In science, it is important to have clear and precise definitions of terms. This is to remove ambiguity, making it easier for different researchers to compare their work. Ultimately, the goal of science is to be as objective as possible. This leads to terms like energy, force and momenta—which in common parlance are often interchangeable—describing quite different things in physics. The planetary issue was that no one had ever actually defined planet. This made it impossible to determine if a new discovery was a planet, or simply a large asteroid, wayward moon or something else.
The IAU's decision was to adopt the following definition of a planet:
- It primarily orbits the Sun;
- It is large enough that gravity forces it to be spherical;
- It has cleared its orbit.
The first criteria removes moons, which do orbit the Sun but clearly primarily orbit a planet. The second point removes asteroids, comets and other smaller stuff. Neither of these was particularly controversial. The third point was what lead to Pluto (and other objects like Ceres) to fail to meet planetary status. Further, it is very unlikely that there are any other objects that meet all three conditions, so our list of planets is complete.
In his paper, though, David Russell argues that this choice was the wrong one for a number of reasons. He points out that, if we moved Earth to Pluto's orbit, it would no longer be classified as a planet (for essentially the same reason as Pluto, the presence of Neptune). That raises immediate questions about the definition chosen. He also notes the problem of nomenclature compared to other astrophysical objects. Pluto is a "dwarf planet", but not a planet; in contrast,
A dwarf galaxy is still a galaxy. A dwarf star is still a star.A related problem lies in the choice of the word dwarf:
The word "dwarf" also implies an object of notably smaller size. Pluto is 5.5x smaller in radius than the Earth, but the Earth is 11x smaller in radius than Jupiter.
For these and other reasons, Russell advocates abandoning the last criteria from the list of three instead; instead, whether or not that is satisfied should help inform what type of planet we are dealing with. We end up with four classes of known planets, three of which are necessarily complete, plus a possible fifth class to be discovered in the future. There are 27 currently known planets by this choice. We have
- Terrestial Planets: Mercury, Venus, Earth and Mars.
- Cerian or Asteroid Belt Planets: Ceres, Pallas, Vesta and Hygiea.
- Jovian Planets: Jupiter, Saturn, Uranus and Neptune.
- Kuiperian Planets: Pluto, Eris and thirteen others, five of which lack names in the vernacular sense.
- Oortian Planets: A hypothetical class of planet-like bodies in the Oort cloud.
As I said at the beginning, I'm not an astronomer. So I can't really judge if this idea is likely to gain any traction. And of course, by making Pluto a planet again we are forced to substantially increase the number of planets total. But this idea seems to make a certain amount of sense to me on its own merits.
The biggest problem with this proposal is the likely media response. I can envision stories about "Scientists change their minds", and you can be sure that the creationist and climate-denialists will seize on this as justification for their actions. Not to mention the Facebook group, "When I Was Your Age, Pluto Wasn't a Planet".