A neutron star–black hole merger (GW200105) occurred with an oval, highly eccentric orbit just before merger, marking the first robust evidence of such a path.

The GW200105 event shows that pre-merger orbits of mixed binaries can be elliptical and precessing, challenging the assumption of circular orbits.

Mass estimates for the black hole and neutron star in GW200105 have been revised using new LIGO/Virgo data and Birmingham University modeling.

The discovery undermines the circular-orbit assumption and highlights gaps in current models, pointing to the need for new theories and possibly future detectors like LISA to capture diverse signals.

Contrary to standard formation theories, the orbit remained highly eccentric at the end of the system’s life, suggesting alternative formation channels or environmental influences.

Analyses show the eccentricity and the lack of observed precession were imprinted early in the system, rather than arising from late-stage evolution.

Bayesian analysis indicates a circular orbit is extremely unlikely for GW200105, with about 99.5% confidence against circularity.

GW200105 occurred roughly 910 million light-years away, producing a final black hole around 13 solar masses, as detected by LIGO and Virgo.

A new Birmingham Institute of Gravitational Wave Astronomy model enabled measurement of orbital eccentricity and precession together, marking the first time these effects are jointly constrained in this type of merger.

The results imply multiple formation channels for black hole–neutron star mergers, explaining observed diversity and motivating more advanced waveform models.

The oval orbit points to formation in a densely interacting environment, likely involving gravitational interactions with other stars or a third body, rather than isolated evolution.

Overall, the study highlights how gravitational-wave astronomy will reveal unexpected orbital dynamics, opening new windows into extreme physics.