When Earth was young and received its first-ever moon, it might also have received a smattering of mini moons. New research on the Earth-Moon system’s orbit over time suggests that so-called “moonlets” once circled our planet. Though those moonlets no longer orbit Earth, scientists believe similar ones might orbit distant exoplanets and their moons.
Called the giant impact hypothesis, scientists’ working theory of the Moon’s formation revolves around Theia, an early planet thought to have collided with Earth 4.5 billion years ago. Though the impact likely ejected pieces of proto-Earth into the cosmos, some of those pieces are believed to have merged to form our Moon. But it’s unlikely that all of proto-Earth’s fragments joined in on that lunar group project. Instead, some remnants were ejected from the Earth-Moon system entirely, while others probably ended up revolving around our planet and its new rocky companion.
In a preprint paper published to the arXiv, astrophysicists at the University of Nevada describe how circumbinary orbits might have once facilitated the temporary keeping of moonlets. Right now, the Moon orbits Earth from a respectable distance of about 239,000 miles (384,600 kilometers). But that’s because of tidal friction, which pulls the Moon about 1.5 inches away from our planet each year. Immediately following Theia’s impact, the cluster of debris that made up the Moon was only 11,890 miles (19,135 kilometers) away. The Moon’s tilt with respect to Earth’s orbit has also decreased by about 5 degrees since the celestial body first formed.
Artist’s illustration of Theia smashing into Earth.
Credit: NASA
By examining the Moon’s early orbit, the researchers found that the Earth-Moon system’s non-zero orbital eccentricity—AKA its deviation from a perfect circle—could have allowed for two different types of misaligned polar orbits. Circulating orbits would have enabled an object to march around the angular momentum vector of the Earth-Moon system. In contrast, librating orbits would have caused an object to “wobble” around a point of gravitational equilibrium. (In fact, the Moon librates today, causing its north and south poles to alternate in tipping slightly toward Earth.) Either of these orbits could have allowed debris not incorporated into the Moon to instead circle the entire Earth-Moon system in what’s called a circumbinary orbit.
Per the researchers’ simulations, either of these orbit types would have remained stable until the distance between Earth and the Moon reached 59,381 miles, or 7.5 times Earth’s diameter. But those orbits—and the moonlets that would have used them—don’t exist today. “As the separation of the Earth-Moon [system] increased through tides, the region of space for which the polar orbits exists decreased,” they explain in their paper. But that doesn’t mean similar moonlets might orbit other planets and their moons.
“The existence of stable polar orbits around the young Earth-Moon system has implications for exoplanets,” the researchers write. “The range of initial inclinations for which librating orbits exist increases with the binary eccentricity…. Therefore, a more highly eccentric planet-moon orbit is more likely to [be] able to host polar circumbinary material.” In other words, a distant planet and its moon with an imperfect orbit just might possess its own mini moon.