A lot happened in 2006. Saddam Hussein was killed, Google purchased YouTube for $1.65 billion, and Pluto was downgraded to the status of “dwarf planet,” a new classification created by the International Astronomical Union (IAU.) In the dozen years since the controversial reclassification, the argument over Pluto’s status has popped up repeatedly, with polls as recent as 2014 showing that popular opinion is Pluto should have never lost its status as a planet, per CNN.
Philip Metzger, planetary scientist from the University of Central Florida, agrees. He’s the lead author of a study appearing in the journal Icarus arguing that the initial reclassification was invalid due to the inclusion of an arbitrary criterion in the definition of “planet.” When the IAU updated the definition of a planet, they chose the following:
(1) 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 neighborhood around its orbit.
It’s that last bit that caused trouble for Pluto. Because of Pluto’s diminutive size, it’s not able to “clear the neighborhood” the way that larger planets like Jupiter, Earth, or even Mars can. A quick examination of the surface of Pluto shows very few craters, unlike the surfaces of other rocky planets and moons, indicating that it does not encounter much random space debris. It’s also important to note that with Pluto as distant as it is, there’s not much in the neighborhood to clear out, so it makes sense that it doesn’t have as many craters.
Metzger argues that the third criterion should not be applied because it is not a concept used in scientific research. It is also subjective; how does one define clearing out the neighborhood? He has support in his concerns. According to Fox News, Owen Gingerich of Harvard, who was the chair of the IAU in 2006 when Pluto was demoted, told National Geographic something similar in 2014.
I thought it was really dumb that the IAU took as a category “dwarf planet” and then said, “But they’re not planets.” I was disappointed that it happened that way.
If given a choice, Metzger would like to see the neighborhood aspect removed and have the determination for a planet be whether or not its gravity is sufficient to hold it in a spherical shape, similar to item (b) in the current definition. He points out that the ability to do that is an important stage in the evolution of a planetary body, because that’s the point at which pressures become intense enough to initiate “active geology in the body.”
The IAU has noted that there is a pathway to resolving the issue, through a motion raised with the group’s governing body to proposed a new resolution, but so far no one has gone through that process.