Minor-planet moon

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of September 2026, there are 643 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights into their physical properties that are generally not otherwise accessible. Several of the moons are quite large compared to their primaries: 90 Antiope, Mors–Somnus and Sila–Nunam (95%), Patroclus–Menoetius, Altjira and Lempo–Hiisi (90%, with Lempo–Paha at 50%). The largest known minor-planet moon in absolute size is Pluto's moon Charon, which has about half the diameter of Pluto. There are also several known ring systems around distant objects (see: Rings of Chariklo and Chiron).