Why isn't Pluto a Planet Anymore?

What is a "Planet" Anyway?

Don't scoff, it's a good question.  I have heard otherwise rational and reasonable adults deriding the 2006 decision of astronomers to demote Pluto to "dwarf planet."  Why the demotion, and why the stubbornness of many people- even some scientists- to accept that Pluto doesn't deserve the honorific title of Planet?

Let's go back to the astronomical heyday of the mid-1800s.  Then, everyone had a common and common-sense understanding what a planet was.  It was a large and spherical non-stellar body that orbited a host star in a regular ellipsoidal orbit (only because no perfectly circular orbit has yet been discovered).  But then Pluto was discovered by meticulous astronomical observation in 1930, and it was almost instantly blessed with the honor of planet-hood.  Unfortunately this was probably due more to hype and some miscalculations than any serious categorical consideration.  Upon its discovery, Pluto was erroneously supposed to be larger than Earth.  Being also the first such body to be discovered since Neptune's official discovery in 1846 (unofficially discovered by Gallileo way back in 1612), Pluto made a big splash in the scientific and- perhaps of greater historical significance- the layman communities.  Everyone knew it was a planet.  But this caused the first ripples of uncertainty concerning the meaning of "planet" in the minds of discerning astronomers.

Look at the graphics below.  Notice that Pluto (red) describes a single eccentric orbit in the time it takes Uranus (large bluish planet) to orbit three times.  Even these simple graphics should rub you the wrong way enough to wonder whether Pluto really "fits" in with the other planets.

                Inclined overview                                                          Sidereal view                    

                    Graphic source                                                         Graphic source

It wasn't until around 2005 that a really clear and unambiguous definition of "planet" was established.  Until the 1990s we seem to have had a liberal view of who deserved such a title.  Then in 1992 we discovered exoplanets, large objects orbiting stars other than our sun.  In the twenty-first century we discovered about a dozen dwarf planets orbiting at distances and inclinations similar to Pluto, and usually much more extreme.  Eris, discovered in 2005, is about 25% larger than Pluto; some even make incursions approaching the sun at smaller distances than Pluto, as the dwarf planet Orcus does.

So now we are left with a certain intellectual tension.  All these little dwarf planets, Eris and Haumea and Sedna- along with others- clearly belong to the same category as Pluto.  They are tiny, icy, impossibly distant comet-like bodies orbiting in highly elliptical (squashed) orbits at extreme orbital inclinations (tilts).  Some of them probably arrived in our solar neighborhood by being flung from interstellar space by wandering dwarf stars or even black holes.  They take between 100 and 12,000 years to orbit our sun.

So if these cosmic critters belong to the same category as Pluto, and Pluto is a planet, then these Pluto-like dwarfs are also planets.  To be honest, then, we would have to teach our youngsters a new planetary scheme that would look something like this:

Sun

Mercury

                                                     Venus         (Rocky Inner Planets)

Earth

Mars

Jupiter

                                                Saturn      (Gaseous Outer Planets)

Uranus

Neptune
Orcus

Pluto

                                                                    Eris                       (Dwarf Planets)

Haumea

Makemake

Varuna

Quaoar

Pallas

Ceres

Vesta

Ixion

2007 OR10

2002 TC 302

Hygiea

Sedna

(More coming soon...)

Can you imagine having to know 20 or more planets, most of which are so small and dim that not even a powerful private telescope can spot them?  These outer dwarf planets were primarily discovered with space telescopes anyway.  Can you see the arbitrariness of calling Pluto a planet?  Or at least see that to call Pluto a planet, we would have to do the same for a whole new system of tiny icy balls making up distant, cold regions of space sexily named the Kuiper Belt and the Oort Cloud?

Now we can see what would be a more reasonable definition for "planet" than simply any big thing that orbits our sun.  We are led to accept the following: A planet is a cosmic body that is large enough to attain sphericity by its gravity, sweeps its orbit clean of any other smaller bodies, keeps a relatively non-eccentric elliptical orbit around its host star, whose orbital tilt is similar to that of its solar neighbors.  So we have to be content with only eight planets.

It was thus reasoned by the astronomical community in 2006 that Pluto should be stripped of its designation as "planet" and bestowed a new one called "dwarf planet."  Now dwarf planets are really a whole category of cosmic oddballs drifting in the dark creepy neighborhood beyond Neptune.  They are called "trans-Neptian objects," or TNOs.  Let's look at a few of the most interesting.

Trans-Neptunian Objects

As of right now, there are more than 10 accepted TNOs, along with some comets and asteroids that frequent the same places but are too small even to be regarded as "dwarfs."  Shown below are most of them.

Image source

Most of these TNOs have had their orbits and sizes worked out with some precision.  Notice that Eris is noticeably larger than Pluto, but that many of the dwarfs, such as Varuna, Ixion, and Orcus, are even smaller than Pluto's own moon, Charon.  Notice also that Haumea is squashed; this is thought to be due to its rapid rotation.  The semi-circles around Eris and Makemake are margins of error because their sizes are not exactly known yet.  And we don't even have a proper name for poor 2002 TC302.

Here's rather a more artistic rendition of some of these planets, along with the as-of-yet unmentioned 2007 OR10.  Did you know Pluto has five moons?

Image source

The Strangest Orbit

Don't forget that size and distance are not the only qualifiers for being called a TNO or "dwarf planet."  Most of these bodies also have extremely elliptical and inclined orbits, much like many comets.  Let's look below at some notable ones.

Look at the orbits of Pluto (red) and Haumea (yellow).  Pluto has quite a tilt, but Haumea looks almost ridiculous in its inclination and eccentricity.  Its orbit is also nearly twice the diameter of Neptune's (grey.)

Then look at the orbit of Orcus (blue and teal), again compared to Pluto and Neptune.  It has about the same tilt and eccentricity of Heumea, but it is actually tilted the other way.

Eris (blue and teal) gets stranger still.  Seen to the left, its orbit just barely shaves by Neptune's and even intersect's Pluto's when seen from above, and describes a wide ellipse when viewed along the ecliptic (from the side).  As seen in the right images, its orbit completely envelopes that of all the inner planets.

An artist's eerie conception of a visit to Eris can be found here.

Lastly, Sedna's orbit (red) is the most absurdly eccentric of all the dwarf planets.  The tiny Russian doll of colors at the top right describes the orbits of the inner planets, and also the orbit of Pluto (purple).

Sedna is so outstandingly aloof that its orbital designation is officially "Detached," meaning that it can't quite be said to be faithfully orbiting the sun at all.  If another star were to move by our solar system, it might snatch Sedna away as its own little interstellar treasure.

Seen at left are the orbits of a few of the eight planets (orange, yellow, green, and purple), the orbit of Pluto (greyish), the orbit of Eris (red), and the orbit of Sedna (light blue).

This gives you one of the finest perspectives of the extreme bizarreness of the dwarf planets' orbits.

Graphic source

Don't Complain

If you insist that Pluto is a planet, then you must also accept the other dozen-or-so Pluto-like planets, even far-off Sedna and lonely 2002 TC302.  Of course you do not wish to do this.  No one does.  So don't.  Accept that Pluto, like all the other icy dwarfs beyond Neptune, cannot be designated as a planet and therefore needs to be called something else.  We call it a "dwarf planet," which only adds diversity and complexity to our understanding of the universe.  We are grownups and should accept that not everything- not even the tiny dwarf planets- will fit into our tiny, neat box of biases.