Oval Orbit Reveals New Insights into Black Hole-Neutron Star Mergers

Researchers from the University of Birmingham, Universidad Autónoma de Madrid, and the Max Planck Institute for Gravitational Physics have made a significant discovery regarding the merger of black holes and neutron stars. Their study, published in _The Astrophysical Journal Letters_ on March 11, 2026, presents the first robust evidence of a black hole and neutron star orbiting in an oval path prior to their merger, rather than the expected circular orbit.

This finding challenges long-standing assumptions about the formation and evolution of these cosmic pairs. The gravitational-wave event known as GW200105 revealed that the system traveled along an oval orbit before merging into a black hole that is 13 times more massive than the sun. This oval orbit, never before observed in such collisions, provides new insights into the dynamics of these extreme astronomical events.

Dr. Patricia Schmidt from the University of Birmingham emphasized the importance of this discovery, stating, "This discovery gives us vital new clues about how these extreme objects come together. It tells us that our theoretical models are incomplete and raises fresh questions about where in the universe such systems are born."

The research team utilized data from the LIGO and Virgo detectors, applying a new gravitational-wave model developed at the University of Birmingham's Institute of Gravitational Wave Astronomy. This innovative approach allowed the researchers to measure the eccentricity of the orbit and any spin-induced wobbling, marking the first time these effects have been measured together in a neutron star-black hole event.

Geraint Pratten, a Royal Society University Research Fellow, noted that the elliptical shape of the orbit suggests that the system did not evolve in isolation but was influenced by gravitational interactions with other stars or possibly a third companion. A Bayesian analysis comparing thousands of theoretical predictions to actual data indicated that a circular orbit is extremely unlikely, ruling it out with 99.5% confidence.

The implications of this research extend beyond the specific event studied. Gonzalo Morras from the Universidad Autónoma de Madrid and the Max Planck Institute highlighted that this finding is compelling evidence that not all neutron star-black hole pairs share the same origin, suggesting a more complex formation environment where multiple stars interact gravitationally.