A detail that no one had thoroughly examined was hidden somewhere in the LIGO gravitational wave detectors’ data archives, buried in a signal captured on January 5, 2020. The analysis assumed something so well-established that it hardly seemed worth challenging, not because the scientists weren’t cautious—they were. The orbit of a neutron star and a black hole should circularize as they spiral toward one another over millions of years, losing energy to gravitational radiation. Physics predicts that. Every prior observation had demonstrated that. Thus, the initial analysis of GW200105 made the assumption of a circular orbit, calculated the results, and moved on.
A group from the University of Birmingham, the Universidad Autónoma de Madrid, and the Max Planck Institute for Gravitational Physics returned to that signal six years later using a different model that did not make the circular assumption. On March 11, 2026, their findings were published in The Astrophysical Journal Letters: the orbit was oval. Unquestionably, clearly, and quantifiably oval. A circular orbit was ruled out with 99.5% confidence using a Bayesian analysis that compared thousands of theoretical models to the actual gravitational wave data. That is not a hint in astrophysics. That’s the outcome.
| Event designation | GW200105 — gravitational wave event detected January 5, 2020; new analysis published March 11, 2026 |
| Published in | The Astrophysical Journal Letters, March 11, 2026 |
| Lead institution | University of Birmingham — Institute of Gravitational Wave Astronomy |
| Research partners | Universidad Autónoma de Madrid; Max Planck Institute for Gravitational Physics (Germany) |
| Lead author | Gonzalo Morras — Universidad Autónoma de Madrid / Max Planck Institute for Gravitational Physics |
| Key co-authors | Dr. Patricia Schmidt (University of Birmingham); Geraint Pratten, Royal Society University Research Fellow (University of Birmingham) |
| What collided | A neutron star and a black hole — a “mixed merger” — producing a resulting black hole approximately 13 times the mass of the Sun |
| Distance from Earth | Approximately 910 million light-years away |
| The anomaly | The pair orbited on an oval (eccentric/elliptical) path just before merging — directly contradicting the long-held expectation that such systems settle into near-perfect circular orbits before collision |
| Statistical confidence | Circular orbit ruled out with 99.5% confidence via Bayesian analysis comparing thousands of theoretical models |
| Detection instruments | LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo gravitational wave detectors |
| Likely origin theory | The system probably formed in a dense, chaotic stellar environment — shaped by gravitational interactions with other stars or a third companion body |
| Impact on prior data | Earlier analyses of GW200105 (assuming circular orbit) underestimated the black hole’s mass and overestimated the neutron star’s mass — both now corrected |
| Reference / source | https://www.sciencedaily.com/releases/2026/03/260311213432.htm |
The actual event took place 910 million light-years away from Earth. In human terms, that means that before complex animal life existed on this planet, the light from that collision left its source. The last moments of a merger—a neutron star, the compressed remnant of a dead star, being engulfed by a black hole, leaving behind a new black hole about 13 times the mass of the Sun—were what LIGO and Virgo saw. The duration of the gravitational wave signal is merely a few milliseconds. However, the shape of the orbit the two objects were following prior to their collision is imprinted in those vibrations, like a fingerprint pressed into spacetime.
Early in the 1600s, Johannes Kepler discovered that planetary orbits are ellipses rather than circles. In space, oval shapes are typical. While many comets travel on extremely stretched ovals, Earth’s orbit around the Sun is somewhat elliptical. Therefore, an oval orbit may not appear odd at first. The problem is that black hole and neutron star pairs don’t behave like planets. As they orbit one another, they continuously release gravitational waves that drain energy.
This energy loss has a circularizing effect, gradually smoothing the orbit into something that resembles a perfect circle long before the two objects are close enough to merge. This pattern was followed by all detected mixed mergers prior to GW200105. At the time of impact, none had displayed a measurably eccentric orbit.
“The orbit gives the game away,” stated Geraint Pratten, a Royal Society University Research Fellow at Birmingham who co-developed the new model. Its elliptical shape just prior to merger indicates that this system was most likely shaped by gravitational interactions with other stars or possibly a third companion rather than evolving quietly in isolation.” “Did not evolve quietly in isolation” is a powerful statement. A binary neutron star–black hole system is typically depicted as two objects that form together, orbit one another for eons, and eventually spiral inward.
Calm and predictable, controlled only by the constant drain of gravitational waves and their mutual gravity. Instead, the oval orbit points to a more chaotic origin—possibly a dense stellar cluster where gravitational interactions between several bodies can introduce eccentricity into an orbit late in the game, upsetting what should have been a smooth circularization.
Gonzalo Morras, the Max Planck Institute’s lead author, called it compelling evidence that “not all neutron star–black hole pairs share the same origin.” That may seem modest, but it’s a big change in perspective. This indicates that mixed mergers originate from multiple formation channels. While some form in relative isolation, others are shaped by the chaotic dynamics of dense environments, such as galactic centers, globular clusters, and locations where stars are so close to one another that their gravity interferes.
The distinction is important because the two pathways predict different orbital characteristics, different mass distributions, and different rates of merger, all of which influence how astrophysicists model the larger population of collisions between compact objects.
Reading the specifics of this discovery gives me the impression that the correction it imposed on everything else is more fascinating than the oval orbit itself. The initial GW200105 analysis underestimated the black hole and overestimated the neutron star due to its circular assumption. Both are corrected by the new analysis. This begs the quiet, slightly unsettling question: how many other gravitational wave events in the past were examined using presumptions that were so clear-cut that no one bothered to test them?
The true frequency of eccentric mergers is still unknown. This is just one instance thus far. However, the sensitivity of gravitational wave detectors continues to increase, the signal archive continues to expand, and more advanced models are being developed to analyze the signals. GW200105’s oval orbit anomaly may prove to be uncommon. Alternatively, it might prove to be something that was always present, just waiting to be discovered.
