Pluto is not a planet.
Hear me out, I'll explain why!
Merriam-Webster has this to say about planets.
1 a : any of the seven celestial bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars b (1) : any of the large bodies that revolve around the sun in the solar system (2) : a similar body associated with another star c : EARTH -- usually used with the
2 : a celestial body held to influence the fate of human beings
3 : a person or thing of great importance : LUMINARY
Interestingly, they don't even specify the 9 generally accepted planets! Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto - My Very Excellent Mother Just Served Us Nine Pizzas. Mercury Venus Earth Mars (Asteroids) Jupiter Saturn Uranus Neptune Pluto - Most Voters Earn Money (Always) Just Showing Up Near Polls.
Dictionaries start off with the oldest definition, which here is objects that appear to move throughout the sky, hence the Greek name of "wanderer." The ancient Greeks could observe through Saturn with the naked eye. You can too, even in a city. Even if you don't know where to look, if you occasionally see a "star" that's especially bright, see it before the stars come out, more colored than the others, and doesn't seem to twinkle as much, and doesn't move, that's probably a planet.
Then in the 1800's telescopes helped astronomers realize that there were a couple other things wandering through the sky - Uranus and Neptune. Calculations of irregularities in Mercury's briefly made people think there might be a planet inside its' orbit, named Vulcan, but a test of Einstein's general relativity circa 1913 proved (with large margins of error) that GR could do it alone. Calculations of irregularities in Neptune's orbit (by Percival Lowell) led to the discovery of Pluto (by Clyde Tombaugh), but only because the two errors in the calculations happened to cancel out and Pluto was in just the right spot.
Somewhen in there asteroids were discovered - the first of them, Ceres in 1801, was briefly considered a planet until we realized there were lots of them orbiting at roughly the same distance from the sun. Then in the 1990's, Geoff Marcy and various others started discovering planets in other solar systems (we've topped 100). From 1992 through to the present day this was topped by Mike Brown (and others') discovery of three Pluto-sized objects further out than Pluto: Sedna, Quaoar, and 2003_UB313 (aka Xena). These objects are variously called trans-Neptunian objects (things that sometimes orbit w/in Neptune's orbit, as does Pluto), Kuiper Belt Objects (aka KBOs, like another asteroid belt but outside Neptune's orbit), and some call them more planets. Do we really want to add more planets? One is larger than Pluto; if it's not a planet, then is Pluto? If there's lots of them, like another asteroid belt, shouldn't we drop Pluto like we did Ceres? But we've been calling it a planet for so long, we shouldn't drop it now.
This all begs the question:
What is a planet?
Unfortunately, astronomers haven't ever officially declared a definition of a planet, so we're left with a conundrum. There's an assortment of characteristics that seem to be generally agreed upon, but not universally.
- Orbits the sun (or star, or primary star, or pair of stars). In our solar system this includes a shit load of things. This includes asteroids, comets, planets, but not moons.
- Small enough to not be undergoing nuclear fusion. There is a class of objects known as brown dwarfs, nicknamed "failed stars," that are larger than Jupiter but too small to be stars. They typically have tiny amounts of fusion (of deuterium, for example), but are considered somewhere between stars and planets. This isn't an issue in our Solar System (Jupiter is 100x too small to be a brown dwarf), but it's important in other systems.
- Large enough to be round from self-gravitation. Moons included, but not asteroids and comets.
Only thing in its orbit.Excludes planets with moons, excludes planets that have asteroids trailing or leading in the same orbit (such as Earth and Jupiter).
Largest or dominating body in its or a similar orbit. Somewhat vague, excludes moons, but not clear when multiple bodies the same size. Some have suggested that "dominates its orbit" be clarified as "the body's mass must be greater than the masses of all objects in similar orbits." It would exclude asteroids, comets, and KBOs (such as Pluto).
The five definitions above are generally accepted, but not officially accepted. The International Astronomical Union (IAU) is the governing astronomy body that keeps track of objects, and they've had a "working group" that Wikipedia claims will announce a definition by the end of the year. I personally hope that rather than just handing it down from on high, that they allow a period of public commentary from both the astronomical community and the general public. It seems like such an interesting topic with so many differing opinions that it would be a good thing to get more general input. Additionally, if it's controversial, publicizing things in that way would help stir up more public interest in astronomy. *grin*