News & Opinion | October 15, 2018 5:00 am

What Do You Call the Moon of a Moon?

Recent research suggests that smaller moons orbiting bigger moons is theoretically possible.

If moons theoretically could have their own moons, what do we call those smaller moons? The question is one that has animated the online science community, while scientists themselves ponder whether such a thing even exists. A recent report on the pre-peer review server arXiv.org speculated that submoons, as they chose for the name, could exist, but none have yet been discovered.

The recently-published study found that most moons do not allow enough space for a smaller moon’s orbit. A theoretical submoon would need to be six miles maximum in diameter, and to orbit a moon with a significant gravitational pull, and to keep its distance from a host planet that could interfere with its orbit. The conditions that researchers describe do exist in our solar system, creating the scientific basis for further submoon research. “We’re really just scratching the surface here with how we can use the absence of submoons to figure out early history,” Juna Kollmeier, one of the scientists behind the paper, told Gizmodo.

Submoon is not yet the official name for such theoretical lunar forces. Another recent paper on the same server referred to them as moon-moons, while the preferred term on Twitter and in New Scientist magazine is moonmoon, sans hyphen. The debate is surely far from over.