Oval Orbit Revealed in LIGO’s 2020 Neutron Star–Black Hole Crash

On Jan. 5, 2020, a ripple in spacetime washed through a single 4-kilometer machine in Louisiana, briefly nudging its laser beams by less than the width of a proton. At the time, the signal was filed as a solid but unremarkable catch: a black hole swallowing a neutron star nearly a billion light-years away.

Six years later, that same event has taken on new significance. A fresh analysis finds that the doomed pair did not orbit each other in a neat circle, as astronomers had assumed, but in a distinctly oval path — a sign that they were likely brought together in a crowded, chaotic region of space rather than evolving quietly as a lifelong stellar couple.

The result is the first robust measurement of orbital eccentricity in a neutron star–black hole merger detected by the LIGO and Virgo observatories. Researchers say it challenges a core assumption behind many models of how such systems form and points toward a more diverse, and more violent, origin story for some of the universe’s most extreme collisions.

“This discovery gives us vital new clues about how these extreme objects come together,” said Patricia Schmidt, an astrophysicist at the University of Birmingham and co-author of the new study. “It also shows that our theoretical models are incomplete.”

The work, led by Gonzalo Morras of Universidad Autónoma de Madrid and the Max Planck Institute for Gravitational Physics, was published this month in The Astrophysical Journal Letters.

A signal first seen by one detector

The event, known as GW200105, was first reported in 2021 as one of the earliest confirmed mergers between a black hole and a neutron star. The signal was picked up mainly by the LIGO detector near Livingston, Louisiana; its twin in Hanford, Washington, was offline, and Europe’s Virgo detector near Pisa, Italy, saw only a marginal trace.

Analysis by the LIGO–Virgo–KAGRA collaboration showed a black hole about 8.9 times the mass of the sun merging with a neutron star roughly 1.9 solar masses, at a distance of about 280 megaparsecs (about 910 million light-years). Like virtually all low-mass systems analyzed at the time, GW200105 was modeled under the assumption that the two objects were in a nearly circular orbit when they entered the detectors’ sensitive band around 20 hertz.

That assumption was not arbitrary. General relativity predicts that as compact objects orbit each other and emit gravitational waves, they lose energy and angular momentum. Over millions to billions of years, that radiation tends to shrink and circularize their orbits. For pairs that start life as ordinary massive stars born together in relative isolation, theory suggests any initial ellipticity should be almost completely erased by the time the system is emitting gravitational waves detectable on Earth.

“The standard picture says that by the time a system like this reaches the LIGO band, its orbit should be almost perfectly round,” said Geraint Pratten, a Royal Society research fellow at the University of Birmingham and co-author of the study. “Here, the orbit gives the game away. Its elliptical shape just before merger shows this system did not evolve quietly in isolation.”

Reanalyzing the waveform for hidden structure

To uncover that shape, Morras and colleagues reanalyzed the original GW200105 data with a new theoretical model of gravitational waves that, for the first time in such a system, includes both orbital eccentricity and the wobbling effect of spin-induced precession. Rather than assuming a circular orbit from the start, they allowed the eccentricity to vary and used Bayesian statistical methods to see which combination of masses, spins and orbital parameters best matched the recorded signal.

In simple terms, an eccentric orbit leaves telltale fingerprints on a gravitational wave. Instead of a perfectly smooth “chirp,” in which the wave’s frequency and amplitude rise in lockstep as the objects spiral together, an eccentric pair speeds up and slows down during each orbit, rushing closest at periastron and swinging farther out at apoastron. That motion imprints small modulations and extra harmonics on the waveform and subtly alters how its phase evolves over time.

These effects are too faint to see by eye in the noisy data from a single detector, but they add up over many orbital cycles. With a bank of accurate theoretical templates to compare against, the analysis can discriminate between a truly circular inspiral and one with a measurable oval.

A modest oval — but strongly noncircular

The new study finds that GW200105 had an orbital eccentricity of about 0.145 when the gravitational-wave frequency reached 20 hertz. On a scale where 0 is a perfect circle and values close to 1 describe highly elongated ellipses, an eccentricity of roughly 0.15 is a modest oval — not a comet-like orbit, but far from circular by the standards of late-stage mergers.

Crucially, the researchers report that orbits with eccentricity below 0.028 at that stage are excluded at 99.5% confidence. In other words, a nearly circular orbit — the default assumption in earlier work — is statistically very unlikely.

“We see convincing proof that not all neutron star–black hole pairs share the same origin,” Morras said in a statement released by the University of Birmingham. “The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally.”

Why eccentricity matters: two origin stories

Astrophysicists distinguish broadly between two formation channels for compact-object binaries:

• Isolated formation: Two massive stars are born together in the galactic field, evolve as a binary, and each collapses into a compact remnant. If the system survives both supernova explosions, it gradually loses energy through gravitational radiation until the neutron star and black hole collide. In this case, the long inspiral efficiently erases any initial eccentricity.

• Dynamical formation: Pairs are assembled later through gravitational interactions in dense stellar clusters, the crowded regions around supermassive black holes, or within triple systems where a distant third companion perturbs the inner pair. Close encounters and exchanges can throw objects together on orbits that plunge rapidly toward merger, leaving less time for circularization.

Because of that difference, measurable eccentricity in the frequency band probed by LIGO, Virgo and Japan’s KAGRA detector is widely considered a smoking gun for a dynamical origin.

“The eccentric orbit is telling us that GW200105 most likely formed in a region where stars and compact objects are constantly influencing each other, rather than in a quiet patch of the galaxy,” Schmidt said.

Hints of that conclusion had emerged before. In 2025, another team using different eccentric waveform models reported moderate statistical support for nonzero eccentricity in GW200105. But the evidence at the time was not strong enough to claim a detection. By employing a more sophisticated model that simultaneously handles eccentricity and spin effects, Morras and colleagues have now strengthened the case.

Implications for future gravitational-wave searches

The finding adds a new piece to the growing gravitational-wave catalog, which already includes binary black holes of unexpected mass, the first neutron star merger tied to a visible explosion, and other neutron star–black hole collisions. It suggests that the population of such systems is a mixture: some likely form through quiet binary evolution, while others, like GW200105, may come from gravitational pinball in stellar clusters or galactic centers.

It also has practical implications for how future signals are searched and interpreted. Many of the search algorithms used during LIGO and Virgo’s first observing runs relied mainly on circular-orbit templates. As detectors become more sensitive and their catalogs grow, researchers say accounting for eccentricity will be increasingly important, both to avoid biasing parameter estimates and to use orbital shape as a tracer of cosmic environments.

The result underscores another lesson as well: major discoveries can emerge years after an event, as theory and modeling catch up.

“Events we thought we understood can still surprise us when we look again with better tools,” Pratten said. “GW200105 shows that the universe doesn’t always draw its orbits as perfect circles, and that difference in shape can tell us where these systems came from.”