars. IT Home reported that these first-phase results were published in Nature Astronomy in April 2026. [ 3 , 4 ]

Han's team, working with researchers from five Chinese research institutions, then analysed FAST radio data together with open archival infrared and optical data. IT Home reported that these included data from Gaia, 2MASS and PanSTARRS, plus 160 effective gamma-ray photons accumulated by the Fermi satellite over 18 years. The researchers identified the pulsar's optical companion and confirmed the triple system. The second companion is a Sun-like star still evolving, with nearly the same mass as the Sun, orbiting the pulsar-white dwarf binary every 73.5 years on an eccentric orbit with eccentricity 0.6. This star caused the anomalous increase in the pulsar's spin period measured earlier by FAST. [ 3 , 4 ]

Based on stellar evolution theory, the system originated as primordial "triplets" formed in the same cloud, Jiemian reported. The most massive star, about 20 solar masses at formation, ended its evolution and became the pulsar; the second, initially several solar masses, became the white dwarf. The smallest, at one solar mass and far from the other two, was not affected by their early supernova explosions and is still evolving. Eventually it will also burn out, leaving a long-term stable triple of a pulsar and two white dwarfs. [ 3 ]

Apart from pulsar planetary systems, the only pulsar triple system found before consisted of a pulsar and two white dwarfs. Jiemian said it was found by a foreign team; IT Home said it was a US team using the GBT, and that all three stars had reached the end of their evolution. IT Home also reported that a European team of scientists from Germany, the Netherlands, France and the UK independently reached the same conclusion from similar archival data, and that the Chinese team's orbital geometry and parameters were an order of magnitude more precise because it accounted for gravitational interactions within the system and several relativistic effects. [ 3 , 4 ]

The observatories called the discovery a landmark result of relay exploration by multiple Chinese and foreign teams. They said the system shows clear three-body gravitational interaction effects and is a natural platform for testing fundamental gravity theories such as the strong equivalence principle. [ 3 , 4 ]