Researchers report unseen 2.2-solar-mass companion orbiting red giant in Pisces

Researchers reported Nov. 11, 2026, that an unseen companion in the binary system J0008+0128, in the constellation Pisces thousands of light-years away, has an estimated mass of roughly 2.2 solar masses. A typical neutron star has a mass of about 1.4 solar masses, so the companion’s estimated range sits well above that norm.

The companion reveals itself only through gravity. The red giant it orbits wobbles as the two objects move around each other, and the companion itself emits no detectable light. Researchers identified J0008+0128 using the LAMOST and APOGEE surveys with archival data from the retired ESA Gaia mission. After ruling out other explanations, the team concluded the object is most likely a neutron star, the crushed core left behind when a massive star explodes.

Neutron stars squeeze more mass than the Sun into a city-sized sphere and should be scattered throughout the Milky Way, but most are effectively invisible. Only a few thousand have been detected so far, mostly as pulsars, even though scientists estimate tens of millions to hundreds of millions exist in the Milky Way. To date, researchers have only been able to measure neutron-star masses in binary pairings. Such kicks at birth in supernova explosions can send neutron stars racing through the galaxy at speeds of hundreds of miles per second.

Research on neutron stars helps explain how stars live and die, how they spread heavy elements, and how matter behaves under extreme pressure and density. That is where gravity-only detection comes in. When a compact object passes in front of a distant background star, its gravity warps spacetime and deflects the background star’s light — gravitational microlensing, in which a foreground object’s gravity bends and magnifies light from a star behind it. The lensing briefly brightens the background star and shifts its position in the sky.

NASA’s Nancy Grace Roman Space Telescope can measure both the temporary brightening, called photometry, and the tiny positional shift, called astrometry, with exceptional precision. Many telescopes can catch the brightening, but Roman’s combination of the two lets it detect neutron stars and sometimes weigh them. Roman’s main survey hunts for exoplanets with photometric microlensing, but its astrometric capabilities open the door to discoveries through astrometric microlensing.

“What’s really cool about using microlensing is that you can get direct mass measurements,” said paper co-author Peter McGill of Lawrence Livermore National Laboratory. McGill said a star’s positional shift reveals the mass of the lensing object, giving researchers a direct way to weigh something otherwise unseen.

The team plans to point the future Galactic Bulge Time Domain Survey at millions of stars at once, using high-frequency sky images taken repeatedly. Zofia Kaczmarek of Heidelberg University in Germany led the study and it appeared in Astronomy and Astrophysics; it found that Roman could spot and measure dozens of isolated neutron stars using gravitational microlensing. If those predictions prove right, Roman could deliver the first large sample of isolated neutron stars found by gravity alone, exposing a hidden population researchers could not reach.

Roman’s measurements could help astronomers test whether a real gap separates neutron-star masses from black-hole masses, and could gauge how quickly neutron stars move. Researchers think a neutron star collapses into a black hole at about two to three times the Sun’s mass, but the exact threshold is unknown. NASA expects Roman to reshape the study of microlensing and hidden galactic populations, covering everything from rogue exoplanets to neutron-star remnants.

A 2024 case shows how uncertain that boundary remains. Researchers reported June 7, 2024, that the MeerKAT Radio Telescope in South Africa had spotted an unidentified object circling pulsar PSR J0514-4002E while monitoring pulsars in a star cluster 40,000 light-years from Earth. Pulsars spin and emit beams of light that flash past Earth as regularly as a precise clock; when flashes arrive off schedule, researchers can infer that another object is affecting the pulsar’s motion and use the timing to calculate its mass.

PSR J0514-4002E’s flashes revealed an invisible companion with a mass of 2.1 to 2.7 solar masses — heavy for a neutron star but lightweight for a black hole. Astronomers cannot be entirely sure of the object’s identity. The findings appeared in the Jan. 19 issue of Science, and the team hopes to resolve the object’s identity by continuing to track the pulsar’s flashes.

The companion’s estimated mass falls in that same uncertain territory between the heaviest neutron stars and the lightest black holes. Researchers say the origin of the object’s extra mass remains unknown, though the finding could help explain neutron-star physics.

NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages Roman, with participation from NASA’s Jet Propulsion Laboratory, Caltech/IPAC, the Space Telescope Science Institute, and a science team. Zofia Kaczmarek said the neutron stars detected so far are too small a sample to represent the big picture, and that a single isolated mass measurement would be powerful. Peter McGill said that even a handful of confirmed microlensing detections could improve models of stellar explosions and extreme matter. McGill also said researchers expect to begin flagging promising microlensing events within the first months after Roman’s commissioning.