The outer regions of the solar system include; small cold objects and one object that's been the cause of some controversy because it was demoted as a planet, Pluto. No I'm not a Pluto-hater, but I align with my fellow professional astronomers in thinking that it's better categorized as a dwarf planet. So let's look at poor misbegotten Pluto and see why astronomers in their cold-hearted way demoted this from the pantheon of planets. The first part of the case against Pluto comes from the fact that we've already discovered at least one object larger than Pluto and pending surveys that are currently underway in that outer part of the solar system there may be between 3-5 other objects that are Pluto-sized or larger. Either children will have to forget one of the planets they know and love or have to remember the names of another three to five. Michael Brown at Caltech discovered the first larger than Pluto object in the outer part of the solar system a decade ago. It's called Eris now, brown for a while wanted it to be called Xena and thought he could name it himself. The planet indeed and logically has a small moon called Gabrielle, but the astronomers decided in their stubborn and somewhat old-fashioned way, that you're not allowed to name planets yourself, you have to go through naming conventions sanctioned by the International Astronomical Union and the tradition is; that planets and their moons should be named after mythological beings. So Xena was nixed and we have Eris. The real case against Pluto comes from the fact that it's not like the other four giant planets. It's far smaller and shows signs of having been captured and put into its high inclination high eccentricity orbit. The final part in the case against Pluto and the formal reason why it was demoted, is that a planet needs to be large enough to be round by its own gravity, and that's satisfied by Pluto, but also needs to have cleared out its zone of the solar system of debris early in the formation process. Pluto does not have its own zone in the solar system for the reason that it was probably captured from interstellar space. So in the solar system, we have four major planets in the outer part, four and the inner part and then a large set of rocky objects at very large distances, beyond 40 or a 100 astronomical units. In addition to the Trans-Neptunian dwarf planets of which there are a handful, there are very large number of rocks not much bigger than the asteroids in the asteroid belt. These extend to hundreds of astronomical units and are located in the plane the ecliptic the same disc out of which the solar system objects formed. The solar system formally contains eight planets. Pluto has been demoted to the status of a dwarf planet and if we look at the outer solar system, we find that there are a number of rocky objects, a couple of which are larger than Pluto and many of which are likely to be similar in size. These Trans-Neptunian objects are part of the debris disc out of which the planets formed. Pluto itself appears to have been captured into its eccentric and inclined orbit at sometime early in the history of the solar system.
