Outer solar system formation. Artwork showing the formation of the unusual orbits of bodies in the outer solar system. The inset shows three stages in the formation of the solar system. Early on (top) it is a disc of small bodies (planetesimals), which collide and accrete into gradually larger bodies. These accrete smaller ones until their orbital paths are nearly clear of debris (middle), but some large bodies do remain, and collisions (orange) still occur. Some smaller bodies are thrown out of the solar system by the gravity of larger bodies (lower right of middle stage). The result (bottom) is that some bodies, such as the dwarf planet Pluto, have highly eccentric orbits (yellow lines).