Have We Detected a Gravitational Wave Signal from a Black Hole Moon?
A giant impact of a Mars-size proto-planet named Theia with proto-Earth, 4.5 billion years ago, may have ejected debris that coalesced to form our Moon. Computer simulations support this giant impact hypothesis, as discussed here.
Could a moon form through a similar process when two neutron stars collide?
Dense star clusters, known as globular clusters, contain of order a million stars and are known to have stellar remnants segregated near their centers. These remnants include stellar-mass black holes and neutron stars. They are typically more massive than the background stars and hence settle towards the cluster center through a gravitational segregation process resembling the separation of heavy dust particles from air molecules under the influence of the Earth’s gravity. Near the cluster center these remnants find each other, creating pairs of black holes that coalesce through the emission of gravitational waves. This natural process could explain the origin of many of the gravitational wave sources detected by the LIGO-Virgo-KAGRA (LVK) collaboration over the past decade, as originally proposed here.
The cores of globular clusters are known to contain an abundant population of neutron stars which appear as pulsars or X-ray sources with a mass of up to twice the mass of the Sun (as discussed recently here). These neutron stars, remnants from core collapse of massive stars, have a mass density of an atomic nucleus and a characteristic size of a city, about 12 kilometers (as discussed here).
The dense core of a globular cluster can lead to the formation of pairs of black holes or neutron stars but also to three-body systems that are dynamically unstable, and can result in a head-on collision between two neutron stars under rare circumstances (as discussed here).
When two neutron stars collide head-on, their merger is expected to form a black hole carrying most of their combined masses. However, just as in the collision between Theia and Earth, a fraction of the mass might be ejected as debris that coalesces to form a moon made of neutron star matter.
Equilibrium configurations of stable neutron stars exist down to 0.09 of the mass of the Sun (as discussed here). Therefore, the debris from a head-on collision between two neutron stars could potentially lead to the formation of a central black hole or neutron star accompanied by a low-mass neutron-star moon. As a result of the emission of gravitational waves, the moon will ultimately merge with the central object. The lifetime of the system depends on the initial separation of the moon from the central object as well as their masses. An alternative channel for creating a black hole moon is from the core collapse of a single progenitor star to a black hole and a debris disk that coalesces into a neutron star moon, as discussed here.
On November 12, 2025, the LVK collaboration reported the detection of a gravitational wave signal from a compact merger candidate named S251112cm (as reported here). This event is statistically compelling due to its relatively low False Alarm Rate, estimated at about 1 per 6.2 years (as noted here). The source luminosity distance is estimated to be in the local Universe, measuring about 300(± 87) million light years (93 ± 27 Mpc), but a search for an electromagnetic counterpart did not yield a detection (as reported here). The analysis of the gravitational wave signal implies that the source chirp mass falls predominantly in the range of 0.1 to 0.87 solar masses, indicating a sub-solar mass object at the 99% confidence. The more massive object could be in the range of 1–3.5 solar masses, based on Figure 1 here. The inference of a low-mass object raises the possibility that S251112cm may have been produced by the coalescence of a neutron star moon and an order of magnitude more massive companion in the form of a black hole or a neutron star.
Our own Moon might also crash back on Earth. This would not be the result of gravitational wave emission but rather its drag on the envelope of the Sun once it expands as a red giant to engulf the Earth-Moon system (as discussed here).
Some moons are doomed to crash back on their birth place, just like adults settling back to their childhood home.
***
For the research paper related to this essay, click here.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb
