I have changed my mind about Pluto. I have decided that it should be a planet after all.
Part of the reasoning that went into the new definition of a planet is that by making Pluto a planet, eventually there might be another dozen or so. Many astronomers were uncomfortable with that idea. Planets shouldn't be so common, they said.
The definition made in 2006 by the International Astronomical Union demoted Pluto from the ranks of planethood. The planet-killing part of the definition says that a planet must have “cleared the neighborhood around its orbit.”
In IAU-speak, that means “it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence.”
Those bodies, like Pluto that don't meet the “clearing the neighborhood” requirement, are now called “dwarf planets.”
Earth's orbital neighborhood is the region between Venus and Mars, a span of about 35 million miles.
Pluto orbits the sun from 3.7 billion miles away in an area of the solar system known as the Kuiper belt.
Three other dwarf planets also orbit the sun in the Kuiper belt. There are millions of comets and other small objects in the Kuiper belt, so obviously neither Pluto nor any of the other three dwarf planets there have “cleared their neighborhoods.”
But look at the size of that neighborhood. It's a hundred times wider than Earth's neighborhood. Put Earth out there and, even though it is 500 times more massive than Pluto, it would have a hard time “clearing its neighborhood.”
But ignoring that, Pluto has another characteristic that makes me want to drop the “dwarf” from its description. Astronomers using the Hubble Telescope have recently discovered Pluto's fifth moon. Fifth!
Earth has only one; Mercury and Venus have none.