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.

The Unusual Inclination of Uranus

Remember way back in June when we talked about the strange inclination of Venus? Believe it or not, there is one planet that has a stranger inclination. Here, the Sun does not rise in the west and set in the east. But then, it doesn't really rise in the east or set in the west all year long. For a portion of its year, Uranus has a hemisphere completely illuminated and another hemisphere completely in the dark.

Uranus inclination is about 98°, which means that its rotational axis is nearly perpendicular to its orbital axis. Another way of saying this, is that Uranus rotates on its side. This means that for a portion of its year, the north pole of Uranus is almost pointed straight at the Sun and the northern hemisphere is almost completely sunlight while the south pole is pointed away from the Sun and therefore, the southern hemisphere is dark. Vice versa, halfway around its orbit, the south pole is pointed towards the Sun and the north pole away.

This phenomena happens on Earth, but not as extreme. There are two latitudes, one north of the equator and one south of the equator, where in the northern summer, at any location north of the northern latitude the Sun will never set and in the northern winter, the Sun will never rise. The same thing happens in the southern latitudes near Antarctica. These latitudes are called the Arctic Circle and the Antarctic Circle. If you have heard of "The Land of the Midnight Sun", this is in reference to the Arctic Circle. On Earth, these circles are at 66.5° North for the Arctic Circle and 66.5° South for the Antarctic Circle. If you recall from science class, that the Earth is tilted 23.5° with respect to the Earth's orbital plane, those two angles equal 90°. On Uranus, it is a little more complicated than that.

When the north pole of Uranus is pointed towards the Sun (or the northern "summer" on Uranus), any latitude north of 8°N will never see the Sun set and any latitude south of 8°S on Uranus will never see the Sun rise. Vice versa, in the southern "summer" on Uranus, any latitude south of 8°S will never see the Sun set and any latitude north of 8°N will never see the Sun rise.

Why does Uranus have such an extreme inclination? Much like Venus, it is believed that soon after formation, Uranus was hit by a large object that knocked it on its side, but because it has a much larger mass than Venus, it did not flip upside down, but rather just onto its side.

A Day on Venus

Venus has a unique day.  Granted, Mercury can have a day where the Sun travels from west to east in the sky, can even set again in the east only to rise again later, but this does not happen every day on Mercury.  Venus day is even weirder.

VENUS HAS A DAY LONGER THAN ITS YEAR



Yes, that is correct. It takes Venus less time to complete one revolution around the Sun than it takes to complete one full rotation on its axis.  Its orbital period is 224.7 Earth days.  Its rotational period is 243.0 Earth days.  Recall that both these are in reference to distant stars.

The orbital period, or sidereal period, of Venus is how long it takes for Venus, the Sun, and a distant star to be in the same configuration.  This is generally called its year.

The rotational period, or sidereal day, is how long it takes for a star to appear at the same longitude in the sky.  For Venus, this is 243 Earth days.  However, the solar day is how long it takes for the Sun to go from noon to noon.  For Venus, this is actually 117 Earth days.  How are these so different?

*This is my awesome artistic skills

 

As you can see, as Venus goes around the Sun, it's rotation is slow enough that it takes just over half a solar year for Venus to go from noon to noon.  However, it has just completed over half a rotation in that time.  Therefore, it takes almost another half a solar year to complete one full rotation on its axis.

 

There is another strange phenomena about the rotation of Venus.  On almost all the planets, the Sun generally rises in the east and sets in the west.  We've already discussed Mercury's strange day and Uranus day is strange as well, but Venus is the weirdest.  The Sun exclusively rises in the west and sets in the east.  We call this retrograde rotation.  If we view the solar system from above (i.e. looking down on the Earth's north pole), all the planets rotate counterclockwise (also called anticlockwise).  Venus, however, rotates clockwise.  

The way this is explained is by describing the inclination of Venus' orbit.  The Earth is tilted 23.5° with respect to its orbital axis.  Venus's inclination is 177°.  Why not 3°?  A 3° inclination would suggest that Venus rotates like all the other planets, counterclockwise (west to east).  By saying Venus has an inclination of 177°, we know that Venus rotates clockwise (east to west).  That is why the Sun rises in the west and sets in the east on Venus. 

 

 

Both the slow rotational speed of Venus and its almost 180° inclination are probably explained by the same thing: early in its creation, Venus was hit but a large planetoid body which caused it to flip upside down and considerably slowed down its rotation.

 

 

*Forgive my horrible drawings, as I am not an artist.  But I feel I should probably start using my own images instead of resorting to Google to find them.