­­­LIGO has detected another black hole merger, raising the tally to five.

On November 15th, five months after the spacetime ripples jiggled LIGO’s instruments, astronomers announced the detection of their sixth gravitational-wave discovery, which is the fifth from the merger of two black holes. The event, GW170608, came from the union of the smallest black holes scientists have yet “seen” using this technique.

The waves hit LIGO at 02:01:16 Universal Time on June 8th, during the project’s second observing run (November 30th to August 25th). Their passage triggered the alarm at the site in Livingston, Louisiana, but the detector in Hanford, Washington, was under routine maintenance and had its alert system turned off. Even with the ongoing tinkering, the Hanford interferometer detected GW170608, too.

Although Europe’s Virgo gravitational-wave observatory was still in its commissioning phase and didn’t observe the event, the Virgo team contributed to the analysis, which appears in a preprint paper on arXiv.org.

Based on the signal’s characteristics, the two teams infer that the initial black holes were roughly 7 and 12 solar masses and created an 18-solar-mass black hole, radiating away a Sun’s worth of energy in gravitational waves. The marriage happened more than a billion light-years away. With only two detectors, the team can only say that the signal came from somewhere in a huge, 520-square-degree swatch of sky in the Northern Hemisphere.

The spin of the final black hole is 69% of the maximum value it could be — once again matching the predicted 70% rate for black holes that have been created by the merger process. There’s also no sign that the two initial objects were wildly tilted in their orbit as they spiraled into each other.

The most interesting thing about this latest detection, however, is the black holes’ small sizes. They’re similar to those from LIGO’s second discovery, GW151226, which combined objects of about 8 and 14 solar masses to create a black hole of 21 Suns (the rest was radiated away). These initial masses are also similar to black holes discovered in binary systems with stars, which astronomers can find due to the X-ray glow of the gas the black holes are tearing from their stellar companions.

This is exciting because of a tantalizing possibility: If the black holes discovered with LIGO and Virgo start falling into two distinct mass groups, then it’s possible that they’re made different ways. With enough black holes — and the teams say that they’ll need to find on the order of 100 — astronomers could start figuring out where each group comes from.

Why are we finding out about GW170608 five months after it happened? The teams were too busy analyzing the two 3-site detections from August: the fourth black hole merger, GW170814, and the first-ever neutron star merger, GW170817.

What tickled me about the paper — besides the growing evidence that merger-made black holes all have similar spins, which gives me a little jolt of glee — was this sentence in the conclusion: “With expected increases in detector sensitivity in the third . . . observing run, projected for late 2018 . . . detection of black hole binaries will be a routine occurrence.”

Ladies and gentlemen, we have now hit the point where gravitational waves from merging black holes are no longer a big deal. ­Think about that: ripples in the fabric of spacetime, radiating away from bizarre gravitational potholes inside which physics as we know it breaks down, are becoming a routine detection.

Science is stranger than fiction.

Reference: The LIGO Scientific Collaboration and Virgo Collaboration. “GW170608: Observation of a 19-solar-mass Binary Black Hole Coalescence.” Posted to arXiv.org on November 15, 2017.

Learn how the first black holes might have formed in Sky & Telescope's January 2017 cover story.