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My Pluto Page

Yes, that's Pluto, brother of Jupiter and Neptune, Roman god of the dead. Quit thinking of Mickey's dog.

There has been a lot of discussion lately about Pluto's status as a planet. Is Pluto a planet? It depends on who you ask. Before you read my answer, I suggest you read what Marc Buie has to say.

Now, go to my rebuttal.

Pluto is a Roman god and what else?

Notice that Marc Buie's argument basically gave you three choices, asteroid, comet or planet. There are two types of planets, jovian and terrestrial. Following Marc Buie's method, I ask which type of Planet is Pluto, jovian or terrestrial? I know he proposed a third type, but he also said not to re-write our text books.

Marc Buie is right when he says that the International Astronomical Union has called Pluto a planet and only they have the authority to change its' status. This was decided in the 1930's when very little was known about Pluto. It was thought that Pluto was nearly as massive as Earth. We now know that Pluto's mass is about 1/400 that of Earth. Would they still have classified it a planet if they had known this 60 years ago?

The problem is not with Pluto. It is with the word, planet. It is poorly defined. Look it up. I find, 1a: 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): one of the bodies except a comet, meteor, or satellite that revolves around the sun in the solar system (2): a similar body associated with another star c: EARTH --usu. used with the 2: a celestial body held to influence the fate of human beings 3: a person or thing of great importance: LUMINARY. Then there is a table listing nine planets. The table does not include the sun or moon. It does include Earth and Pluto.

Definition 1a says Pluto is not a planet and definition 1b says it is. You'll have to ask your fortune teller if definition 2 includes Pluto. Pluto is certainly covered by definition 3, as is Clyde Tombaugh.

It is time to remind ourselves that planet means wanderer, because they wander around the sky following complex paths. This means that Pluto, and all the asteroids, are planets and those other bodies discovered around distant stars are not.

All this tells me that we need new words to describe these things. I propose that we re-write our textbooks and dictionaries, which is done constantly anyway. We should drop the word planet completely. Instead of saying terrestrial planets, we will say terrestrials. Instead of jovian planets, we will says jovians. For Pluto and other things like Pluto we might say plutonoids. Marc Buie wanted a catchy word and I'm offering one.

Naming a new class of object after Clyde Tombaugh's discovery is a more fitting tribute to his discovery of an entirely different type of object. This would elevate the status of Pluto to that of prototype. Earth is the largest terrestrial. Jupiter is the largest jovian. And, Pluto is the largest plutonoid, as well as the prototype. There are others in our solar system that may be classified as plutonoids. There are 1992 QB1, 1993 FW, 1993 RO, 1993 RP, 5145 Pholus and a few others.

Go back to Lloyd's Home Page.

This page last updated January 16, 2001 by Lloyd Johnson.

Is Pluto a terrestrial planet?

Pluto is certainly not a terrestrial planet. Its' size is comparable to terrestrial planets, about half Mercury's diameter, and it has a solid surface, but that's not enough. Consider this data table.
Mercury Venus Earth Mars Pluto
distance from sun 0.3871 AU 0.7233 AU 1 AU 1.5237 AU 39.5294 AU
mass 0.0553 Earth mass 0.8150 Earth mass 1 Earth mass 0.1074 Earth mass 0.002461 Earth mass
density 5.43 g/cm3 5.24 g/cm3 5.515 g/cm3 3.94 g/cm3 2.13 g/cm3

Notice that Pluto is completely different in terms of mass, density and distance from the sun. The four terrestrial planets have rocky surfaces. Pluto's surface is icy with some dirt and rocks mixed in. Pluto is colder than liquid nitrogen. Pluto gets so cold that its' methane atmosphere freezes out and is deposited as a frost on the surface, when it is farthest from the sun.

Is Pluto a jovian planet?

Pluto is certainly not a jovian planet. Its' distance from the sun is comparable to that of jovian planets, but that's not enough. Consider this data table.
Jupiter Saturn Uranus Neptune Pluto
diameter 142,800 km 120,660 km 52,290 km 49,500 km 1,151 km
mass 317.896 Earth masses 95.185 Earth masses 14.537 Earth masses 17.151 Earth masses 0.002461 Earth mass
density 1.33 g/cm3 0.70 g/cm3 1.30 g/cm3 1.64 g/cm3 2.13 g/cm3
moons 16 23? 15 8 1

Notice that Pluto is completely different in terms of size, mass, and number of moons. It really doesn't fit in terms of density either. Pluto's density is more like that of a jovian moon. The jovian planets are massive gas giants with lots of moons. Pluto is a small body of ice and rock, very similar in size and composition to Triton, a moon of Neptune.