The Planet debate seems to be ongoing (subscription required). While I agree with the IAU that Pluto should not be considered a planet, I find myself in disagreement with their current guidelines for a planet.
Right now the requirements include:
1. For an object to be defined as a planet, it must be restricted to this solar system.
This unfortunately would invalidate all the research behind extra-solar planets, including the proto-planet that was announced on Science Daily a few days ago. This proto-planet is only 100,000 years old!!! This requirement would also remove planetary status from this planet. This is the “earth-like” planet that was discovered approximately a year ago.
2. The object must have cleared its own orbit.
This is perhaps the most galling of the requirements, seeing as how several planets within this solar system haven’t cleared their orbits. The article uses Jupiter as an example of a planet that hasn’t cleared its own orbit. However, the example that jumps first and foremost to my mine is THE EARTH! Every year we cross paths with extra-terrestrial debris which results in meteor showers. If we seriously consider this to be a requirement for an object to be a planet, do we really have ANY planets?
I understand the spirit of the definition, but there are better ways to go about it. Other proposals have included an object that has enough gravity to pull itself into a “spherical shape”. Unfortunately as things bounce around, they break. When they break enough, they begin to take on spherical shapes. I don’t think this is a useful definition. For a while, I thought any object that has enough gravity to initiate differentiation within its own composition could be considered a planet, but apparently certain large asteroids can do this as well.
Maybe we are beating around the bush. The IAS is just trying to set up guidelines to identify the “objects that are gravitationally dominant in [a] solar system”. Perhaps, it should just be defined as some threshold of gravitational pull. Mercury is the smallest non-contentious planet. It is 0.38 Earth’s gravity. Anything with gravity greater than that could be considered a planet. Alternatively, we could even make it a requirement that would include the mass of the parent star and the distance to the star. We could then just use our solar system as a template.
Whatever we do, all these definitions are subjective. They don’t reflect any meaning of the universe. They are merely a human construct to facilitate science. Pluto doesn’t give a rat’s ass if it is a planet or not, neither do any other celestial bodies. I have stated my opinion on the subject (for what it is worth) and many others will do the same. However, while it is easy to talk about your opinions (I notice I do that… A LOT) it is important not to get bogged down in them and get on with science.
As far as the debate itself is concerned, it will continue in August 2008 at an IAS convention. So stay tuned.
PLANETARY SCIENCE: The Planet Debate Continues Mark V. Sykes (28 March 2008) Science 319 (5871), 1765. [DOI: 10.1126/science.1155743]
Royal Astronomical Society (2008, April 2). Youngest Planet Ever Discovered Offers Unique View Of Planet Formation. ScienceDaily.