When the planets were first forming, they were orbiting within a disc of gas, and while inside that disc, they can actually slide around and adjust their orbits to their neighbors until they're perfectly in tune.
Quando os planetas se estavam a formar estavam a orbitar num disco de gás. Dentro desse disco, eles podem deslizar e ajustar a sua órbita à dos vizinhos até estarem completamente sintonizados.