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.

Triton and the Other Moons of Neptune

Neptune has two moons that can easily be seen from Earth with the aid of a telescope. One of those moons is one of the few moons in our Solar System with an atmosphere, Triton. The other moon Nereid is smaller, but still visible in a large enough telescope. Proteus is actually larger than Nereid, but not as easily seen as it does not reflect as much light.

Triton with Neptune in the Background

Image Credit:

NASA/Voyager



 

Triton is the largest moon orbiting Neptune, though its size is only 78% that of the Moon. It is one of the few moons (and the largest of all solar system moons) to have a retrograde orbit around its planet. It is believed that some interaction early in the formation of the Neptunian system caused it to reverse its orbit around Neptune. It also may be a Kuiper Belt object, which we will discuss later. Despite having a retrograde orbit, however, it does have a very circular orbit. It is very cold and because it is cold, it has an atmosphere. Our Moon, despite being larger, does not have any atmosphere as it is much closer to the Sun than Neptune. The atmosphere is dominated by molecular nitrogen and methane, but is 10 million times less dense than the Earth's atmosphere. Because it is so cold, there are no clouds in Triton's atmosphere and is transparent, allowing us to see the surface. Triton has an icy surface of nitrogen ice, methane ice, carbon monoxide ice, and carbon dioxide ice. It was found by William Lassell in 1846, just after the discovery of Neptune itself.

Triton has few craters, which tells us that its surface is relatively young, though it does have dark spots on the surface. Those dark regions are from organic molecules, created when methane is exposed to light. And because of the methane on its surface, there are regions near its southern pole where methane geysers can erupt through cracks in the surface. Around these cracks, dark smudges are found which are created in the same way as the dark spots on Triton.

Fuzzy Image of Nereid seen from Voyager 2

Image Credit:

NASA/Voyager

Nereid is unique as it has a very eccentric orbit, which may mean that it is a captured comet, based on its composition.. However, it does orbit prograde, and despite being smaller than Proteus, was actually the second moon discovered around Neptune in 1949 by Gerard Kuiper.

Proteus

Image Credit:

NASA

 

Proteus is the second largest moon but was not discovered until Voyager 2 spotted it in 1989, which is odd considering Nereid was discovered by an Earth-based telescope

Neptune, The Other Blue Planet

Neptune with a Great Dark Spot prominent in the center

Image Credit:

NASA

Earth is not the only blue planet in our Solar System. The planet farthest from the Sun (and Pluto is still not a planet, nor should it be), is also blue. However, Neptune's color does not come from water but from the atmosphere composed mainly of methane. Neptune is also the fourth largest planet in diameter (smaller than Uranus and larger than Earth), but the third largest in mass (between Saturn and Uranus). It orbits at a semi-major axis distance of 30 AU which gives it a orbital period of 165 Earth years. Based on its eccentricity of 0.0087, it is by far the planet with the most circular orbit.

Neptune is also known as the first object to be predicted to exist before it was discovered. In the 1840s, two different astronomers predicted another planet orbited outside the orbit of Uranus and those predictions were confirmed in 1846. Since its discovery, Neptune has only completed one full orbit, way back in 2011.

Neptune also is home to one of the larger moons in the Solar System, Triton. It also has 13 confirmed moons, and may have more awaiting discovery. And just like the other gas planets, Neptune does have a ring system.

As shown in the above image, Neptune has a series of prominent storms on its surface that have persisted since their discovery in 1989 by Voyager 2. Much like Jupiter's Great Red Spot, they are a fascinating set of storms that we are still learning about.