No I am not kiddin’ ya. Pluto and some selected Kuiper Belt Objects and possibly some Oort Cloud objects should be considered planets. Here is why. First some of the variables to consider; mass, orbital divergence from the plane of the ecliptic (remnants of the solar disk are generally found here), whether there is a solar orbit, composition, and temperature.

Why is mass imprtant? With enough mass, there is what is called hydrostatic equilibrium. There is enough gravity to overcome structural rigidity to form a fairly spherical body, subject to inertial deformation from rotation. Simple enough, the possible planet should be ball like in shape.

The plane of the ecliptic is very important. Bisecting the sun through the solar equator and extending it possibly 1-2 light years would contain almost all relevant mass as far as planetary building material is concerned. Just for scale; One A.U. is the distance between the sun and earth, 100 A.U. is  9.3 billion miles, 10000 A.U. is 930 billion miles and one light year is about 5.9 trillion miles. The furthest reasonably well calculated orbit of a plane of ecliptic object, is Sedna at perihelion about 95 billion miles. Pluto’s semi major axis is about 3.6 billion miles the “average” distance. Sedna’s “average” distance is 48.9 billion miles. There aren’t that many objects out there to interact with and eccentric orbits can be much more stable as a result.

In this plane are five rocky metallic objects including the moon. Mars and the moon differ in size and mass, but in density they are much lower than Mercury, Venus and the earth. Only the earth and Mercury have very strong magnetic fields. There are striking diferrences between all these objects. The moon is in hydrostatic equilibrium formed from proto-mantle blasted from the earth when a Mars sized object hit more than 4 billion years ago. It is possible that the heat from this collision and radioactive decay are the reasons why the core has a region of liquid iron/cobalt that gives the earth a magnetic field. I suspect that a similar process took place with Mercury, possibly in more than one collision with no mass retained in orbit. The fact that Mercury has a relativley eccentric orbit, is at least very good circumstantial evidence supporting the idea. Mars and the moon never had enough of a metallic core to have strong magnetic fields. Venus with a nearly matching mass and density of the earth has never had a collision with another world of a size that would generate the heat necessary to keep a region of it’s  metallic core liquid. Nevertheless, despite differences, all are considered terrestrial planets with the earth/moon system a genuine double planet and they all have surfaces. The moon is the only one of these objects lacking both an atmosphere and a magnetic field and in orbit around a non stellar object. Most of the mass that was similar in compostion to stellar spectra has been cooked away by proximity to the sune relative to the mass.

The asteroid belt contains objects including Ceres and Vespa in hydrostatic equilibrium, but on a scale relative to the terrestrial objects, much smaller in mass and diameter. They are also extremely numerous with no clear demarcation in size with objects in the belt, where they could be classified based on mass. There are no atmospheres or magnetic fields to speak of.

Next are the Jovian planets with no surfaces to “stand” on. These are relatively huge compared to anything else in orbit around the sun. Jupiter and Saturn are much more alike that they are to Neptune and Uranus which have striking similarities. All radiate more heat than they receive from the sun, due to gravitational contraction to a much larger degree than the terrestrial worlds. There are very few similarities with the terrestrial worlds in any way with with whole number multiples in mass greater than any of them, no surfaces, and that heat source. They also all have strong magnetic fields. Their distance from the sun also allowed them to retain mass more easily. Even though some extra solar planets are much more massive than Jupiter and are close in to the parent star it might be better to see this class of planets as failed stars; clumps of mass too small for fusion to start. Incidentally these Jovian worlds are nearer to stellar compostion as it is.

Now to the “problem children”; KBO trans-Neptunian objects including Pluto. The Plutonian type of object has a very great distance from the sun, usually close to the plane of the ecliptic and in orbital resonance with Neptune. It is said that thse objects have been flung about the solar system and for many that may be true, but there are mathematical indications that Uranus and Neptune have switched places in order from the sun, but Neptune still has one of the most circular orbits that is most concentric. But they are still planets. Pluto has three moons, Haumea, Orcus and Eris also have moons. They have surfaces. Eris is more massive than Pluto. These objects certainly are massive enough to have rocky cores and may have small amounts of metal although almost certainly no magnetic fields. Atmosheres on these objects would be very thin with a definite predisposition for having helium present and for the warmer ones methane, hydrogen and nitrogen.

Because of the extreme distance, it is cold enough for low molecular weight mass, that would otherwise be blasted away by solar wind and radiation to coalesce into these objects. This also aided the formation of the Jovian worlds.

There is greater justification for increasing the number of planets than demoting Pluto into the Hadean realm of mere KBO/Plutinoid/TNO status. Lets protest for Pluto!