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Most scientists agree that a major collision 4.5 billion years ago with a protoplanet named Theia formed the Earth-Moon system.
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Now, a new study from the University of Nevada argues that, out of the chaos of that collision, polar circumbinary “moons” might’ve existed in the early days of that Earth-Moon system.
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While the changing conditions of the Earth-Moon system no longer make this possible, this finding could have a big impact for astronomers studying exoplanets—some that maybe even mimic Earth’s early conditions.
Some 4.5 billion years ago, when Earth was only 100 million years old or so, a Mars-sized protoplanet named Theia smashed into our planet, ejecting loads that eventually returned to the Earth’s surface, formed the Moon, or left the newly-formed Earth-Moon system entirely.
Piecing together this dramatic smash-up hasn’t been easy, but evidence of possible remnants of Theia within the Earth’s mantle and the analysis of lunar rocks have collectively confirmed as much. Now, a new study from scientists at the University of Nevada is adding some complexity to this immensely chaotic moment in Earth’s history by suggesting that, for a brief time, the Earth-Moon system played host to a variety of polar moons—also known as circumbinary particles. However, these “moons” were only possible due to the close proximity of the Moon to Earth right after the planet’s collision with Theia. The results of this study have been uploaded to the non-peer reviewed preprint server arXiv.
“Circumbinary orbits that are polar or highly inclined to the Earth-Moon orbit are subject to two competing effects: nodal precession about the Earth-Moon eccentricity vector and Kozai-Lidov oscillations of eccentricity and inclination driven by the Sun,” the paper reads. “While we find that there are no stable polar orbits around the Earth-Moon orbit with the current day semi-major axis, polar orbits were stable immediately after the formation of the Moon, at the time when there was a lot of debris around the system.”
Nodal precession means that particles in orbit around the Earth-Moon system slowly ‘precess’ (a slow, continuous change in a body’s orbital parameters) around the angular moment vector. Kozai-Lidov oscillations, on the other hand, largely pertain to a dynamical phenomenon involving a binary system and the perturbations introduced by a distant third body.
After making calculations taking these competing effects into account, particles orbiting in the polar region were the most stable. This is because when the Moon first formed (which some scientists think may have taken only a few hours), it was only 5 percent as far from Earth as it is today at around 238,900 miles on average. However, over the course of billions of years, tidal forces began to push the Moon farther and farther into its modern orbit, which eventually erased these stable polar regions entirely. It’s possible that this polar material also pushed along the growing eccentricity of the Earth-Moon system.
“Polar circumbinary material can drive eccentricity growth of the binary,” the paper reads. “If a significant mass of material ended up on polar or librating orbits, then the eccentricity of the Earth-Moon binary could have been increased as a result of its interaction.”
While these polar moons are of little-to-no impact today, the confirmation of their existence during Earth’s early years has profound implications for the study of exoplanets beyond our Solar System. The researcher team explained that a planet-moon system with a highly eccentric orbit is more likely to host one of these circumbinary polar “moons,” which would in turn drive retrograde procession—a.k.a. orbiting backwards.
To find one such moon among the billions of stars in the galaxy would be like catching a glimpse of the very creation of our home planet.
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