Pluto: A Classification by any other Name would smell as sweet

Or more precisely, Pluto is just as interesting a world no matter how it is classified.

Is Pluto still a Planet? Should I call it a Dwarf Planet or a Kuiper Belt Object? Does calling it a “dwarf planet” bother you? Does it bother your children? I think I can understand your feelings on the matter and that of your children. The question of whether Pluto is or is not deserving of the important title of “planet” still exists as a significant topic of discussion in Astronomy in the mainstream news and has for around 20 years. The “debate” made enough noise that in 2006, the International Astronomical Union decided to hold a meeting and redefine the word “planet” with a formal definition. Their definition say that to be a planet, an object must:

Orbit the Sun
Be massive enough that gravity has it form a round shape that has hydrostatic equilibrium
That it clear out the objects in its orbital neighborhood

Pluto failed the 3^rd requirement. As a result, Pluto was demoted to the less important title “dwarf planet”. Did that end the debate? Not in the mainstream. Many children and adults still argue and want Pluto to be called a planet. Since it was recently discovered that Pluto is such an interesting world with a possible undersea ocean and has mountains of ice and glaciers, there is an emerging grassroots effort to promote it back to its previous coveted classification.

This is nonsense. Few things in science could be so ridiculous as this debate. The word “planet” lost its meaning centuries ago and we should’ve left it behind back then and moved on.

A Little History

The word “planet” is traced back to the ancient Greeks, who, like everyone else at the time, believed the Earth was stationary at the center of the universe while objects in the sky revolved around it. The Greek term asters planetai meant “wandering stars” and described the tiny lights that moved across the sky differently than all the other stars when observed over weeks and months. These wandering stars, back then, were the bodies known as Mercury, Venus, Mars, Jupiter and Saturn. Our understanding of the universe didn’t change much for hundreds of years. Then the Spyglass (a.k.a. telescope) was used for observations of the night sky and things have never been the same.

In January, 1610, Galileo Galilei discovered that Jupiter had these 4 lights moving with it as it moved across the sky. They were the major Jovian moons: Io, Europa, Ganymede and Callisto. They moved across the sky with Jupiter and were apparently bound to it. Galileo discovered that Jupiter had moons of its own. What wasn’t discussed at the time is that these moons were wanderers among the stars just like Jupiter and the other planets. According to the Greek definition of “planet”, they should be called planets because they wandered differently from the other stars. It was decided to just call them moons since, like Earth’s moon, we now discovered that moons can exist around other planets. It was at this time in history that the word “planet” lost its meaning but few, if any, even noticed. We changed the definition without officially changing it. It was convenient to simply refer to these objects as moons and then re-define a planet as a spherical celestial body that orbits the Sun similar to the way the “original five” orbit the Sun. That was simple and it worked for a few more centuries. Except it didn’t.

Since then, astronomers discovered asteroids, comets, binary star systems and that stars themselves were wandering around the sky if you watched them long enough. Everything in the sky wanders. Technically, everything is a “planet” by the original definition. The word became meaningless. The word “planet” was applied to celestial objects when we wanted to apply it. If we discovered an object that we wanted to call a planet, it was a planet. Otherwise, we classified it as something else. So, we kept the classification with no good scientific definition like we have when we want to classify other objects. It was kept out of tradition because we like the word.

Uranus and Neptune were classified as planets when discovered because they were like the other major planets that orbited the Sun. When Pluto was discovered in 1930, it was just natural to call it a “planet” even though it was smaller and on a more eccentric orbit than the other large bodies. But it was circular and orbited the Sun and no one objected to calling it a “planet” so we kept the definition. Why didn’t we call asteroid Ceres a planet? That’s probably because it was small and surrounded by lots of rocky irregular asteroids in the same orbit so we made an exception.

