The seventh inner moon of Neptune

The rings and small moons of Uranus and Neptune

All four giant planets are encircled by distinctive systems of rings and small, inner satellites. These all reside within or near their central planet's Roche limit, the rough boundary within which bodies held together by self-gravity will be disrupted by tidal forces. However, the similarities of the four ring-moon systems end here; in most other regards, they are remarkably diverse. We study these systems for three key reasons: (1) for the information they reveal about the properties, history and ongoing evolution of the planetary systems of which they are a part; (2) as dynamical analogues for other astrophysical systems such as protoplanetary disks; and (3) for the wealth of fascinating properties and origin scenarios that make them worthy of study in their own right. The inner Uranus system is characterized by 10 narrow rings, some quite dense, as well as a variety of more tenuous structures. These are accompanied by 13 known moons all orbiting interior to Miranda. Nine of these, Bianca through Perdita, comprise the most densely packed set of moons in the solar system, with orbits so close that their interactions appear to drive chaos over time scales approximately 10⁶ years. Neptune has five named rings, all optically thin, interleaved with seven inner moons. The most notable feature is a set of arcs embedded within the Adams ring; two of these arcs have been stable for time scales of decades. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.