Many planets, including our own, are orbited by one or more moons. But what if these rock and ice celestial bodies were themselves circled by smaller objects? Do such things exist, and, if so, what are they called?
According to astronomers Juna Kollmeier from the Carnegie Institution of Washington and Sean Raymond from the University of Bordeaux, the answer to the first question is: yes, maybe. As to the second question, they have a few ideas, from the scientific-sounding "submoon" to the much more entertaining "moonmoon", but more on that later.
In an analysis published in the pre-print database arXiv, Kollmeier and Raymond calculate the Goldilocks conditions that would allow a submoon to stably orbit its moon without being thrown off course or sheared into pieces by the gravitational pull – also known as tidal force – from the moon’s planet.
After making a few assumptions about the densities of moons and submoons based on what we know about the players in our own Solar System, the authors conclude that only large moons – those with a radius of 1,000 kilometers (621 miles) or larger – with wide orbits could accommodate long-lived submoons, whose size would be proportionate to 10 kilometers (6.2 miles) and larger.
“Tidal [energy flux] destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar System's moons,” they wrote.
“A handful of known moons are, however, capable of hosting long-lived submoons: Saturn's moons Titan and Iapetus, Jupiter's moon Callisto, and Earth's Moon.”
They add that the newly discovered moon orbiting the exoplanet Kepler-1625b (a gas giant about six to 12 times the size of Earth, orbiting a Sun-like star located about 8,000 light-years away) may be able to host a submoon as well, though they don’t know enough about the object to be sure.
“This system where you’ve got a giant planet and a Neptune-sized moon that’s kind of far away from the planet is sort of the best-case scenario for a moonmoon,” Raymond told New Scientist.