Is Pluto a Planet?

In 2006, a collection of scientists known as the International Astronomical Union produced a scientific definition for a planet. This definition came with certain criteria, which Pluto failed to meet, and so Pluto was not a planet anymore.

Recently, a different group of scientists, deciding that the IAU definition was incorrect, and spurred on by public outcry over Pluto's "demotion", published an article with a different definition for planets, which now not only includes Pluto, but 109 other objects in our solar system, including our Moon!

So, which is it? Is Pluto a planet, or not? What makes a planet? Why should we care?

Before we answer these questions, let's take a look at the history of our understanding of the universe. This sounds like a grand concept, spanning millennia of human history, but we can actually narrow it down to the last 100 years.

While cultures the world over have been creating stories and theories explaining the cosmos, the last 400 years have been far better at creating and explaining models of our solar system and universe. Copernicus' heliocentric model of the universe, in the 16th century, put the planets in perspective, orbiting the Sun. While many planets had been discovered centuries ago, Pluto wasn't discovered until 1930. In recent decades, we have begun observing other solar systems, exoplanets and their moons, and expanding our understanding of planets and celestial bodies, but I'll leave that for another article.

So, what's the problem? Why is the definition going back and forth? Well, the first problem is that Pluto is unique, and does not act like Jupiter, Earth, or other planets. 

Why was Pluto's status changed?
The 2006 IAU definition stated that, in order to be a planet, a body had to do meet three criteria:

1. Be in orbit around a star,
2. Have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (be massive enough that its gravity pulls it together into a round shape),
3. Has cleared the neighbourhood around its orbit

Since Pluto crosses Neptune's orbit, it wasn't the dominant part of its orbit, and since it failed that last criterion, Pluto failed the test. It was "demoted", no longer considered a planet.

The more recent definition finds problems with the IAU criteria, namely Pluto's "demotion" and the fact that many other planets do not exactly fit the definition, so their new definition is as follows:

• A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

...which basically amounts to "round objects in space that are smaller than stars".

Under this definition, Pluto would be back, bringing 109 other objects under the status of "planet", including our Moon. 

I am not in favour of either definition.

The IAU definition is flawed, and even if it weren't flawed within our own solar system, it would very likely not work as a definition for all planets in all solar systems.

Within our own solar system, planets such as Earth, and even Jupiter, have not satisfied the final criterion, of clearing their orbits. There are small asteroids, known as trojans, which move along the orbit of many planets, and would cause these "planets" to fail the IAU test/definition.

Outside our solar system, we are discovering more objects everyday, and should any of them form in different ways than our own, it may be tough to classify them as planets, under the definition. One concept in particular, rogue planets, objects which have no star and fly alone through space, may also fail this definition. If Earth, for example, had all the same physical properties as today, but was just flying through space, alone, it would not classify as a planet. Of course, in that case, there would likely be no scientists on Earth to (dis)agree with any definition of anything.

The recent definition has merit, especially for planetary scientists. Those in the field, studying these objects, like the definition because it focuses on the intrinsic properties of objects, not the extrinsic properties. Rather than focus on how the body moves through space, which is subject to great change, the definition focuses on the properties of the object itself.

This makes sense. If a large asteroid were to settle into the same orbit as Mars, would Mars then not be considered a planet anymore?

Moreover, planetary scientists also study the Moon, and other moons, so again, this definition makes sense.

However, I feel the semantics are important, and the definition is not complete.

First, let me just say that I do not think emotional opinion is a very good basis for scientific nomenclature or change. When Pluto's status was changed, people were very upset. There were protest signs, bumper stickers, people acted as if they had lost a friend. They called the change a "demotion", and lamented it. I think this response is inappropriate and I am glad that the IAU did not change simply to meet this opinion.

Pluto's change was not a demotion, because while the general public may only have enough interest to remember the planets in the solar system, other bodies are equally, if not more, fascinating, and deserve our attention. For example, I have studied the planets, and I can think of more interesting facts about Pluto than about Neptune, planetary status or not. This is not to say that "interest" is the only drive and reason for study, but I do not think planets should be considered more special than other objects, just different.

The recent article stated that "many members of the public, in our experience, assume that alleged “non-planets” cease to be interesting enough to warrant scientific exploration...a common question we receive is, “Why did you send New Horizons to Pluto if it’s not a planet anymore?”"

If this were the only reason for their proposed definition, I would have dismissed this article entirely. Why send New Horizons to Pluto? How about before New Horizons snapped that beautiful picture of the celestial object with the biggest heart, all we had to rely on were a few pixels? 

That was all we could see, and we weren't able to study Pluto at all. How about the fact that we have discovered, and continue to discover, fascinating geological properties like ice volcanoes?

The correct response to these members of the public is not to give them a title to focus on, but rather to educate them on both the interesting places in our universe, and the need to study them. People should know that these missions are important, and that even the smallest, oddest-shaped, asteroid could unlock clues related to how celestial objects are formed, how solar systems develop, and even how life on Earth evolved.

Next, my problem with semantics. I do not like that the Moon would be considered a planet. I believe that moons and planets are different and should fall under different definitions. However, if they did...would planetary scientists still study them?

However, belief should not guide me, but rather logical reasoning. The trouble with these definitions is that there will likely always be exceptions. 

For example, assuming that Pluto had a more standard orbit around the Sun, but also assuming that Charon (one of Pluto's moons) had Pluto's size and mass and that the two orbited each other as they waltzed around the Sun, would Pluto and/or Charon be considered planets? By the 1st definition, no, by the second definition yes, but then, there's no distinction between that system and the Earth-Moon dance.

All celestial objects are important, but I think the 2nd definition removes the "function" of planets. In all cases I can currently think of, except rogue planets, a consequence of a planet's existence is the clearing of orbits around a star. The clearance doesn't have to be, and often isn't, complete, but it is a function, both of the planet's formation, and of its continued existence. In this way, I view planet's as unique in that they clear orbits, just as I view moons as unique for orbiting a planet, and asteroids are unique in that they do not fit the above and often move throughout a solar system. These definitions serve to not only define the object, but understand their typical role in space.

Of course, these simple ideas break down if things move. If our Moon leaves Earth's orbit, is it an asteroid? 

Personally, I think the 2nd definition would work a lot better if it added the IAU's 1st criterion, and added, "and whose orbiting barycenter does not lie within any non-stellar celestial body."

Simply put, the barycenter is the centre of gravity of orbiting objects. While the Moon orbits the Earth, it does not orbit Earth's centre, but rather a point off-centre. In this way, both the Earth and the Moon orbit their shared barycenter, but since the Earth is so much more massive than the Moon, the barycenter is located inside Earth. For clarity, please have a look at the embedded link above for animations explaining barycenters.

So, these additions would come down to simply saying, "round objects in space, smaller than, and orbiting, a star." A little more complicated, but a little better, because it doesn't group everything together, but recognizes celestial objects as different.

This definition would mean that Earth and Mars are planets, the Moon is a moon unless it left Earth's orbit, Mars' moons would still be moons even though they are much smaller than asteroids, and Pluto...well, Pluto would be a planet, waltzing with Charon while Neptune occasionally chaperoned. 

What do you think? What definition makes sense for you, and why? I am sure I've stirred up some emotions here, we all love Pluto, but let's try to keep this positive.

Thank you very much for reading!