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Is Pluto a Planet?
Dateline, Earth 2010
In June 2010, Earth Magazine ran an interview with Neil DeGras Tyson, which was mostly a story about Pluto being demoted to a dwarf planet. A lot of people were disappointed by this demotion, apparently feeling that as the cold lonely underdog of the solar system, it was unfair to insult Pluto even more by kicking it out of the game with the big guys. I agree with the classification, and wrote a letter to Earth Mag in support. They printed the letter in the Sept 2010 issue, which I repeat below.
Earth Magazine June 2010
Interview with Neil DeGras Tyson
In your interview with Neil deGrasse Tyson, the question kept coming up as to why people were so concerned that Pluto was demoted. One reason that wasn't mentioned was that the criteria used by the IAU appeared contrived for the purpose of excluding particular members.
For example, "massive enough to be round", ignores composition. A liquid body, such as Europa, could be the size of Ohio and fit that description. And how round does it have to be? Saturn is about 5% oblate, but what if it was 20% oblate - where is the cutoff? And we don't even know the shape of Kuiper Belt objects.
And "Massive enough to clear its orbit", is equally meaningless. How many Kuiper Belt objects have cleared their orbits? No one knows. And again, where's the cutoff? Neptune still hasn't cleared its orbit of Pluto. If mass is important, they should just quantify mass.
Statisticians use a simple trick for catagorizing objects like this: they find an obvious dividing line in the ranking. If you list the planets in order of size and composition, you'll find a big gap in both between Pluto and the rest of us. And isn't that what Neil said: wrong size, wrong composition?
Pluto has a long happy history of being a planet and is a favorite with children everywhere. But in reality, it's just not the same as the rest of the inner 8. Pluto is a small ice-body in the Kuiper Belt, just like the other recently discovered planetoids.
And Pluto is not even the biggest of the Kuiper Belt objects. Eris is slightly larger, and that is what caused this comotion in the first place. When Eris was discovered (in 2005) astronomers had to decide whether Eris was the tenth planet or Pluto was not a planet. So astronomers around the world got into a lengthy discussion and decided, more than anything, they needed to define just what a planet is in the first place. So they came up with a list of criteria, which in my mind was mighty lame, but the correct conclusion.
The IAU's definition of a planet is:
A planet 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.
Just for a reference
Here is a list of planets and dwarfs listed by size with biggest first.