The KBO Shakes Things Up

In 1992, the first Kuiper Belt Object was discovered and more KBOs after that and the classification of Pluto as a “planet” came into question. When the KBO Sedna was discovered in 2003, too many astronomers just couldn’t allow Pluto to keep that classification as a “planet”. For 62 years Pluto was classified as a “planet” without question. That’s probably why this is so contentious among the public. For 2-3 generations people had a simple understanding that there were 9 planets in the solar system. Children who grow up with only a passing interest in astronomy still knew that there were 9 planets and Pluto is one of them. There were coloring books and children’s placemats in diners and cartoons that educated everyone on the 9 simple planets. People were comfortable with this definition. It was one of the few astronomical trivia items that everyone knew. Also, Pluto shared a name with a popular Disney character and who wants to mess with that?

If Pluto were discovered in 1980 and other Kuiper Belt Objects like Sedna shortly afterwards, then the not calling Pluto a planet would probably not be an issue. Or if the Kuiper Belt and large objects like Sedna were discovered in the 1940s, then they all would’ve just been classified originally as KBOs and there would be no fuss. But history often doesn’t go like it should and we have to accept this and the cultural disruption it causes.

It will probably be another 2-3 generations before the Pluto’s reclassification as a “dwarf planet” is fully accepted.

The Future of Planets and other Celestial Bodies

Objects must be classified in science to reflect our understanding. As we learn more about the Universe, we must create new classifications and sometimes reclassify objects. That is what I find personally annoying about the debate and discussion around Pluto. When I hear someone complain that Pluto has been demoted, I want to educate them. Don’t they understand that we humans are constantly exploring and getting smarter? Scientists work for years to understand a few more scraps of information. The least we can do is not complain when they teach us something new. It would be better to step up and learn the hard fought knowledge. Is it so hard to teach children and adults the term “Kuiper Belt Object”? The IAU (International Astronomical Union) has much more important things to focus on than trying to hold onto a simple word like “planet” that may not have relevance in modern times even though it had an important place in history.

July, 2018 marked 3 years since the historic flyby of the KBO Pluto and its moons by the New Horizons Probe. Because of this flyby, we’ve learned that Pluto is a beautiful, dynamic and unique world with tall ice mountains, flat plains with few craters, interesting cracks, a thin atmosphere and possibly an underground ocean. Its largest companion Charon is just as interesting with its own ice mountains. As humans explore more of this region of the outer solar system, we may need to reclassify worlds further. Is it worth holding onto an archaic word like “planet” just to satisfy people who don’t want to learn about the wonders of the universe that science has to teach them?

Consider other celestial bodies and how we may need to reclassify them. For example, we classify comets and asteroids by a simple mechanism: if it has lots of frozen gases near the surface, such that the ice sublimates into gas as the object nears the Sun, then it is a comet. Otherwise, it is an asteroid. However, if the object is very small, about the size of a rock that you could hold in your hand, then it is not an asteroid, it is a meteoroid. As we’ve learned more about asteroids, we’ve classified them into different types, such as C-types (dark carbonaceous) or S-types (stony, silicaceous). Someday, we will discover much more about the composition of these objects and we may lose the term “asteroid” because it is no longer relevant. Or lose the term “comet” for some comets because of more specific criteria and composition. Will we hear an uproar from the public if Halley’s comet is one day “demoted” to being something else? I can already imagine the letters being written from 8 year-olds to Neil deGrasse Tyson of the American Museum of Natural History. They have already written to complain about Pluto no longer being called a planet. Here is my polite answer to those 8 year olds who complain about Pluto:

Thank you for your letter. We’re so glad that you have taken an interest in astronomy and exploring the universe. We understand that you want us to keep classifying Pluto as a “planet” despite the fact that we’ve learned it has properties that make it much more similar to the icy bodies that orbit beyond Neptune that we call Kuiper Belt Objects. So it needed a new classification. We must allow scientific knowledge to best dictate how objects are classified. We encourage you to expand your astronomical vocabulary and start using the proper terms that reflect the enormous scientific efforts which expanded our understanding of the Universe. If you ever become a scientist someday and make discoveries to improve human knowledge, we’re sure you will want others to do the same. We hope you understand the logic behind our decision. Keep exploring and learning!

For those adults out there who still want us to call Pluto a “planet”, I want to say: “We reclassified it. Get over yourself!”