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# Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa, and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large Solar System bodies to clear the neighborhood around their orbits by ejecting nearly everything else around them; this effect is used in the current definition of a planet.

Except as noted in the Laplace resonance figure (below), a resonance ratio in this article should be interpreted as the ratio of number of orbits completed in the same time interval, rather than as the ratio of orbital periods (which would be the inverse ratio). The 2:3 ratio above means Pluto completes 2 orbits in the time it takes Neptune to complete 3.

## History

Ever since the discovery of Newton's law of universal gravitation in the 17th century, the stability of planetary orbits has preoccupied many mathematicians, starting with Laplace. The stable orbits that arise in a two-body approximation ignore the influence of other bodies. The effect of these added interactions on the stability of the Solar System is very small, but at first it was not known whether they might add up over longer periods to significantly change the orbital parameters and lead to a completely different configuration, or whether some other stabilising effects might maintain the configuration of the orbits of the planets.

It was Laplace who found the first answers explaining the remarkable dance of the Galilean moons (see below). It is fair to say that this general field of study has remained very active since then, with plenty more yet to be understood (e.g. how interactions of moonlets with particles of the rings of giant planets result in maintaining the rings).

## Types of resonance

In general, an orbital resonance may

• involve one or any combination of the orbit parameters (e.g. eccentricity versus semimajor axis, or eccentricity versus orbit inclination).
• act on any time scale from short term, commensurable with the orbit periods to secular (measured in 104 to 106 years).
• lead to either long term stabilisation of the orbits or be the cause of their destabilization.

A mean motion orbital resonance occurs when two bodies have periods of revolution that are a simple integer ratio of each other. Depending on the details, this can either stabilize or destabilize the orbit. Stabilization occurs when the two bodies move in such a synchronised fashion that they never closely approach. For instance:

Orbital resonances can also destabilize one of the orbits. For small bodies, destabilization is actually far more likely. For instance:

• In the asteroid belt within 3.5 AU from the sun, the major mean-motion resonances with Jupiter are locations of gaps in the asteroid distribution, the Kirkwood gaps (most notably at the 3:1, 5:2, 7:3, and 2:1 resonances). Asteroids have been ejected from these almost empty lanes by repeated perturbations. However, there are still populations of asteroids temporarily present in or near these resonances. For example, asteroids of the Alinda family are in or close to the 3:1 resonance, with their orbital eccentricity steadily increasing due to interactions with Jupiter until they eventually have a close encounter with an inner planet that ejects them from the resonance.
• In the rings of Saturn, the Cassini Division is a gap between the inner B Ring and the outer A Ring that has been cleared by a 2:1 resonance with the moon Mimas. (More specifically, the site of the resonance is the Huygens Gap, which bounds the outer edge of the B Ring.)
• In the rings of Saturn, the A Ring's outer edge is maintained by a destabilizing 7:6 resonance with the moon Janus.

A Laplace resonance occurs when three or more orbiting bodies have a simple integer ratio between their orbital periods. For example, Jupiter's moons Ganymede, Europa, and Io are in a 1:2:4 orbital resonance.

A Secular resonance occurs when the precession of two orbits is synchronised (usually a precession of the perihelion or ascending node). A small body in secular resonance with a much larger one (e.g. a planet) will precess at the same rate as the large body. Over long times (a million years, or so) a secular resonance will change the eccentricity and inclination of the small body. A prominent example is the ν6 secular resonance between asteroids and Saturn. Asteroids which approach it have their eccentricity slowly increased until they become Mars-crossers, at which point they are usually ejected from the asteroid belt due to a close pass to Mars. This resonance forms the inner and "side" boundaries of the main asteroid belt around 2 AU, and at inclinations of about 20°.

The Titan Ringlet within Saturn's C Ring exemplifies another type of resonance in which the rate of apsidal precession of one orbit exactly matches the speed of revolution of another. The outer end of this eccentric ringlet always points towards Saturn's major moon Titan.

A Kozai resonance occurs when the inclination and eccentricity of a perturbed orbit oscillate synchronously (increasing eccentricity while decreasing inclination and vice versa). This resonance applies only to bodies on highly inclined orbits; as a consequence, such orbits tend to be unstable, since the growing eccentricity would result in small pericenters, typically leading to a collision or (for large moons) destruction by tidal forces.

