This is a thoroughly stale topic, but it manages to come up again and again. In 2006, through a perhaps rather questionable procedure, the International Astronomical Union (IAU) defined the word “planet” in such a way as to kick Pluto out of the category, which apparently struck a nerve with some people. I have yet to hear a compelling reason as to why this struck a nerve (I suspect that it’s that people don’t like statements to contradict what they learned in elementary school), but it did.
Personally, I was very glad. The more one learns about Pluto, the less one should really feel that it merits the same category as the eight now official planets. The first point is that Pluto is significantly smaller than the big 8. In fact Mercury, the smallest planet, is 25 times more massive than Pluto.
Now, it’s fair to point out that there is great variation within the ranks of the planets and that some body has to be the smallest planet and that Earth is 18 times as massive as Mercury, so it’s a pretty arbitrary lower bound.
However, it seems to be a much better lower bound when one looks at the sizes of moons. Fully 7 of our solar system’s moons (Ganymede, Titan, Callisto, Io, Earth’s Moon, Europa, and Triton) are both more massive and of greater dimensions than Pluto, as well as Eris, the drawf planet that precipitated the IAU’s decision.
In fact, here’s an nice illustration courtesy of “Tony g100” at the Wikimedia Commons (click to got there for a larger version):
Now you may notice that Mercury is in fact smaller (in terms of dimensions) than Ganymede and Titan, but it should be noted that these ice moons are much less dense than Mercury and in fact each have less than half the mass of Mercury, which is most rock and iron. It’s much like comparing a snowball to a shot put shot.
Now, the final distinction is in the IAU’s definition and that is the relationship between planets and other objects in the solar system. Here is the definition with the relevant distinction between planets and dwarf planets in bold
(1) A "planet"1 is a celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
Now, the main objection is that this is excessively vague. However, within our solar system, there is a pretty bright line for this measurement. Quantitatively, all planets have neighborhoods are multiple orders of magnitude more clear than all other solar system objects.
In the case of Neptune and Pluto in particular, Pluto’s orbit is dominated by Neptune. It actually has an orbit that specifically avoids Neptune and the dwarf planet actually gets closer to Uranus than it ever does to Neptune, orbiting the sun three times for every two revolutions of Neptune. None of the 8 planets have orbits that specifically avoid another one and I think that that is of note.
Finally, there is the case of the multiple other solar system bodies that are similar to Pluto. Eris, another dwarf planet, is larger than Pluto. Closer to Earth, Ceres was made a dwarf planet by the IAU’s decision, as it is round (because of its size) and orbits the Sun.
In fact, while Pluto is significantly larger than Ceres, there aren’t really any specially qualities that Pluto has that Ceres doesn’t have. And in fact, Ceres was also once considered a planet. Both of them had been named as such when they were initially thought to be larger and more unique that they actually were (Pluto, which is actually 0.2% Earth’s mass was once thought to be larger than the Earth).
I think the most telling part of this whole drama has been the lack of any public outcry for Ceres to be reinstated as a planet. That tells me that this is really a circus of ignorance and petty nostalgia, not one of a genuine desire to classify our solar system’s objects by common sense guidelines.