Black holes and neutron stars are the universe's most extreme objects. When they collide — merging in violent cataclysms that release energy equivalent to entire galaxies for a fraction of a second — they send ripples through space-time itself. We detect them as gravitational waves.
Until now, it was assumed that these pairs always merged in nearly circular orbits. Today, scientists at the University of Birmingham, working with colleagues from the Universidad Autónoma de Madrid and the Max Planck Institute for Gravitational Physics, have published evidence that one such pair didn't follow the rules.
The event, GW200105, was first detected by the LIGO and Virgo gravitational-wave observatories in January 2020. At the time, scientists classified it as a neutron star-black hole merger and moved on. Now, using a new and more sophisticated gravitational-wave analysis model developed at Birmingham's Institute of Gravitational Wave Astronomy, the research team has gone back to GW200105 and found something extraordinary.
The black hole and neutron star were orbiting each other in an **oval — an eccentric orbit** — just before they merged. That has never been seen before in a neutron star-black hole collision. It challenges fundamental assumptions about where and how these systems form.
**Why Circular Orbits Were Expected**
Under standard theory, when a black hole and a neutron star form a binary pair and begin orbiting each other, gravitational wave emission gradually radiates away the orbital energy. Over time — potentially millions or billions of years — this circularises the orbit. By the time the two objects are close enough to merge, they are expected to be in a nearly perfect circle.
GW200105 broke that rule. The orbit was detectably eccentric right up to merger. Something disrupted the expected progression.
**What the Oval Orbit Tells Us**
Dr. Patricia Schmidt, Associate Professor at the University of Birmingham, explained the significance: '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.'
An eccentric orbit in the moments before merger suggests the two objects didn't evolve in isolation — spending billions of years slowly spiralling toward each other. Instead, the system was likely shaped by gravitational interactions with other stars in its environment.
Geraint Pratten, a Royal Society University Research Fellow from Birmingham, noted: 'The elliptical shape of the orbit indicates the system likely was not evolving in isolation but was shaped by gravitational interactions with other stars.'
This points to a birthplace in a dense stellar environment — a globular cluster or a galactic nucleus teeming with stars, where close gravitational encounters are common. Such environments can 'toss' compact objects into tight, eccentric orbits with each other — bypassing the billions of years of gentle in-spiral.
**A First in Measurement**
The new analysis did something no previous neutron star-black hole study had achieved: it simultaneously measured both the **eccentricity** (how oval the orbit was) and the **spin-induced precession** (the wobbling effect caused by the objects' rotational spin). Measuring both together, in the same event, provides a far richer picture of the system's dynamics.
The merged remnant — what's left after the collision — is a black hole approximately **13 times the mass of the Sun**.
**Why This Changes the Picture**
The discovery demonstrates that not all neutron star-black hole pairs share the same origin story. Some pairs form in relative isolation and evolve on predictable timescales. Others — like GW200105 — are dynamically assembled in crowded environments, their chaotic paths reflected in their unusual orbits at the moment of merger.
It's a reminder that the universe writes its own rules — and that when we listen carefully enough, we still find ones we didn't know existed. 🌌
*Sources: University of Birmingham (March 11, 2026) · The Astrophysical Journal Letters · Dr. Patricia Schmidt · Geraint Pratten · LIGO/Virgo collaboration · LiveScience*