## Mean motion resonances in the Solar System

There are only a few known mean motion resonances in the Solar System involving planets or larger satellites (a much greater number involve asteroids, Kuiper belt objects, planetary rings and moonlets).

The simple integer ratios between periods are a convenient simplification hiding more complex relations:

As illustration of the latter, consider the well known 2:1 resonance of Io-Europa. If the orbiting periods were in this relation, the mean motions $n,!$ (inverse of periods, often expressed in degrees per day) would satisfy the following

$n_\left\{rm Io\right\} - 2cdot n_\left\{rm Eu\right\} = 0$

Substituting the data (from Wikipedia) one will get −0.7395° day−1, a value substantially different from zero!

Actually, the resonance is perfect but it involves also the precession of perijove (the point closest to Jupiter) $dotomega$ The correct equation (part of the Laplace equations) is:

$n_\left\{rm Io\right\} - 2cdot n_\left\{rm Eu\right\} + dotomega_\left\{rm Io\right\} = 0$

In other words, the mean motion of Io is indeed double of that of Europa taking into account the precession of the perijove. An observer sitting on the (drifting) perijove will see the moons coming into conjunction in the same place (elongation). The other pairs listed above satisfy the same type of equation with the exception of Mimas-Tethys resonance. In this case, the resonance satisfies the equation

$4cdot n_\left\{rm Th\right\} - 2cdot n_\left\{rm Mi\right\} - Omega_\left\{rm Th\right\}- Omega_\left\{rm Mi\right\}= 0$

The point of conjunctions librates around the midpoint between the nodes of the two moons.

### The Laplace resonance

The most remarkable resonance involving Io-Europa-Ganymede includes the following relation locking the orbital phase of the moons:

$Phi_L$$= lambda_\left\{rm Io\right\} - 3cdotlambda_\left\{rm Eu\right\} + 2cdotlambda_\left\{rm Ga\right\}$$= 180^circ$

where $lambda$ are mean longitudes of the moons. This relation makes a triple conjunction impossible. The graph illustrates the positions of the moons after 1, 2 and 3 Io periods.

### Pluto resonances

Pluto is following an orbit trapped in a web of resonances with Neptune. The resonances include:

• A mean motion resonance of 2:3
• The resonance of the perihelion (libration around 90°), keeping the perihelion above the ecliptic
• The resonance of the longitude of the perihelion in relation to that of Neptune

One consequence of these resonances is that a separation of at least 30 AU is maintained when Pluto crosses Neptune's orbit. The minimum separation between the two bodies overall is 17 AU, while the minimum separation between Pluto and Uranus is just 11 AU (see Pluto's orbit for detailed explanation and graphs).

## Coincidental 'near' ratios of mean motion

A number of near-integer-ratio relationships between the orbital frequencies of the planets or major moons are sometimes pointed out (see list below). However, these have no dynamical significance because there is no appropriate precession of perihelion or other libration to make the resonance perfect (see the detailed discussion in the Mean-motion resonances in the Solar System section, above).

Such near-resonances are dynamically insignificant even if the mismatch is quite small because (unlike a true resonance), after each cycle the relative position of the bodies shifts. When averaged over astronomically short timescales, their relative position is random, just like bodies which are nowhere near resonance.

For example, consider the orbits of Earth and Venus, which arrive at almost the same configuration after 8 Earth orbits and 13 Venus orbits. The actual ratio is 0.61518624, which is only 0.032% away from exactly 8:13. The mismatch after 8 years is only 1.5° of Venus' orbital movement. Still, this is enough that Venus and Earth find themselves in the opposite relative orientation to the original every 120 such cycles, which is 960 years. Therefore, on timescales of thousands of years or more (still tiny by astronomical standards), their relative position is effectively random.

The presence of a near resonance may reflect that a perfect resonance existed in the past, or that the system is evolving towards one in the future.

