To continue with a discussion that we started a couple of years ago, I will again visit the question of how many planets are there in the Solar System. The answer today is more complex than it was three or four years ago when we could say, simply, "Nine."
The International Astronomical Union (hereafter referred to as the IAU) created another category for objects that are large enough to be relatively spherical shape, that orbited the sun but that aren’t large enough to have cleared their obits of additional debris... which was the new, third criterion for a planet and which always struck me as a nonsensical way of demoting Pluto from its status as a planet.
Pluto and another Kuiper Belt object (Pluto on left, Eris on right), Eris, are now known as Plutoids rather than dwarf planets. That distinction belongs to Ceres which once was considered a planet and then the largest of the asteroids. It was demoted to asteroid when more and more of them were discovered. In fact, Ceres might be the only object of its kind and at the moment is the lone dwarf planet that is not also a Plutoid.
Plutoids, then, are objects that a spherical, orbit the sun beyond Neptune, and are objects in the Kuiper Belt known as Kuiper Belt Objects (KBO which seems to be broken down into Classical KBOs or Cubewanos and Resonant KBOs) and are also Trans-Neptunian Objects (TNO) and I’m sure that if I looked farther I could find more qualifications. Oh, yes, the moons of Plutoids (or more properly, the satellites) are not Plutoids and therefore not KBOs or TNOs. The question that springs to my mind is which is the satellite and which is the Plutoid in the Pluto - Charon system.
There are about another dozen objects that are being considered as Plutoids. These include 2003 EL 61, Sedna, 2005 FY 5, Quaoar, Orcus and Ixion, all larger than Ceres and then 2002 AW 197, Varuna and 2002 TC302 which are smaller. Now some of these are cubewanos, or SDOs (which are scattered disc objects) or plutinos (which, apparently are TNOs that orbit the sun twice for every three orbits of Neptune)... Are you confused now? Yeah, me too.
Well, let’s see if we can’t complicate this even more. There are two additional objects that are in the process of being named Plutoids. There are called Easterbunny and Santa and both have other, long designations. And, I seem to remember something about an object way out, maybe beyond the Kuiper Belt and into the area of the Oort Cloud. It’s about the size of Mars. What are they going to do about that if the size is verified?
In fact, in the last couple of days, astronomers have suggested a large object out there that would be, currently, the largest of the Plutoids, but this is not based on observation. It’s based on the gravitational permutations of the various objects out there. This is how Pluto was discovered in the first place. They were looking for a planet that caused disturbances in the orbits of the outer planets which makes me wonder if all that debris out there might not account for the trouble. What this means is that there might be something else out there that is the size of a "real" planet... or if this new speculation is true, there might be two.
What we can now say about the Solar System is that there are eight planets. These range in size from those small, rocky planets of the inner Solar System (Mercury, Venus, Earth and Mars) and the large gas giants of the outer Solar System (Jupiter, Saturn, Uranus and Neptune). It strikes me that we could further define planet if we wanted to, breaking them into two additional categories because the inner planets are certainly different than the outer planets in size, structure and number of satellites (there are only three known for the inner planets and about 150 for those in the outer system... in fact two of these satellites are larger than Mercury, but I digress).
Beyond Neptune are the Plutoids, that is, Pluto and Eris (which between them have more satellites than the four inner planets combined). They might be joined by a dozen others, including two that are in the process of being accepted as Plutoids, as I mentioned. There are some estimates that suggest there might be fifty of these objects, some about a light year or so from the Sun. These objects, like Pluto and Eris (which is thought to be slightly larger than Pluto) have diameters between 2000 kilometers and about 700, making the smallest, smaller than Ceres.
Ceres, because of what it is, the size it is, and the location of it, is currently the lone dwarf planet that is not also a Plutoid. There is speculation that there will be other Ceres-class dwarf planets, though I don’t know where they would be hidden.
So, the answer to the question of how many planets, is still eight. But now we have several other large objects that are being grouped under a variety of categories, which falls under planet... more or less. If you have questions about this, don’t ask me. I’m as confused as the rest of you.