Quantum orbits of the planets. Conceptual computer artwork showing the planets of the solar system on a model of atomic orbitals. This represents the theory that the orbits of the planets were determined by similar physical principles to those governing the orbits of electrons. Electrons can only occupy distinct energy levels around an atom, rather than having the capability to be anywhere. These quantum levels are determined by the electron's energy. The theory that similar effects occur at the solar system scale is controversial and not widely accepted, as the orbits can be explained by well-understood gravitational theory.