Reference Work Entry

Encyclopedia of Astrobiology

pp 1280-1284

Planetary Rings

  • Françoise RoquesAffiliated withLaboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris Email author 


Circumplanetary disk


Accretion, collisions, giant planets, Jupiter, Neptune, particles, resonance, Saturn, tide, Uranus, waves


Planetary rings are systems of dust, particles, and small satellites orbiting around the giant planets of the solar system, in the nearby vicinity of a planet, where large satellites cannot form because of tidal effects.


Before observations were possible, the ancients imagined that stars were similar to the Sun and were surrounded by exoplanets, but no one imagined the possibility of planets surrounded by rings. In fact, all the giant planets are surrounded by a ring system located close to the planet. Giant planets own also large and regular satellites with prograde and circular orbits and, farther away, irregular satellites with orbits possibly retrograde and/or inclined.

The first telescope allowed Galileo Galilei to observe in 1610 that Saturn has a strange shape, which changed in course of time. It was only 46 years later that Huyguens (1656) proposed that “Saturn is surrounded by a thin flat ring, nowhere touching and inclined to the ecliptic.” This correct interpretation of poor quality observations was based on the philosophical model of Descartes that a vortex is a natural shape in the Universe. It was only in 1857 that theoretical considerations lead JC Maxwell to conclude that the ring is composed of small orbiting particles and not a solid ring, as initially thought.

In 1977, the occultation of a bright star by Uranus was observed by Elliot and his colleagues to study the planet’s atmosphere. The star disappeared very briefly nine times before and after the occultation by the planet, revealing Uranus’ ring system. This was the discovery that Saturn was not the only planet to be surrounded by rings and that narrow planetary rings can be stable. Subsequently, Goldreich and Tremaine (1982) showed that gravitational interactions between ring particles and nearby satellites can account for narrow and eccentric rings. This shepherding mechanism is one of the numerous and complex phenomena due to ring-satellite interactions.

The next ring was discovered around Jupiter by a space mission, Voyager 1, in 1979. This third ring system is very different from the first two; it is broad and ethereal, a million times less opaque than the dense Saturn rings. The dust particles, governed by nongravitational forces, have short lifetimes and must be permanently resupplied by moons. Dusty rings exist in the four planetary systems and are associated with satellites.

The last giant planet hid its ring system until 1984. Again, it was a stellar occultation that revealed a ring structure around Neptune (Hubbard et al. 1986). This discovery included another surprise, as the ring was detected on one side of the planet but not on the other side, showing an incomplete ring. An intensive campaign of stellar occultation observations revealed that one-tenth of the ring length is denser than the rest. This was confirmed when Voyager 2 imaged, in 1989, three dense arcs embedded in dusty ethereal rings. From this time, the three arcs have evolved to four arcs, two of which have partially vanished, showing the short lifetime of these structures. Resonances with moons on eccentric and/or inclined orbits explain the stability of incomplete rings. However, recent observations found that the arcs are not at the position predicted by the theory, leaving a puzzling situation.

The four ring systems and embedded satellites are described in Fig. 1. Particle characteristics are summarized in Table 1. The Saturn system is composed of five rings, with most edges confined by observed satellite resonances. In the ethereal Jupiter system, a 1,000-km wide region between the Adrastea and Metis orbits could be the location of invisible (smaller than 0.5 km) bodies, “parents” of dusty rings. Shepherding by satellites of the narrow Uranus rings and the Neptune arcs is not yet completely explained.
Planetary Rings. Figure 1

A comparison of the four planetary ring systems, including the nearby satellites, scaled to a common planetary equatorial radius. Density of cross-hatching indicates the relative optical depth of the different ring components. Synchronous orbit is indicated by a dashed line, the Roche limit for a density of 1 gcm−3 by a dotted line (From Esposito 2006)

Planetary Rings. Table 1

Planetary rings Characteristics (from Esposito 2006)


Planetocentric distance (width) (km)

Optical depth

Dust fraction (%)

Power-law index (particle size distribution)








12,500 km thick

Main ring


3 × 10−6


q < 2.5

Bounded by Adrastea

Amalthea Gossamer





2,000 km thick

Thebe Gossamer


3 × 10−8



4,400 km thick


D ring





Internal structure

C ring





Some isolated ringlets

B ring





Abundant structure

Cassini division





Several plateaus

A ring





Many density waves

F ring

140,200 (W ≅ 50 km)




Narrow, broad components

G ring






E ring





Peak near Enceladus


1986 U2R


10−4 – 10−3



Still unnamed

Dust belts





Fine internal structure





q > 3.5






q > 3.5






q > 3.5






q > 3.5






q >3.5






























2.5 < q < 3.0

Adjacent to Cordelia




4 – 10 × 10−5





53,000 (W = 10 km)




Adjacent to Despina



1 – 3 × 10−4





62,930 (W = 50 km)




Adjacent to Galatea

Adams arcs

62,930 (W = 10 km)





Basic Methodology

Observations from ground and from space (survey and orbiter missions) of the last 50 years have led to a good knowledge of the ring systems. After Pioneer surveys of Jupiter and Saturn, Voyagers I and II in the 1980s explored the four systems and led us to the current conception of young and dynamical rings. The Cassini orbiter is now in orbit around Saturn after having flown by Jupiter. Evolution in ring structures can also be monitored from the ground, thanks to high angular resolution observations. Special events such as stellar occultations and ring-plane crossings provide unique information on the ring structures and particle sizes.

N-body simulations with tens of thousands of particles, taking into account mutual gravitational forces, can simulate formation of viscous instability, collective wakes, and accretion phenomena (Salo 2001). The rings and their evolution can also be fruitfully simulated as fluid (hydrodynamics) or gas (kinetic theory).

