Pluto is a small planet. Actually, it is useful to distinguish between big planets, medium planets and small planets. Big planets and medium planets differ in

Pluto is a small planet. Actually, it is useful to distinguish between big planets, medium planets and small planets. Big planets and medium planets differ in chemical composition, caused by the intensity of their gravity. Only the big planets can retain large quantities of the very abundant chemical elements that form mostly volatile compounds (H, He, C, N, O, Ne and S). The medium planets are strongly depleted in these chemical elements, in comparison with the average composition of their stellar system. The medium planets and the small planets differ in their capacity of clearing their orbits of any other big bodies. The celestial bodies which orbit the star, but which are not big enough to become quasi-spherical, are not considered planets. By these definitions, the Solar system has 4 big planets, 4 medium planets and a large number of small planets, including Pluto and Ceres. It is likely that the configuration of the Solar System, with some medium planets close to the star and some big planets far from the star, is typical for most star systems, due to higher temperatures closer to the star, which prevent the condensation of the volatile elements that contain most of the available mass for forming a planet and due to lower amounts of bodies condensed around the smaller orbit, which can be accreted into the final planet.