It’s easy to be brand loyal to the moon. We’ve only got the one, after all, unlike Jupiter and Saturn, where you’d have dozens to choose from. Here, it’s luna or nada. Or not. The fact is, there’s another sorta, kinda moon in a sorta, kinda orbit around Earth that was discovered only in 2016. And according to a new study in Nature, we may at last know how it was formed.
The quasi-moon—named Kamo’oalewa, after a Hawaiian word that refers to a moving celestial object—is not much to speak of, measuring less than 50 m (164 ft) across.
It circles the Earth in a repeating corkscrew-like trajectory that brings it no closer than 40 to 100 times the 384,000 km (239,000 mi.) distance of our more familiar moon. Its odd flight path is caused by the competing gravitational pulls of the Earth and the sun, which continually bend and torque the moonlet’s motions, preventing it from achieving a more conventional orbit.
“It’s primarily influenced just by the sun’s gravity, but this pattern shows up because it’s also—but not quite—on an Earth-like orbit. So it’s this sort of odd dance,” says graduate student Ben Sharkey of the Lunar and Planetary Laboratory at the University of Arizona, the lead author of the paper.
None of this means that Kamo’oalewa has to have especially exotic origins. The solar system is littered with asteroids, some of which are captured by the gravity of other planets and become more conventional—if fragmentary—moons. Others don’t orbit other planets in the common way but fall into line in front of them or behind them and pace them in their orbits around the sun, like the flocks of so-called Trojan asteroids that precede and trail Jupiter.
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Either way, Kamo’oalewa was bound to get attention because its composition posed a stubborn mystery. Asteroids tend to reflect brightly in certain infrared frequencies, but Kamo’oalewa just doesn’t.