Shepherd moon

• 1. Introduction

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  A shepherd moon is a small natural satellite that clears a gap through a planet's ring system. In other cases, it holds a band of ring particles together, keeping them from drifting apart. Astronomers named the phenomenon after the way a shepherd limits a herd's movements, since these moons limit the herd of ring particles around them. They do this purely through gravity, nudging nearby particles out of their paths or pulling on them through repeated orbital resonances. Before 1979, nobody had ever confirmed that such a moon existed. What led scientists to suspect unseen satellites were sculpting rings they could only observe from Earth, and what turned up when a spacecraft finally looked?

• 2. Discovery

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  Goldreich and Tremaine proposed the idea in early 1979, after noticing that Uranus's rings appeared unusually thin and sharply bordered, with clean gaps separating them. The two researchers suggested that a pair of small, still-undetected satellites might be squeezing each ring into that narrow shape. Later that same year, Voyager 1 captured the first images ever taken of a shepherd satellite, confirming a theory that had existed for only months. Saturn's ring system, as later probes would show, held far more of these moons than Uranus ever did.

• 3. Saturn's Many Shepherds

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  Prometheus, one of several small moons shaping Saturn's rings, keeps the thin F ring pinned into a tight band rather than letting it spread apart. Daphnis carves a gap called the Keeler Gap, while Pan maintains a wider break known as the Encke Gap. Janus and Epimetheus, meanwhile, both shepherd material within Saturn's broad A ring. Uranus, with its far dimmer rings, would rely on shepherd moons of its own for that same kind of confinement.

• 4. Feeding and Decaying

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  Metis and Adrastea orbit Jupiter so closely that both sit inside the planet's Roche limit, the distance within which gravity can pull a body apart. Jupiter's tides may be pulling material directly off the surfaces of these two moons to help form the rings. Impacts from ring particles striking their surfaces may be helping that process along. Cordelia and Ophelia perform a similar job around Uranus's narrow epsilon ring, shepherding it from the inside and outside respectively. Both moons orbit well within Uranus's synchronous orbit radius, so tidal deceleration is slowly dragging their orbits inward. Neptune's rings, once probes finally reached them, would present a stranger puzzle still.

• 5. Neptune's Clumpy Rings

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  Earth-based telescopes once showed Neptune's rings as broken arcs rather than full circles, a picture that puzzled astronomers for years. Voyager 2's images overturned that picture, revealing complete rings dotted with bright clumps of material. Galatea, a shepherd moon orbiting nearby, is thought to cause that clumping, possibly alongside other shepherd moons not yet discovered. Shepherding, it turns out, is not limited to planets at all.

• 6. Rings Beyond the Planets

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  Chariklo, a small centaur orbiting far beyond Neptune, carries remarkably well-defined rings. Those rings are suspected to be either very young or held in place by a shepherd moon close in mass to the rings themselves. Chiron, another centaur, is thought to carry rings similar in form to Chariklo's. J1407b, a free-floating brown dwarf or rogue planet, shows a major gap in its enormous disk. That gap sits about 61 million kilometers, or 0.4 astronomical units, from the object's center. Astronomers treat that gap as indirect evidence of an unseen moon or planet, one that could carry up to 0.8 times the mass of Earth.