Bound together by gravity

When the International Astronomical Union (IAU) defined planets in 2006, part of that definition included the requirement that a planet has enough mass that its self-gravity causes it to reach hydrostatic equilibrium. The planet is able to resist compressive forces in space to hold together and stay rounded in shape.

Planets also "clear the neighbourhood" around their orbits. This means that there are no other bodies of the same size in its orbit. The Sun has a strong enough pull to keep the planets and their bodies around it.