New Delhi: The LIGO and Virgo gravitational wave observatories have detected a merger between two compact objects designated as S251112cm on 11 December
2025. Most of these mergers are between black holes or neutron stars, which are the dead cores of massive stars. Most of these objects contain between five and 200 solar masses, and are extremely compact. The new detection is exciting to scientists because one of the two objects in the merger appears to be less massive than the Sun. Such an object has never been observed before, but scientists have a few theories on what it could be.
Oo! Interesting #GravitationalWave candidate #S251112cm potentially from a *subsolar* mass source
If real, the source is probably has chirp mass ~0.1–0.87 solar masses
False alarm rate 1 in 6.2 yr
GraceDB gracedb.ligo.org/superevents/…
GCN gcn.nasa.gov/circulars/42…
Rating 📏🍬[🧪🔭⚛️]
— Christopher Berry (@cplberry.bsky.social) November 12, 2025 at 11:59 PM
Now, LIGO and Virgo can only detect interactions between extremely compact objects. Neutron stars are the remnant cores of massive stars, and measure about 20 km across, packing in more mass than the Sun in the object about the size of a city. The most massive stars collapse into black holes when they run out of nuclear fuel, with a 200 mass star producing a black hole that measures about 1,200 km in diameter. White dwarfs are about 10,000 to 20,000 km wide, their interactions would produce signals in frequencies lower than the detection limits of the gravitational wave detectors. So, the new detection has to be a compact object, but the conventional theories do not allow for such a low-mass compact object!
What can the mysterious object be?
Scientists have some theories though. The substellar mass object in the interaction may be an elusive primordial black holes. These are black holes formed by the direct collapse of matter in the early universe, from the first density fluctuations. While the theory is strong, there are no confirmed primordial black holes detected, with a few candidates. A white dwarf in a binary system can accrete sufficient material from its companion to collapse into a neutron star, or fragmentation during the gravitational collapse of very massive stars may just produce low-mass neutron stars, or a neutron star approaching a black hole may be stripped of some material… but all of these are exotic, theoretical explanations.
Typically, gravitational wave signals are classified into mergers between a pair of neutron stars, a pair of black holes, or a neutron star and a black hole. The newly detected signal defies this classification, because the nature of the substellar mass object is unknown. The LIGO and Virgo detectors measure the compression and expansion of the spacetime continuum itself because of the passage of gravitational waves using mirrors and lasers dampened from external influence. However, terrestrial noise from earthquakes or even traffic can result in false positives.
