A shepherd moon, also called a herder moon or watcher moon,[citation needed] is a small
natural satellite that clears a gap in
planetary ring material or keeps particles within a ring contained. The name is a result of their limiting the "herd" of the ring particles as a shepherd.
Due to their
gravitational influence, shepherd moons deflect ring particles from their original orbits due to proximity or through
orbital resonances. This can carve gaps in the ring system, such as the Encke Gap maintained by Saturn's moon
Pan, or lead to the confining of narrow ringlets, such as Saturn's F ring.
Discovery
The existence of shepherd moons was theorized in early 1979.[1] Observations of the rings of Uranus show that they are very thin and well defined, with sharp gaps between rings. To explain this,
Goldreich and
Tremaine suggested that two small satellites that were undetected at the time might be confining each ring. The first images of shepherd satellites were taken later that year by Voyager 1.[2]
Examples
Jupiter
Several of
Jupiter's small innermost moons, namely
Metis and
Adrastea, are within Jupiter's
ring system and are also within Jupiter's
Roche limit.[3] It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiter's
tidal forces, possibly facilitated by impacts of ring material on their surfaces.
Uranus also has shepherd moons on its
ε ring,
Cordelia and
Ophelia. They are interior and exterior shepherds, respectively.[8] Both moons are well within Uranus's synchronous orbit radius, and their orbits are therefore slowly decaying due to
tidal deceleration.[9]
Neptune
Neptune'srings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but
Voyager 2's images showed them to be complete rings with bright clumps.[10] It is thought that the gravitational influence of the shepherd moon
Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.[11]
Minor planets
Rings around some
centaurs have been identified.
Chariklo'srings are remarkably well-defined and are suspected to either be very young or kept in place by a shepherd moon similar in mass to the rings.[12]Chiron is also thought to have rings similar in form to those of Chariklo.[13]
Exoplanets
A major gap in the large ring system of the
V1400 Centauri b object at about 61 million km (0.4
AU) from its center is considered to be indirect evidence of the existence of an
exomoon with mass up to 0.8
Earth masses.[14][15]
^Karkoschka, Erich (2001-05-01). "Voyager's Eleventh Discovery of a Satellite of Uranus and Photometry and the First Size Measurements of Nine Satellites". Icarus. 151 (1): 69–77.
Bibcode:
2001Icar..151...69K.
doi:
10.1006/icar.2001.6597.
^Miner, Ellis D.; Wessen, Randii R.; Cuzzi, Jeffrey N. (2007). "Present knowledge of the Neptune ring system". Planetary Ring System. Springer Praxis Books.
ISBN978-0-387-34177-4.