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Is Pluto a planet?
Category: Space
Published: August 5, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
It depends on how you define the word "planet". Science is fundamentally all about numbers and equations. Words are used in science almost grudgingly in order to communicate. The reason that numbers trump words in science is that numbers can be exact and universal while words are always culture-dependent and evolving in meaning. As humans, we love to use words to classify objects into broad groups, and thereby acquire great efficiency in communication. A proper scientific answer to the question, "What is Pluto?" would be "Pluto is a spherical rocky object in orbit around our sun with a radius of 1153 kilometers and a mass of 1.3 x 1022 kilograms." But it's a lot easier to say "Pluto is a planet" or "Pluto is an asteroid" than spout a list of numbers, so scientists use words and classifications as crutches to more quickly communicate the facts. The debate surrounding Pluto's status is really a linguistic debate about the meaning of certain words and less a scientific debate about hard facts. With that said, scientists still make an effort to use the right words and define them accurately so as to minimize confusion. When it comes to giving an exact, scientific definition to every-day words, the issue is more "How can we minimize confusion?" and not so much "What are the scientific facts?" You are free to call Pluto whatever you want, if you are aware that certain names can lead to confusion.
Here are the facts:
- Pluto orbits the sun like planets, asteroids, and comets.
- Pluto is roughly spherical like planets, and unlike asteroids and comets.
- Pluto has its own moons like planets, and unlike asteroids and comets.
- Pluto's orbit around the sun is irregular like a comet or asteroid and unlike a planet.
- Pluto is similar in size, location, and orbit to many recently-discovered asteroid-like bodies beyond Neptune.
- Pluto has failed to gravitationally clear its neighborhood of other bodies. In this respect Pluto is like an asteroid and unlike a planet.
You are free to call Pluto whatever you want and these facts will not change. But calling Pluto a planet could mislead someone into thinking, 1) it has a regular orbit, 2) it is somehow different from the other asteroid-like bodies beyond Neptune, 3) it has cleared its neighborhood, and 4) it is very big. At the same time, calling Pluto an asteroid could mislead someone into thinking, 1) it is not spherical, 2) it is very small, and 3) it has no moons. Either classification leads to misunderstanding. To deal with this problem, the International Astronomical Union (IAU) created a new classification in 2006 called a "dwarf planet". Pluto and all the similar bodies beyond Neptune were put into this new category of dwarf planet in order to communicate the fact that they are between asteroids and planets in their physical characteristics. Some scientists believe calling Pluto and its neighbors "dwarf planets" only creates more confusion, and prefer to call all these objects simply "planets", thereby expanding the notion of what constitutes a planet. The bottom line is that the debate between scientists of what to call Pluto is really a language debate and not a science debate. The scientific facts surrounding Pluto are not controversial or being debated.
The debate over Pluto arose in the first place because little was known about it for several years. Without knowing any better, scientists assumed Pluto was a standard planet just like the other eight planets in our solar system. But as further scientific discoveries were made, it became clear that Pluto was different. The turning point involved the discovery of many rocky and icy bodies beyond Neptune, some of them as big as Pluto or even bigger. To be consistent, scientists had to either promote all these asteroid-like bodies to the status of "planet", or somehow designate Pluto as somewhat less than a standard planet.