Planetary rings are a laboratory where dynamical mechanisms including waves, wakes, braids, clumps, confinement, or migration are observed and studied at all scales (Fig. 2). These mechanisms are certainly involved in others astrophysical disks, such as occur in galaxies, accretion disks, protoplanetary disks, disks of second generation, etc.
Planetary Rings. Figure 2

Shadows on the Saturn rings: bright upper part is the outer part of the B ring showing several wave patterns. The outer B ring is confined by the 2:1 resonance with Mimas. The lower dark region is the Cassini division. The bright structures along the B ring edge are 3 km in height above the ring and throw laced shadows on the ring. The longest shadows are 100 km long. The Sun is 1.9° above the ring plane. The long shadow is caused by Mimas, which is 70,000 km away from this image (© NASA/JPL/SSI)

The natural evolution of colliding particles orbiting around a planet is to aggregate into a satellite because of inelastic collisions. However, within the Roche limit (D R ) of
$$ {D_R} = 2.456*R{\left( {\frac{{{\rho_P}}}{\rho }} \right)^{{1/3}}} $$
where R is the planet radius, \( \rho \) is the particle density, and \( {\rho_P} \) is the planet density, differential gravity (tides) caused by the planet can overcome the gravitational attraction between the particles. As a result, satellites can orbit inside the Roche limit only if they are small enough to be held together by their tensile strength, rather than by their own gravitation.

As inelastic collisions dissipate energy but overall angular momentum is conserved, the initial particle orbits flatten within a short timescale into the equilibrium plane, that is, the equatorial plane of the planet.

The spreading timescale is longer than the flattening one. The relative motion between particles is dominated by Keplerian shear (i.e., particles closer to the planet move faster than more distant ones). Collisions between a more rapid inner particle and a slower outer particle make the inner particle slow down and lose angular momentum and the outer particle accelerate and win angular momentum.

Most images of rings, and in particular of dense rings, show very small structures, down to the limit of definition of the images. They are caused by different size particles gravitationally interacting. Perturbations of a satellite create visible structures in the ring at locations of resonance. At these locations, successive gravitational perturbations are cumulative. Resonance effects are complex, because they concern three parameters for each orbit: mean motion, apsidal precession of an eccentric orbit, and node of an inclined orbit. The mean motion of the satellite orbit is \( {n_S} = \frac{{2\Pi }}{P} = {\left( {\frac{{GM}}{{{a^3}}}} \right)^{{1/2}}} \), where P is the revolution period, G is the gravitation constant, M is the planet mass, and a is the semi-major axis of the orbit. The epicyclic frequency, κ S , is connected to the motion of the elliptical orbit apsidal line, and the vertical frequency, μ S , to the motion of the node of an inclined orbit. If the planet is oblate, \( {\mu_{{_S}}}>{n_S}>{\kappa_S} \), the apsidal line regresses and the node precesses. Resonance occurs when the forcing satellite frequency, \( {W_f} = m.{n_s} + p{\kappa_S} + k{\mu_S} \), where m, p, and k are integers, matches one of the three particle natural frequencies (Fig. 2).

The most spectacular effect of resonances is the shepherding of a narrow ringlet by two satellites as observed for the ε ring of Uranus or the F ring of Saturn. The resonances accumulate near the satellite orbit, and the cumulative torque of a satellite on a ringlet of width w, at a distance d is
$$ T \approx \frac{{{G^2}{m_S}^2\sigma ar}}{{{n_S}^2{d^4}}}, $$
where m S is the satellite mass and σ is the ring surface density. Note that dissipation is essential even if the torque intensity does not depend on the dissipation mechanism.

Dusty ring particles are sensitive to nongravitational forces. Solar radiation pressure, plasma effects, and magnetic perturbations limit the lifetimes of particles. Consequently, these rings must be replenished continuously by micrometeoroidal bombardment or collisions of “parent” bodies. Dusty radial structures, called “spokes,” have been observed in the Saturn rings, but remain partially unexplained.

Key Research Findings

Rings orbit in the sense of the planet’s rotation, as do most of the satellites. This makes one think that they are formed in the same protoplanetary disk. However, the short timescale of involved mechanisms favors a recent creation from a disrupted satellite. Moreover, the very short lifetimes of observed structures require active processes for maintaining them.

Modeling shows that dense ring materiel is subject to very rapid (week timescale) growing mechanisms, with smooth collisions and sticking of small particles, until house-sized objects are formed and disrupted. These structures are called DEBs (dynamical ephemerical bodies) or rubble piles.

The origin of the Saturn rings is still controversial. It is difficult to give a scenario explaining at the same time the spreading timescales, which are smaller than the age of the solar system, and the large mass of Saturn’s rings, as capture and/or disruption of a satellite sufficiently massive to make the rings is very unlikely after the end of the planet formation period.

The ethereal Jupiter rings are clearly the by-product of small satellites. If the Saturn ring is coeval with the planet, the question is why Jupiter has not a similarly large ring system.

The Uranus and the Neptune rings are more easily explained by a captured object. The disruption inside the Roche limit would have left a mixture of small satellites and particles reorganized in the observed configuration.

Future Directions

The next generation of high-contrast imaging instruments will provide the first unresolved image of an extrasolar planet. While the emitted infrared light from the planet in thermal equilibrium should show almost no phase effect, the reflected visible light will vary with the orbital phase angle. A ring around an extrasolar planet, both obviously unresolved, can be detected by its specific photometric signature (Arnold and Schneider 2004).

See also

Giant Planets




Roche Limit



Voyager (Spacecraft)

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