How A Child's Question Sparked Research: Can Moons Have Moons?

A four-year-old's question inspired a recent scientific paper: Can moons have moons?

Researchers from the Carnegie Observatory and Laboratoire d'Astrophysique de Bordeaux studied whether moons can have moons and how that might happen. Published on the preprint research resource arXiv on Tuesday, the study showed that moons can have moons—called submoons or moonmoons—and that they're possible even around our own moon.

"In 2014, my son, who was 4 years old at the time, asked if moons can have moons, which is the title of the article. I was sort of taken aback by that question because of course, we know that we can put satellites in orbit around moons, but we also know that there are no known submoons around any moons in our solar system," Juna Kollmeier, an astrophysicist at the Carnegie Observatory and author on the paper, told Newsweek.

A moon-orbiting moon hasn't been discovered yet, so there's no official name. In Kollmeier's paper, she and her co-author Sean Raymond call them "submoons," however other people have suggested the alternative name "moonmoon." Kollmeier said her household is split—one of her sons prefers submoon, while the others prefer moonmoon.

"My initial response was that no, [submoons] probably can't survive very long because there's not a lot of phase space that's stable. It's not so easy to put something in orbit in a hierarchical system like this and survive for a long period of time," Kollmeier explained. It's difficult to find the perfect spot where a moon is far enough away from its planet and big enough that it can have its own object orbiting it, but close enough that it's still orbiting that planet.

The astrophysicists found that four moons in our solar system can actually have moonmoons: Jupiter's moon Callisto, Earth's Moon, and two of Saturn's moons, Titus and Iapetus. Kollmeier thinks it's unlikely that we just haven't spotted the moonmoons yet, but that there might have been moonmoons orbiting them a long time ago which were destroyed.

"There's sort of a danger zone for satellites and planets and moons. They interact dynamically and it causes instability that cause the submoons to either crash into the moon or get lost from the gravity of the moon," Kollmeier said.

The research that started off as just a question from her son could lead to even more findings for Kollmeier and Raymond. Kollmeier said, "I think it would be really interesting to do more detailed calculations of the dynamical evolution of submoons. Starting with primordial systems and seeing how they evolve would be the logical next step."