If you’ve ever looked up at the sky, you have almost certainly seen our moon. The Moon shines bright and large in the sky, making it a constant companion going through its phases throughout the years. Did you know, however, that the Earth sometimes has other moons?

Well, technically speaking, it sometimes has micro-moons that get caught in the Earth’s gravity for a time. In the past, it was believed that these micro-moons were simply asteroids that got pushed into our path from the asteroid belt, but a new study that is available in pre-print on arXiv has found that this is not always the case.

In August of 2023, astronomers got to look at the object named 2024 PT5, which traveled alongside the Earth for a time. Their analysis found that its origins may have been much closer to home than previously thought. In a paper, the team of researchers explained:

“Our spectroscopic results on the surface composition of 2024 PT5 are not conclusive due to the lack of a near-infrared reflectance spectra, but they are suggestive of a Lunar origin. We confirm that 2024 PT5 is a natural object with a visible spectrum consistent with that of an Sv-type asteroid, although it could also be classified as a Lunar mare breccia.”

That’s right, they think that this ‘mini-moon’ object might be a part of our actual moon that got ejected at some point in the past. This could have happened after an asteroid slammed into the moon, or any number of other points in the distant past.

Objects like this are typically called temporarily bound objects (TBOs) when they remain within about three Earth radii. Depending on the size, speed, and trajectory of the object, they could remain in near-Earth for some time, or simply pass through relatively quickly.

In the paper, the team wrote:

“Some recent studies have shown that lunar ejecta can evolve onto quasi-satellite orbits. In particular, (469219) Kamo‘oalewa is an ∼ 50 m diameter asteroid with a spectrum consistent with being space-weathered lunar basalt. Both papers suggest that not only did it originate from the Moon’s surface but that it can be associated with the formation of the ∼ 22 km-diameter Giordano Bruno crater within the last 1 to 10 Myr. If this is the case, then it is likely that many more quasi-satellites with lunar-like spectra will be identified in the future.”

To try to better understand how these objects escape from the moon, the researchers simulated a number of lunar impacts of various types. What they found is that objects are likely ejected from the moon more often than expected, and somewhere around 20% of those that are ejected may go on to become ‘mini-moons’ at some point in the future. The team explains:

“In general, the fraction of prompt and delayed TBOs decreases rapidly with a particle’s ejection speed with the only exception being that the fraction of delayed TBOs increases with ejection speed just above the lunar escape speed, presumably because most of the barely-escaping objects promptly become bound.”

To see the path of the 2024 PT5 mini-moon that was the main subject of this study, check out this brief video:

The team does acknowledge that further study of these objects is needed to confirm that they are indeed from the moon. The initial analysis shows that they are similar to the lunar makeup, but there could be other reasons for that.

Fortunately, micro-moons like this come into our path on a fairly regular basis.

If you thought that was interesting, you might like to read about a second giant hole has opened up on the sun’s surface. Here’s what it means.