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SCIENCE
Earth always has more moons than you think
Astronomers recently detected a stray quasi-moon in our planet’s orbit, but it's hardly the first stowaway to hang out around Earth.

An illustration of an asteroid in close proximity to Earth. Earth's Moon can be seen in the distance.
Unlike the Earth's moon, quasi-moons are not gravitationally bound to our planet. Some may be asteroids, which orbit the sun but beco...Read More
DETLEV VAN RAVENSWAAY, SCIENCE PHOTO LIBRARY

The solar system has some exciting local news: A mysterious space rock, about the size of a building, is coasting alongside Earth on its journey around the sun. Unbeknownst to astronomers until this summer, the object has been shadowing the planet for decades, in a celestial configuration that makes it a "quasi-moon."

When Ben Sharkey, an astronomer at the University of Maryland, first heard about PN7, as scientists now call it, their first thought was: "Oh cool, another one." That's because PN7 is just the latest find in what is a perpetual parade of tiny, moon-ish objects in Earth's vicinity.

Our planet has other quasi-moons like PN7; these orbit the sun, but their looping path through space—sometimes gliding ahead of Earth, other times drifting behind it—make them appear as if they are really circling the planet. And there are mini-moons, which are actually captured by Earth's gravity and temporarily orbit the planet before breaking free.

None compare to the moon, Earth's only natural satellite, the numinous crown jewel of the night sky. These other objects are only visible to powerful telescopes, particularly the kind designed to catch the faint sunlight glinting off miniscule, fast-moving rocks in the darkness. But each new discovery is a reminder of a delightful reality about our cosmic neighborhood: Earth always has more moons than we think.

"They really make you reconsider a nice, orderly, static view of the solar system," Sharkey says.

What are quasi-moons?
In the solar system, Earth isn’t the only planet with stowaway satellites; astronomers detected the very first known quasi-moon around Venus in 2002. The discovery of PN7 brings our planet's count of known quasi-moons to at least seven. (There are likely more, moving undetected.)

These small bodies can slide in and out of a shared orbit with Earth by gravitational happenstance, Sharkey says, and they experience tiny gravitational tugs from our planet. The quasi-moons discovered so far have ranged in size from 30 feet to 1,000 feet; PN7 is currently suspected to be one of the smallest of the bunch.

PN7, which was detected by the Pan-STARRS Observatory in Hawaii in late August, synced up with Earth sometime in the mid-1960s, before the first humans set foot on the moon. Scientists predict that PN7 will wander into a different kind of orbit around the sun in 2083. The duration of such arrangements varies; another object discovered by PAN-STARRS in 2016, Kamoʻoalewa, has held quasi-moon status for about a century, and will maintain it for the next 300 years.

Mini-moons come about by gravitational chance too, except Earth truly snaps them up. These purloined rocks usually circle the planet for less than a year; their orbits are quite unstable, and they can easily fly off. Astronomers have only observed four mini-moons so far, the latest one, about the size of a school bus, ditched Earth last year after a few months.

(Read more about Earth’s latest mini-moon.)

Most mini-moons are "quite small, like boulders," which means that they are difficult to detect, says Grigori Fedorets, an astronomer at the University of Turku in Finland. There are no known mini-moons currently lassoed around Earth, but an analysis by Fedorets predicts that Earth has a mini-moon measuring several feet across at any given time, and another analysis suggests that the planet could have six of similar size.

By Marina Koren
November 13, 2025

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