Why Pluto is NOT a planet

First of all, I'm just going straight out and telling you. PLUTO IS NOT A PLANET. The International Astronomical Union made that clarification in 2006 and nothing will change that. I'm going to give you the reasons why I think Pluto is not a planet and why the IAU made the correction determination.

Secondly, when I was teaching college astronomy at the University of Pittsburgh, one of the things I also taught my students was that Pluto was not a planet. I last taught in 2005, a full year before the IAU made the announcement. Everything that I will talk about in here were the reasons I gave as to why Pluto is not a planet. Demoting Pluto did not diminish what Clyde Tombaugh accomplished in 1930 when he found Pluto at Lowell Observatory. There will be more of the discovery of Pluto in a later post.

One thing that argues against Pluto being a planet is its inclination to the equator of the Sun. In general, a planet should have a low orbital inclination as the Sun and planets were formed in the same nebula. As the nebula rotates and shrinks, all larger objects should stay in the same general plane. Here are the inclinations of the eight planets, Ceres, and Pluto

• Mercury 3.38°
• Venus 3.86°
• Earth 7.155°
• Mars 5.65°
• Ceres 17.75°
• Jupiter 6.09°
• Saturn 5.51°
• Uranus 6.48°
• Neptune 6.43°
• Pluto 11.88°

As you can see, Ceres (the largest asteroid or a dwarf planet in the Asteroid Belt) and Pluto both have orbital inclinations to the solar equator of more than 10 degrees. Earth has the largest of all the planets, but is inclined three degrees shallower. It would make sense since all the planets formed at the same time as the Sun and are the most massive bodies in the solar system after the Sun, that they would orbit within the equatorial plane of the Sun and not deviate much within that plane.

Another reason why Pluto is not a planet is it has a highly eccentric orbit. Objects that form in the same cloud as a star should be in a relatively circular orbit. (There really is no such thing as a perfect circle in science or nature. Variations in conditions can distort objects to make them less than perfect.) The nebula rotated which caused the cloud to collapse into a disk-like shape with the protosun at the center. Therefore, anything forming in that cloud will have a nearly circular orbit. Let's look at the different eccentricities of the same nine objects.

• Mercury 0.206
• Venus 0.007
• Earth 0.017
• Mars 0.093
• Ceres 0.076
• Jupiter 0.049
• Saturn 0.056
• Uranus 0.047
• Neptune 0.009
• Pluto 0.249

Looking at these eccentricities, all of them except Mercury and Pluto have eccentricities less than 0.1. Mercury's orbit is eccentric because of its proximity to the Sun and relativistic effects of the space that Mercury orbits in. Pluto is so much farther out, that relativistic effects do not affect it as much. Curvature of spacetime is a consequence of massive bodies that won't be explained here. You can always google the topic, or if you would like me to explain general relativity, comment below.

The density of Pluto is also a dead giveaway that Pluto is not a planet. 

Terrestrial Planets:

• Mercury 5.427 g/cm³
• Venus 5.243 g/cm³
• Earth 5.514 g/cm³
• Mars 3.934 g/cm³

Jovian Planets:

• Jupiter 1.326 g/cm³
• Saturn 0.687 g/cm³
• Uranus 1.27 g/cm³
• Neptune 1.638 g/cm³

Dwarf Planets:

• Ceres 2.077 g/cm³
• Pluto 2.03 g/cm³
• Eris 2.52 g/cm³ (estimate)
• Haumea 2.6 g/cm³ (estimate)
• Makemake 2.3 g/cm³ (estimate)

Pluto's density is too low to be terrestrial, but too high to be Jovian. Based on its density, we know that Pluto is a combination of both icy material and rocky material, with slightly more ice than rock.

The last argument I can make about Pluto not being a planet is its size relative to multiple moons in our Solar System.

 

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

 

If we were to include Ceres on this image, it would be smaller than Orcus (~800 km for Orcus and ~500km for Ceres).

 

The IAU has given a definition for a planet and a dwarf planet. 

• A planet is a spherical body that orbits the Sun and has cleared its orbit of other objects, i.e. it does not share an orbit with other bodies (not including moons).
• A dwarf planet is a spherical body that orbits the Sun but has not cleared its orbit of other objects. They may co-orbit with other bodies. Many of the Trans-Neptunian Objects, Kuiper Belt Bodies, Oort Cloud comets may have the same semi-major axis as other objects, therefore are not planets.