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 planet is itself a contentious matter). Beyond that, a more precise, official definition doesn’t exist. The International Astronomical Union has been in charge of planetary nomenclature for more than 100 years, but “surprisingly, they have not defined what a moon is,” Jean-Luc Margot, a UCLA astronomer, told me. This has created “total ambiguity regarding what is or isn’t a moon,” Jacqueline McCleary, an observational cosmologist at Northeastern University, told me.
Consider moon size. Technological advancements have enabled scientists to better identify ever smaller objects in space, but they have not agreed where to draw the line between a moon and a (relatively) tiny rock. When Galileo cast a homemade telescope toward the night sky and discovered Jupiter’s four largest moons, he at first mistook them for distant stars; the planet now has 95 official moons. Astronomers also have found thousands of other satellites in the gas giant’s orbit: Should those count as moons? Some have taken to calling them “moonlets” instead.
In recent decades, scientists have discovered more moonlike objects in proximity to our own planet, too, including a handful of small asteroids that temporarily enter our orbit. Some people call them “mini-moons,” although it’s “not a term that has a widespread definition,” cautioned Teddy Kareta, an astronomer at Villanova University. Last year, one such object entered Earth’s orbit but left after just two months. Many put that satellite in the mini-moon category, but Kareta worried that, although fun, the term was misleading. That “mini-moon” didn’t even complete a full rotation around Earth.
“Quasi-moons” are not actually very moonlike. These satellites can appear as if they are orbiting Earth, but in reality they are orbiting the sun. That makes the term “a bit of a misnomer,” Margot told me. Other planets have quasi-moons too: The first one to be discovered, a temporary satellite of Venus, was named Zoozve after an artist erroneously transcribed its original name, 2002VE.
Earth has roughly half a dozen quasi-moons, and in this pantheon, our new one is a bit of a flighty dweeb—roughly the size of a small office building, McCleary said. “If the other quasi-moons are like albatross circling our ship on our journey around the sun, then this one is maybe more of a hummingbird,” Asa Stahl, an astronomer and science editor at the Planetary Society, told me. Although it was only just discovered, this asteroid has been running in an Earth-like orbit for some 60 years; in another 60 or so, it will lose its quasi-moon status. Its ultimate path is somewhat mysterious, but Coffinet’s simulations suggest that thousands of years from now, it might cross Mars’s orbit.
In the coming years, astronomers are likely to locate more ambiguous moonlike objects. New imaging technology at the Vera C. Rubin Observatory, for instance, is supercharging our ability to peer into the sky, while progress in software development is improving analysis of the images that observatories capture. As these maybe-moons pile up, “the lack of a clear definition of moon—or quasi-moon, or mini-moon—causes about as much confusion and consternation among scientists as it does in the public,” Kareta explained. In a recent paper, he added an entire “terminology” section to help define various terms. Margot suggested that the International Astronomical Union “revisit these issues and come up with proper definitions that are precise and general for planets and also for moons.” But he’s not expecting the group to act anytime soon: There’s still a lot of “trauma”—to use Margot’s words—left over from the aughts, when the IAU changed the definition of a planet. (RIP Pluto.)
Earth’s own moon—the one that hangs above us, whose light “stretches over salt sea equally and flowerdeep fields,” as Sappho once noted—is itself astounding. No other planet in our solar system has exactly one moon. The leading theory behind the moon’s formation suggests that in the earliest days of our solar system, baby Earth was flying solo until, after some tens of millions of years, a Mars-size object collided with it and sent debris flying into space. After learning of mini-moons and quasi-moons and moonlets, I wondered if the Earth might actually have a second true moon, which has somehow managed to evade detection. When I asked the astronomers, they were skeptical, but none denied the possibility. “It’s certainly conceivable,” McCleary told me. “If there is one, I think we’ll probably find it in the next 10 years,” she said.
