You would assume that Astronomers have manged to make up their minds on what a planet is, especially since we are now discovering them around others stars but you'd be wrong! ATM something is a planet if and only if the IAU (International Astronomical Union) say it is! They are currently working hard on a definition but nothing has been agreed yet. This new interest in an actual definition for a planet is being spurred on by yet more discoveries of large objects out beyond Pluto that the press are immediately dubbing "the 10th planet"(we have had 3 "10th planets" in the past few years!).

So, lets have a look at the facts as I see them.

The first thing to note is that the 8 inner planets all have the same features defining them, namely:

1. They have all been molded into spherical shapes by gravity because they have large masses.
2. They all orbit in near circular orbits.
3. They all orbit in the plane of the solar system (defined by projecting out the Sun's equator).

This would seem to imply a relationship between these objects and a common origin.

So, what about Pluto? Well Pluto is completely different to these objects and does not share the above properties of the 8 inner planets, this can be clearly seen in the diagram below: 

So is Pluto unique? No! It in fact shares the characteristics of a whole other class of objects, the TNOs (Trans Neptunian Objects). Since Pluto was discovered we have also discovered a whole array of smaller but similar objects (including all our '10th planets'). These objects all share the same properties:

1. They tend to be more irregular in shape and are all very dark with low albedos (nothing to do with sex, they just reflect light poorly!).
2. They are all in highly elliptical orbits (Pluto spends a few decades of each orbit inside the orbit of Neptune!).
3. Their orbits are all inclined to the plane of the solar system in a wide variety of angles.

Hence, these objects too would seem to be related and would also appear to have a common origin that is in some fundamental way different to the 8 inner planets.

It is this logical division of the large objects in our solar system which would lead me to split them into two groups, the planets as defined by the three criteria above that the eight inner planets share and the TNOs defined by the three properties of Pluto and the newly discovered large bodies beyond it. This, to me, seems very sensible and a better stab at the definitions astronomers seem to be peddling today which tend to be purely arbitrary and involve just drawing a line in the sand at some nice round number, e.g. "all objects with a diameter greater than 1000km are planets". I'm not a fan of these arbitrary definitions but instead on a definition based on logic but the IAU will find it very hard to bring in such a logical definition because it would involve re-classifying Pluto as the first TNO discovered rather than the 9th planet and there are a lot of Astronomers, particularly in America, who would not allow Clyde Tombaugh's discovery to be 'down graded' (as they would see it).

All in all I'm curious to see what the IAU will come up with but a little fearful of what such a massively large committee might come up with in an attempt to do the impossible and please everyone!

Note

This post was initially posted to my old blog here. Comments may no longer be posted there and should be posted here but there are still some old comments at the original loaction that people may be interested in reading.