Stability of the Solar System essentially the distance from sun to earth). These are now known as the Kirkwood gaps. Some asteroids were later discovered to orbit in these gaps, but their orbits are unstable and they will eventually break out of the resonance due to close encounters with a major planet. Another common form of resonance in the Solar System is spin–orbit resonance, where the period of spin (the time it takes the planet or moon to rotate once about its axis) has a simple numerical relationship with its orbital period. An example is our own Moon, which is in a 1:1 spin–orbit resonance
Inferior and superior planets Interior planet now seems to be the preferred term for astronomers. Inferior/interior and superior are different from the terms inner planet and outer planet, which designate those planets which lie inside the asteroid belt and those that lie outside it, respectively. Inferior planet is also different from minor planet or dwarf planet. Superior planet is also different from gas giant. Inferior and superior planets In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun. In this situation, the latter planet