What's Pluto?

Recently a young person asked me why Pluto isn't a planet. This seemed like a good educational opportunity. The explanation I used is much simpler than the official, scientific explanation - with its "planetary discrimants" and "aggregate masses" - and turned out better than I anticipated, so I thought I would share it with you. It seems appropriate for people with at least a fourth or fifth grade education.

The study of science results in "an organized body of knowledge gained through ... research." Observational science gathers data about the universe and classifies objects, life forms, etc. according to characteristics of those things.

Applying those concepts to our solar system, we can measure those objects and group those with similar characteristics together.

Let's try that.

One of the most obvious characteristics of solar system objects is their composition. All of these objects fall neatly into one of three categories as shown by the accompanying graph:

  1. major rocky objects, e.g. Earth, Mars - these all have densities greater than 3.8 g/cm\^3
  2. gaseous objects, e.g. Jupiter, Saturn, some or all of which have cores of rock and/or metallic hydrogen - these all have densities between 0.69 and 1.6 g/cm\^3
  3. rock-ice bodies - objects with significant amounts of rock and ice, e.g. comets, Pluto and its satellite Charon - their densities range from 1.0 to 3.0 g/cm\^3, with almost all of them in the range 1.0 to 2.0 g/cm\^3.
The clearest distinction is between the three densest inner objects (Mercury, Venus, Earth) and all but one of the outer, rocky/icy objects. Haumea, in case you haven't heard of it, is the newest object to be labeled a "dwarf planet" by the International Astronomical Union.

Although there is overlap in density between the gaseous objects and the rocky/icy objects, no overlap between their sizes (masses) exists, as this next graph shows (Earth is arbitrarily assigned a value of 1,000,000 and the rest are scaled to that value):

Visually, three groupings are discernible: the gaseous objects, the large rocky objects, and everything else. Mathematically, the groupings are separated by more than an order of magnitude. In other words, the smallest member of one group is at least ten times the mass of the largest member of the next group. Uranus is more than 14 times the mass of Earth, and Mercury is almost 20 times the mass of Eris, which is more massive than Pluto.

Besides physical characteristics, the most useful ones are the orbital elements.

All members of our solar system orbit the sun, or orbit another non-stellar object which in turn orbits the sun. These orbits are, almost entirely, described by Newtonian mechanics, the basic elements of which were first described by Johannes Kepler in 1609. Although an orbit has several characteristics, the simplest of them is the semi-major axis, which is often called the "average" distance between the orbiting body and the sun. Although not quite accurate, it's close enough for this purpose.

Here is a graph of 17 of the most important bodies in our solar system. It shows the semi-major axis of each, relative to the semi-major axis of Earth, which is called an Astronomical Unit. It includes the four major rocky objects, the four gaseous objects, all five of the currently recognized dwarf planets, three asteroids, and five other relevant objects. Note that the orbital distances of Eris (68 AU) and Sedna (526 AU) are off the scale of this graph.

Again, three groups appear in the graph: the inner rocky objects, the gaseous objects, and the outer bodies. Separation between objects increases from the inner bodies to the outer ones, but suddenly, starting with Orcus, the separation between orbital distances shrinks considerably.

From those three characteristics: density, mass, and orbital distance, it seems clear that there are at least three groups of major bodies in the solar system:

  1. inner, rocky bodies, each having a mean density greater than 3.8 g/cm\^3, a mass larger than one-tenth Earth's mass, and an orbital distance less than than 2 AU
  2. giant gaseous objects, each with a mean density less than 1.5 gm/cm\^3, a mass larger than 14 times Earth's mass, and an orbital distance between 5 and 32 AU
  3. distant icy, rocky bodies, with a mean density less than 3 gm/cm\^3 (and almost all less then 2), and an orbital distance greater than 35 AU.
Object TypeDensity
Semi-Major Axis
(AU, Earth=1)
Inner, rocky>3.8>0.1<2
Giant, gaseous<1.5>145-32
Distant, icy, rocky<2 (except one)<1/200th>35

All three of those groupings can be displayed in one graph, which shows the distinction between the three different groups at a glance:

In the graph above, the green bars show the range of values for the inner, rocky bodies, scaled so that the highest value is 100. The blue bars show the ranges of values for the gaseous bodies. The purplish bars shows the ranges of values for the outer icy, rocky bodies.

Note that only two ranges overlap: densit for the gaseous and the icy rocky bodies. For all of the other characteristics, there are clear gaps between the ranges of the groups.

Now that we have clear groupings, the question becomes "which of those groups should be planets?" I hope it's obvious that the first two categories should be included in the list of 'planets.' The third group can be included or not, depending on how you want to define the term 'planet.'

However, there are two other factors which help me to decide. Initially, 'planets' were the five wandering lights that weren't the Sun and Moon. These seven wandering lights were so important that early western cultures assigned a day of worship to each, leading eventually to the names of our days.

If 'planets' started with the five wandering stars, it makes sense to add other bodies which have similar characteristics - Uranus and Neptune - yielding a total of eight planets. But none of the others - Pluto, Haumea, Quaoar, etc. - are like the original wandering lights in the sky.

Further, if we were to include the outer, rocky, icy bodies in the list of planets, the list grows significantly. Today, the list would include 13 members, but another 40 known objects might be categorized with Eris, Pluto, et al., and another 150 or more are probably out there. If the category 'planet' can have 8 members or 200, I'll go with 8.

Finally, regarding the question "is it 'right' to 'demote' Pluto?" The list of planets has grown and shrunk several times throughout history. More than 25 bodies have been labeled 'planets' only to be 'demoted' later. Pluto is nothing special in this regard.


You could just have said "because it's not big enough"...

Posted by guest on April 23, 2009 at 02:00 PM EDT #

Yes, but what fun would that have been? :-)

Seriously, if size is the only criterion for distinction as a planet, then you must decide on a threshold. It is difficult or impossible to do that without being arbitrary. Science doesn't like arbitrariness.

Posted by Jeff Victor on April 28, 2009 at 03:22 PM EDT #

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Jeff Victor writes this blog to help you understand Oracle's Solaris and virtualization technologies.

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.


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