Carnegie Science | Spring 2019 16 hat question, by the young son of Carnegie’s Juna Kollmeier, started it. Soon after this bedtime query, Kollmeier was coordinating a program at the Kavli Institute for Theoretical Physics (KITP) on the Milky Way while a former classmate, Sean Raymond of Université de Bordeaux, was attending a program on Earth-like planets. After discussing the question, the two joined forces to solve it. Monthly Notices of the Royal Astronomical Society published their findings. Previously, the duo kicked off an Internet firestorm when they posted online a draft of their article examining the possibility of moons orbiting other moons. The firestorm obsessed over the term to describe the phenomena with options like moonmoons and mini-moons. Putting the nomenclature debate aside, Kollmeier and Raymond began defining the physical parameters for moons that could be stably orbited by other, smaller moons. “Planets orbit stars and moons orbit planets, so it was natural to ask if smaller moons could orbit larger ones,” Raymond explained. Their calculations show that only large moons on wide orbits around their host planets could host submoons, their preferred term. Tidal forces from the planet and moon destabilize the orbits of submoons orbiting smaller moons or moons that are closer to their host planet. They found that four moons in our Solar System are theoretically capable of hosting submoons—Jupiter’s moon Callisto, Saturn’s moons Titan and Iapetus, and Earth’s own Moon. But further calculations are needed to address possible sources of submoon instability, such as the uneven concentration of mass in our Moon’s crust. “The lack of known submoons in our Solar System, even orbiting around moons that could theoretically support such objects, can offer us clues about how our own and neighboring planets formed, about which there are still many outstanding questions,” Kollmeier explained. “The moons orbiting Saturn and Jupiter are thought to have been born from the disk of gas and dust that encircle gas giant planets in the later stages of their formation. Our own Moon, on the other hand, is thought to have originated in the aftermath of a giant impact between the young Earth and a Mars-sized body. The lack of stable submoons could help scientists better understand the different forces that shaped the satellites we do see.” Kollmeier added, “And, of course, this could inform ongoing efforts to understand how planetary systems evolve elsewhere and how our own Solar System fits into the thousands of others discovered.” For example, the newly discovered possible exomoon orbiting the Jupiter-sized Kepler 1625b is the right mass and distance from its host to support a submoon, Kollmeier and Raymond found, although the inferred tilt of its orbit might make it difficult to remain stable. However, detecting a submoon around an exomoon would be very difficult. Given the excitement surrounding searches for potentially habitable exoplanets, Kollmeier and Raymond calculated that the best-case scenario for life on large submoons is around massive stars. Although extremely common, small red dwarf stars are so faint and their habitable zones so close that tidal forces are very strong and submoons (and often moons themselves) are unstable. Finally, the authors note that an artificial submoon may be stable and could serve as a time capsule or outpost. On a stable orbit around the Moon, such as the one for NASA’s proposed Lunar Gateway, a submoon would keep humanity’s treasures safe long after Earth became unsuitable for life. Kollmeier and Raymond agree that there is much more work to be done. Raymond maintains a science blog (https://planetplanet.net) where more details can be found.  Juna Kollmeier and Sean Raymond found that four moons in our Solar System, including our own Moon (above left), could theoretically harbor orbiting submoons. Image courtesy NASA Goddard Space Flight Center Carnegie’s Juna Kollmeier makes a presentation at a recent American Astronomical Society meeting. “Can moons have moons?” SUPPORT: A grant from the Agence Nationale pour la Recherche, NASA Astrobiology Institute’s Virtual Planetary Laboratory Lead Team, and the National Science Foundation supported this research. T