Orbital Resonance
Period Locking & Resonance Chains
Resonance
1 : 2 : 4
Type
Laplace
A period
1T
B period
2T
C period
4T
Cycle length
4T
Two bodies are in mean-motion resonance when their orbital periods form a simple integer ratio. Each time they align — a conjunction — they receive the same gravitational nudge in the same direction.
Repeated nudges at the same orbital phase can either reinforce stability (as with Jupiter's moons) or clear a region (as with Kirkwood gaps in the asteroid belt) depending on the geometry.
In the Laplace resonance, the three-body interaction means the conjunctions are always offset by 120° — no two pairs ever align simultaneously, preventing a destabilising triple conjunction.
Io / Europa / Ganymede
Laplace resonance
Jupiter's three inner moons. The most studied resonance chain in the solar system. Io's volcanic activity is driven by tidal heating from this lock.
Neptune / Pluto
Mean-motion resonance
Pluto completes exactly 2 orbits for every 3 Neptune completes. This resonance protects Pluto from close encounters with Neptune despite crossing its orbit.
TRAPPIST-1 system
Resonance chain
Four of seven planets in a near-perfect resonance chain. This tightly packed system owes its long-term stability almost entirely to the resonance locking.
Kirkwood Gaps
Destabilising resonance
Gaps in the asteroid belt where Jupiter's resonance repeatedly perturbs orbits until objects are ejected. Resonance clears as well as protects.