When a planet is successfully invaded by another empire, it is conquered by that empire and joins it. A conquered world takes a temporary hit to its efficiency. If the conquered world is a sector capital or imperial capital, it changes designation to autonomous. Otherwise, it will retain its original designation. Conquest breaks all trade routes. Structures and stockpiles are not affected by conquest. If the planet is within 250 LY of one of the invader's active sector capitals or imperial capital, it can be issued orders or redesignated.
If a sector capital is conquered or secedes, multiple planets within 250 LY will instantly defect, joining the conquering empire without having to be invaded. It does not seem to be easy to predict which planets will defect. All units of the attacked empire that are in fleets at, or stationed on, a defecting world get transferred to the attacker. This can radically affect the balance of a war