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.
Cando os planetas se formaron, orbitaban nun disco de gas. Dentro dese disco, poden moverse e axustar as súas órbitas ás dos seus veciños ata que estean perfectamente sintonizados.