Many issues have arisen from the debate whether or not Pluto is a planet. Some astronomers say that Pluto should be classified as a "minor planet" due to its size, physical characteristics, and other factors. On the other hand, some astronomers defend Pluto's planet status, citing several key features. Indeed, most of the problem is that there is no formal definition of a planet. Furthermore, it is very difficult to invent one that would allow the solar system to contain all nine planets. I suggest that for an object to be classified as a planet, it must embody three characteristics. It must be in orbit around a star (thus removing the larger satellites from contention), it must be too small to generate heat by nuclear fusion (so dwarf stars are excluded) and it must be massive enough to have collapsed to a more or less spherical shape (which excludes comets, and most of the asteroids). These criteria would admit a few of the larger asteroids and probably some of the Kuiper belt objects as well, but adding a requirement for a planet to have a minimum diameter of 1,000 km would remove the larger asteroids from contention while retaining Pluto. Below are some brief reasons as to why Pluto may not be considered a planet with my rebuttal. Pluto is small compared to the other planets.
Pluto is about half the size of the next smallest planet, Mercury. However, there is no scientific reason whatsoever to pick the size of Mercury as being the size of the smallest object to be called a planet. Mercury itself is less than half the size of Mars, and Mars is only about half the size of Earth or Venus. Earth and Venus are only about one-seventh the size of Jupiter. Why not pick one-tenth the size of Jupiter as the size of the smallest planet, if the cutoff is going to be chosen arbitrarily? In that case, Mars, Mercury and Pluto would all have to be classified as asteroids. If the size-cutoff between asteroids and planets is going to be randomly chosen, the cutoff value should be agreed upon in open debate among interested scientists.
Pluto is smaller than 7 moons in the solar system.
Pluto is smaller than Earth's Moon, Jupiter's moons Io, Europa, Ganymede, and Callisto, Saturn's moon Titan, and Neptune's moon Triton. On the other hand, Pluto is larger than the other 40 known moons in the solar system. There is no scientific reason to arbitrarily distinguish between planets and asteroids based on the sizes of the moons that happen to be present in a planetary system. The only limit on the size of the moons of a planet is that they must be smaller than the planet. Thus, it is coincidence that Jupiter's and Saturn's large moons are as small as they are: if Jupiter happened to have a moon one-fourth of its own size (as Earth does), that moon would be larger than Earth, Venus, Mars, Mercury and Pluto, and all of these "planets" would have to be classified as asteroids. If Jupiter happened to have a moon half its own size (as Pluto does), that moon would be larger than all of the other planets except Saturn, and we would have a two-planet solar system with seven very large asteroids. The problems with this classification criterion are that they are arbitrary and non-general.
Pluto is unlike the other planets in that it has an icy surface instead of a rocky surface, like the inner 4 (terrestrial) planets, or a deep atmosphere, like the next 4 (gas giant) planets. Pluto has a crust believed to be composed mostly of water ice, with a relatively thin layer of nitrogen ice mixed with small fractions of methane and carbon. However, there is no particular scientific reason why this should exclude Pluto from being classified as a planet. It is just as reasonable to claim that all planets must have rocky surfaces, like the terrestrial planets: then Jupiter, Saturn, Uranus and Neptune would have to classified as something other than "planets" (perhaps they would be minor planets?). Alternatively, it could be declared that...