Some orbital frequency coincidences that have been pointed out include:

(Ratio) and Bodies Mismatch after one cycle Randomization time Probability
Planets
(9:23) VenusMercury 4.0° 200 y 0.19
(8:13) EarthVenus 1.5° 1000 y 0.065
(243:395) EarthVenus 0.8° 50,000 y 0.68
(1:3) MarsVenus 20.6° 20 y 0.11
(1:2) MarsEarth 42.9° 8 y 0.24
(1:12) JupiterEarth 49.1° 40 y 0.27
(2:5) SaturnJupiter 12.8° 800 y 0.14
(1:7) UranusJupiter 31.1° 500 y 0.17
(7:20) UranusSaturn 5.7° 20,000 y 0.20
(5:28) NeptuneSaturn 1.9° 80,000 y 0.052
(1:2) NeptuneUranus 14.0° 2000 y 0.078
Mars System
(1:4) DeimosPhobos 14.9° 0.04 y 0.083
Jupiter System
(3:7) CallistoGanymede 0.7° 30 y 0.012
Saturn System
(2:3) EnceladusMimas 33.2° 0.04 y 0.33
(2:3) DioneTethys 36.2° 0.07 y 0.36
(3:5) RheaDione 17.1° 0.4 y 0.26
(2:7) TitanRhea 21.0° 0.7 y 0.22
(1:5) IapetusTitan 9.2° 4.0 y 0.051
Uranus System
(1:3) UmbrielMiranda 24.5° 0.08 y 0.14
(3:5) UmbrielAriel 24.2° 0.3 y 0.35
(1:2) TitaniaUmbriel 36.3° 0.1 y 0.20
(2:3) OberonTitania 33.4° 0.4 y 0.34
Pluto System
(1:4) NixCharon 39.1° 0.3 y 0.22
(1:6) HydraCharon 6.6° 3.0 y 0.037

The most remarkable (least probable) orbital correlation in the list is that between Callisto and Ganymede, followed in second place by that between Hydra and Charon.

The two near resonances listed for Earth and Venus are reflected in the timing of transits of Venus, which occur in pairs 8 years apart, in a cycle that repeats every 243 years.

The near 1:12 resonance between Jupiter and Earth causes the Alinda asteroids, which occupy (or are close to) the 3:1 resonance with Jupiter, to be close to a 1:4 resonance with Earth.

## Possible past mean motion resonances

While Dione and Tethys are not close to an exact resonance now, they may have been in a 2:3 resonance early in the Solar System's history. This would have led to orbital eccentricity and tidal heating that may have warmed Tethys' interior enough to form a subsurface ocean. Subsequent freezing of the ocean after the moons escaped from the resonance may have generated the extensional stresses that created the enormous graben system of Ithaca Chasma on Tethys.

The satellite system of Uranus is notably different from those of Jupiter and Saturn in that it lacks precise resonances among the larger moons, while the majority of the larger moons of Jupiter (3 of the 4 largest) and of Saturn (6 of the 8 largest) are in mean motion resonances. In all three satellite systems, moons were likely captured into mean motion resonances in the past as their orbits shifted due to tidal dissipation (a process by which satellites gain orbital energy at the expense of the primary's rotational energy, affecting inner moons disproportionately). In the Uranus System, however, due to the planet's lesser degree of oblateness, and the larger relative size of its satellites, escape from a mean motion resonance is much easier. Lower oblateness of the primary alters its gravitational field in such a way that different possible resonances are spaced more closely together. A larger relative satellite size increases the strength of their interactions. Both factors lead to more chaotic orbital behavior at or near mean motion resonances. Escape from a resonance may be associated with capture into a secondary resonance, and/or tidal evolution-driven increases in orbital eccentricity or inclination.

Mean motion resonances that probably once existed in the Uranus System include (3:5) Ariel-Miranda, (1:3) Umbriel-Miranda, (3:5) Umbriel-Ariel, and (1:4) Titania-Ariel. Evidence for such past resonances includes the relatively high eccentricities of the orbits of Uranus' inner satellites, and the anomalously high orbital inclination of Miranda. High past orbital eccentricities associated with the (1:3) Umbriel-Miranda and (1:4) Titania-Ariel resonances may have led to tidal heating of the interiors of Miranda and Ariel, respectively. Miranda probably escaped from its resonance with Umbriel via a secondary resonance, and the mechanism of this escape is believed to explain why its orbital inclination is more than 10 times those of the other nonirregular Uranian moons (see Uranus' natural satellites).

In the case of Pluto's satellites, it has been proposed that the present near resonances are relics of a previous precise resonance that was disrupted by tidal damping of the eccentricity of Charon's orbit (see Pluto's natural satellites for details). The near resonances may be maintained by a 15% local fluctuation in the Pluto-Charon gravitational field. Thus, these near resonances may not be coincidental.