FAST Finds Rare Millisecond Pulsar With Near-Circular Orbit
China's FAST radio telescope tracked pulsar PSR J1810-0623 for six and a half years and measured an orbital eccentricity of just 0.000015, one of the most circular ever recorded.
- FAST tracked pulsar PSR J1810-0623 over six and a half years, with the discovery announced on June 11, 2026.
- The pulsar spins about 220 times per second, giving a rotation period of 4.55 milliseconds.
- The binary system's orbit is extremely circular, with an eccentricity of only 0.000015.
- The team used the polarization of the radio signal to measure the magnetic field along the line of sight.
A team of astronomers using China’s FAST radio telescope announced on June 11, 2026, that it had measured the orbit of the millisecond pulsar PSR J1810-0623 and found it to be almost perfectly circular. The system lies within our Galaxy, and the observations spanned six and a half years. What makes the measurement notable is the orbital eccentricity of just 0.000015, one of the smallest values recorded for such a system.
Pulsars are rapidly rotating neutron stars, the remnants of massive stars that ended their lives in a supernova. Millisecond pulsars are a special class: they spin hundreds of times per second. PSR J1810-0623 completes about 220 rotations every second, corresponding to a rotation period of 4.55 milliseconds. According to the research team, that exceptionally stable spin makes the pulsar valuable for precise astrophysical measurements.
Why does such a circular orbit matter to scientists?
The answer lies in how millisecond pulsars are thought to form. They are believed to have gone through a process known as recycling. In a binary system, material from a companion star is gradually transferred onto the neutron star. That material carries angular momentum and, over millions of years, speeds up the neutron star’s rotation to millisecond periods. The same process has a side effect: it gradually smooths out the orbit and reduces its eccentricity.
An eccentricity of 0.000015 suggests this system went through an exceptionally thorough recycling process. For comparison, a value of zero would describe a perfect circle. Such a low measured value gives researchers a rare chance to test models of stellar evolution and mass transfer in binary systems. The more precise the measurement, the tighter the constraints it places on theoretical models.
What else does the pulsar reveal about the Galaxy?
Beyond mapping the orbit, the team used the polarization of the radio signal arriving from the pulsar. As the signal travels through interstellar space, magnetic fields along its path slightly alter its polarization. By measuring those changes, researchers can reconstruct the strength and direction of the magnetic field between us and the source. Pulsars thus serve as natural probes for mapping the hidden structure of the Galaxy’s magnetic field.
It is worth being precise about the reach of this result. This is one system and one set of observations, however long-running. The extremely low eccentricity is valuable precisely because it is rare, but a single measurement does not overturn the existing picture of how millisecond pulsars form. It adds to it a new, highly precise data point. The value of the finding lies in the quality of data that FAST, as the most sensitive single-dish radio telescope, can gather over years of patient tracking.
How might more measurements like this, on even more circular or even stranger orbits, change what we think we know about the lives of neutron stars?
