In 2006, the International Astronomical Union — the global governing body for many official astronomical endeavors, including naming and classification — took a step that had never been taken before: they officially defined the term “planet.” This contentious move, which occurred with only a tiny fraction of the membership present, and notably lacked the contributions of many leading planetary scientists and planetary astronomers, put forth three criteria for defining what gets to be a “planet” versus a non-planet.

It must orbit the Sun and no other body.

It must be massive enough to reach hydrostatic equilibrium: where gravity and rotation primarily determine its shape.

And it must have cleared its orbit, without a substantial amount of leftover primeval material from the solar system’s formation remaining in it.

On the one hand, there are many good reasons to like and support this definition , as it draws a clear distinction between the eight objects — major planets — that achieve planetary status…