Scientists have found the first strong evidence that a neutron star and a black hole merged while travelling in an oval orbit rather than the near-perfect circle scientists long expected
The discovery offers new insight into how these extreme cosmic pairs form and evolve.
The finding comes from an international team of researchers from the University of Birmingham, Universidad Autónoma de Madrid, and the Max Planck Institute for Gravitational Physics. Their study, published in The Astrophysical Journal Letters, analysed gravitational-wave signals from the GW200105 collision.
Gravitational waves are ripples in space-time produced by massive objects accelerating through the universe. Detectors such as LIGO in the United States and Virgo in Europe can observe these signals when compact objects, such as black holes and neutron stars, spiral together and merge.
Before they collide, most neutron star–black hole pairs are expected to move in nearly circular orbits. Over time, gravitational waves drain energy from the system, smoothing out any initial orbital irregularities.
However, the new analysis suggests that the objects involved in GW200105 were still travelling along an elliptical path shortly before merging. This means their orbit was stretched into an oval rather than forming a circle.
The team used an advanced gravitational-wave model developed at the University of Birmingham’s Institute of Gravitational Wave Astronomy to examine the signal in more detail. Their method allowed them to measure both the orbit’s eccentricity and any precession, a wobble caused by the spinning of the objects.
By comparing thousands of theoretical predictions with the real gravitational-wave data, the researchers found statistical evidence that the orbit was not circular. The analysis ruled out a circular orbit with 99.5 per cent confidence.
Revising the masses
The new model also changes scientists’ estimates of the masses involved in the collision. Earlier studies assumed the orbit was circular, which led to slightly different values.
With the updated analysis, the merged system appears to have produced a black hole roughly 13 times the mass of the Sun. The recalculated figures show that earlier estimates slightly underestimated the black hole’s mass while overestimating the neutron star’s mass.
Researchers also found no strong evidence that spin-induced wobbling played a major role in shaping the orbit. This suggests the eccentric shape likely originated during the system’s formation rather than developing later due to spin effects.
The oval orbit suggests that the pair may have formed in a dense stellar environment where many objects interact gravitationally. In regions like close encounters between stars, neutron stars and black holes can dramatically reshape their paths.
These interactions may involve multiple bodies, including a possible third companion star or black hole that perturbed the system before the final merger. Such chaotic environments could leave binary systems with unusual orbital shapes that persist until the moment of collision.
Expanding the picture of cosmic mergers
The discovery adds to a growing list of surprising behaviours observed in gravitational-wave events. As detectors continue to improve, scientists are discovering that compact binary systems are more diverse than originally predicted.
Understanding these differences is important for reconstructing how these extreme objects form and interact across the universe. Improved waveform models that capture effects like orbital eccentricity will also help researchers extract more detailed information from future gravitational-wave detections.
As observatories detect more mergers in the coming years, scientists expect to uncover even more unusual systems, offering a clearer picture of the dynamic, sometimes chaotic environments where black holes and neutron stars are born and collide.
