Pluto's Planethood

The Debate

What is a planet?

*Before discussing Pluto's planethood, it is important to define what a planet is so that it is possible to make an unbiased decision about Pluto.*

A planet is massive body, made round by its own gravity, that orbits a star in an elliptical orbit. A Group called the Committee on Small Body Nomenclature was once in charge of defining the term planet. When doing so, they should have considered the characteristics (physical properties of the body itself; e.i. mass, radius, density, and composition), the circumstances (primarily questions regarding orbital properties), and the cosmogony (formation mechanisms and histories) of all of the celestial objects in question.

Terrestrial planets are known as inner planets. Terrestrial planets are Mercury, Venus, Earth and Mars. They are small, dense and rocky in composition. These planets have a small radius and have a slow rotation. They could be compared to an earth-like composition. All four of these planets lie within about 1.5 A.U of the sun and they all have relatively small masses with solid surfaces. Terrestrial worlds lie close together near the Sun, have weak magnetic fields, have only three moons among them. Finally, all four jovian worlds are thought to contain large, dense “terrestrial” cores some 10 to 15 times the mass of Earth. These cores account for an increasing fraction of each planet’s total mass as we move outward from the Sun.

On the other hand, Jovian planets are known as the outer planets. These planets are Jupiter, Saturn, Uranus and Neptune. These planets have a larger radius and faster rotation. They are large, yet they have low density. They are very gaseous and this makes them have a greater mass than terrestrial planets. The Jovian worlds are widely spaced out through the outer solar system. They all have strong magnetic fields. The Jovian worlds have many moons each, no two of them are the same and none of them like our own. Furthermore, all the Jovian planets have rings, a feature unknown on the terrestrial planets.

There might be many differences between these two planet classes, yet there are some similarities. First off both of these groups are all part of the same ancient solar nebula. Second both of these groups have a gaseous atmosphere as well as the capability of exerting gravitational force. They both have inclinations less than 7 degrees. All planets are almost spherical in shape.

Pluto Versus The World

Pluto's Characteristics

Physical Characteristics of Pluto

There is little that is known about the size of Pluto due to the distance from the sun. It has an estimated diameter less than one-fifth that of Earth which is about two-thirds as wide as Earth’s moon. Pluto’s orbit is highly eccentric and that means that its distance from the sun can vary at times. Pluto’s orbit will take within the orbit of Neptune and when Pluto is closer to the sun the surface’s ice thaws out. This causes it to temporarily form a thin atmosphere composed of mostly nitrogen and some methane. Pluto’s low gravity causes the atmosphere to extend much higher in altitude than the Earths. Most of Pluto’s atmosphere is thought to freeze and all but vanish when it travels far from the sun.

Pluto’s Orbital Characteristics

Pluto has a high elliptical orbit that can take it more than 49 times as far out from the sun as Earth. It gets closer to the sun than Neptune for 20 years out of Pluto’s 248-Earth years long orbit. This allows astronomers a rare chance of allowing them to study Pluto.

Pluto’s Composition & Structure

  • The atmosphere composition consists of methane and nitrogen.
  • Magnetic Field: remains unknown whether Pluto has one. Yet its small size and slow rotation does suggest that it has little to none.
  • Chemical composition: Probably a mixture that is 70% rock and 30% water ice.
  • Internal structure: Probably a rocky core surrounded by a mantle of water and ice, with more striking ices such as methane and nitrogen frost coating its surface.

This information suggests that Pluto doesn’t fit in either category. Terrestrial planets are small and rocky. Pluto is small and rocky. Pluto cant be a Jovian planet because 70% is rock while the other 30 suggests that its probably water ice. In terms of its composition Pluto is very different from the two because it’s made of mostly lightweight materials such as water and the size is smaller than Earth’s moon. Pluto might have a low density which is nothing compared to the Terrestrial planets, however its mass is about the same as Earth.

TNOs, KBOs, and Orbital Resonance

A Trans Neptunian Objects is any minor planet that orbits the sun in our solar system beyond Neptune. TNOs have been only studied using terrestrial or earth-orbiting observations. Pluto is the most popular trans-Neptunian object as well as the first TNO to be discovered. It is relatively easy to find because among the all of the known TNOs it has the highest apparent magnitude. Kuiper Belt Objects are any minor planets that lie outside the orbit of Neptune at the edge of the solar system.

Orbital Resonance is when two bodies are both orbiting around one-parent body in a specific pattern. Gravity often pushes objects into these resonances when the objects are of comparable sizes. However, they eject objects if one is much bigger than the other. There are resonances in our own solar system. Every time Pluto orbits twice, Neptune makes exactly three. This makes it impossible for the two bodies to ever collide despite that their orbits cross. This is the only stable resonance involving two planetary bodies. Other objects that are outside of Neptune’s orbit are in resonances with Neptune and are known as TNOs objects. Most of these are smaller than Pluto.

The Public's Choice

There was a 24-hour email opinion poll among professional and astronomers asking what Pluto's status should be. This sounds like a great idea, since there is so much controversy on Pluto's status, why not just let the public decide the fate of Pluto. I think that American scientists probably voted to demote Pluto, but that the majority of American citizens would have voted to let Pluto continue to be a planet. Although I can’t speak for other countries, I would like to believe that they too would vote to save Pluto, although other countries don't have as strong a connection to Pluto.

A reporter once interviewed the woman who conducted this poll. He asked all the questions that you would expect, except; "What would be the consequences of Pluto's declassification?" and "What about the man who discovered Pluto's widow, how does all this talk about Pluto affect her?"

What would be the consequences of Pluto's declassification? Well, short-term, there would be an unhappy widow and a substantial number of unhappy school children who have chosen Pluto as there favorite planet. Long-term, however, Pluto's declassification would impact the toy and artifact markets, scare money away from more research on Pluto, and possibly drive kids away from the field of science.

So, Is Pluto A Planet?

A planet is a celestial object, made round by its own gravity, that orbits a star. By this definition, Pluto is a planet. However, Pluto is not part of either of the two main classes of planets, Terrestrial or Jovian, instead Pluto is probably part of another class. Perhaps there are other planets with characteristics similar to little Pluto.

Work Cited


  • "Terrestrial Planets, Jovian Planets." Bright Hub. N.p., n.d. Web. 13 May 2013.


  • "Pluto, the Ninth Planet That Was a Dwarf." N.p., n.d. Web. 13 May 2013.