In this episode, we dive into a cosmic mystery that has astronomers buzzing: the detection of the gravitational wave event S251112cm. Detected in November 2025, this event is groundbreaking because it has a 100% probability of containing a compact object with a subsolar mass—an object lighter than our own Sun. Standard stellar evolution models tell us that neutron stars and black holes shouldn't be this light, as modern supernova simulations do not yield remnant objects lighter than roughly 1.17 solar masses. So, what exactly collided out there in the dark?

We explore the massive, multi-telescope campaign launched by the astronomical community to find the electromagnetic "flash" of this merger. Along the way, we discuss the wild theoretical phenomena that might produce such a signal, such as primordial black holes merging within the accretion disks of active galactic nuclei (AGN), massive "super-kilonovae," or "kilonovae-within-supernovae" born from the fragmented disks of collapsing massive stars. Finally, we learn how scientists are using a new framework called TROVE (Multimessenger Tool for Rapid Object Vetting and Examination) to sift through hundreds of transient candidates to separate the true cosmic counterparts from the false alarms.

Key Takeaways:

• The Anomaly of S251112cm: Why a subsolar mass (SSM) merger challenges our current understanding of physics, and how it opens the door to theories involving primordial black holes.
• The Electromagnetic Zoo: A breakdown of the exotic, theorized transients that could accompany an SSM merger, including standard kilonovae, kilonovae embedded within stripped-envelope supernovae, super-kilonovae, and bright flares in AGN disks.
• The Search Effort: How a global network of telescopes (including the Vera C. Rubin Observatory, Swift-XRT, and others) vetted 248 optical and X-ray candidates, and why ultimately none of them were confidently linked to S251112cm.
• Introducing TROVE: How the Multimessenger Tool for Rapid Object Vetting and Examination ranks candidates using location, distance, and photometry to help astronomers efficiently allocate their limited telescope time during future gravitational wave events.

Episode Reference:

Vieira, N., Franz, N., Subrayan, B., Kilpatrick, C. D., Sand, D. J., Fong, W., et al. (2026). Search For a Counterpart to the Subsolar Mass Gravitational Wave Candidate S251112cm. Draft version March 19, 2026.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Astro-COLIBRI