Five years ago today, the International Astronomical Union (IAU) downgraded one of the large objects in the Kuiper belt from “planet” to “dwarf planet” status. That’s right. Pluto just ain’t planetary class no more.
The IAU decided that a planet should be defined as follows:
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 neighborhood around its orbit.
Because Pluto appears inside Neptune’s orbital path for a portion of its orbit, it obviously hasn’t cleared the neighborhood around its orbit. (Has Neptune, though? I suppose so…)
A dwarf planet, according to the IAU, 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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
In other words, the seven moons in our solar system that are larger than Pluto (Io, Europa, Ganymede, Callisto, Titan, Triton and our own Moon) don’t qualify as dwarf planets because they orbit other solar system bodies.
And the fact that Pluto has its own moon doesn’t qualify it as a planet. After all, Eris, an object even farther away from the Sun than Pluto (it’s in the scattered disc), is both larger than Pluto and has its own moon, so if Pluto counts as a planet, Eris has to count as well.
Part (d) of the definition is somewhat troublesome, since all planets are satellites of our sun, but presumably that’s just shorthand for not a satellite of a non-star.
If you’re interested in learning more about how and why Pluto got demoted, I highly recommend the eminently readable book by Mike Brown, How I Killed Pluto and Why It Had It Coming, mentioned not too long ago in this very blog.
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