In August, an amateur French astronomer, Adrien Coffinet, messaged an email list dedicated to asteroid and comet research with an announcement. He’d identified a new quasi-moon: “2025 PN7 seems to be a quasi-satellite of the Earth,” he wrote. Last week, news of the quasi-moon went mainstream, as a surge of headlines declared that Earth officially had a second moon.
This isn’t exactly right: As several scientists reiterated to me, Earth still only has one real moon. But as researchers have discovered more moonlike objects in our solar system—including 128 moons orbiting Saturn just this year—our concept of what counts as a moon has been forced to expand. Now it’s approaching a breaking point.
A moon is generally understood to be an object that orbits a planet (although what counts as a plane
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