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The Pluto Scorecard

Expensive Hardware Lobbing fans, we take on a new opponent in 2006: Pluto.

Pluto was predicted to exist in 1905 when it was discovered that Neptune did not follow its computed orbit. Percival Lowell, in Flagstaff, AZ, began making mathematical predictions of Pluto's position based on perturbations of the orbit of Uranus which was known more accurately at the time. A two year photographic search was begun in 1915 to discover the object, but to no avail. Unfortunately, in 1916 November, Percival suffered a stroke and died at age 61. The search for Pluto was put on hold until 1927.

Clyde Tombaugh joined the staff at Flagstaff in 1929 January with the task of finding Pluto (then called "Planet X"). Using a "blink comparator" to view portions of the sky separated by a span of time, he finally discovered the object we call "Pluto" on 1930 Feb 18. For the next 76 years all the data we've gathered on Pluto and its moon Charon have been from telescopic observations.

But, that's about to change....

Score Launch Date/Time (UTC) Name Player Details
??:?? 2006 Jan 17 19:00:00 New Horizons Pluto Kuiper Belt Flyby USA The first match between Pluto and Earth will begin on 2006 Jan and is scheduled to last until 2015 July 14, when a flyby of the object is planned. Following the flyby, New Horizons will head deeper into the Kuiper Belt to investigate other objects.


End of Scorecard, Commence Pontification

A Side Note: What's in a name?

Pluto lies at a mean distance of about 40 AU from the Sun. Just in case you're not famillar with the finer points of the game, that would be 40 times the distance between the Earth and the Sun.

Pluto's not Alone

Twenty-one years after the discovery of Pluto, an astronomer named Gerard Kuiper (KAI-per) predicted the existence of a belt of debris left over from the formation of the solar system. This belt should lie just beyond the orbit of Neptune. In the 1980's, computer1 simulations of the formation of the solar system made similar predictions.

In 1992, sixty-two years after the discovery of Pluto, the object 1992QB1 was discovered at a distance of 44 AU from the Sun. After that, a number of similar objects have been found. Below is a (very) short list of these Kuiper Belt objects. A much more complete list can be found here off of The Kuiper Belt Page.

Year Discovered Name Mean Distance (AU)
19921992QB144
1998Chaos46
1999Rhadamanthus42
1999Deucalion45
2000Huya40
2001Ixion40
2002Quaoar44
2004Orcus39

An Interesting Analog

So, once this pattern was discovered, the obvious question was ``is Pluto a planet?'' You may recall this argument of a decade or so ago.

Before jumping off your chair and yelling ``Of course its a planet!" at my webpage, switch to decaf and think on this:

On New Year's Day 1801, Guiseppe Piazzi discovered what he thought was a new comet. Once its orbit was determined, it was clear that it was not a comet, but orbited the Sun just like a planet. Piazzi named it Ceres and it lies a distance of 2.8 AU from the Sun.

In rapid2 sucession, three more bodies were found orbiting the Sun near that same distance: Pallas, Juno, and Vesta. At this point, you should realize that we're not talking about planets---but the group of objects called Asteroids. Below is a listing of selected asteroids. A slightly more comprehensive listing with a lot more detail can be found here.

Year Discovered Name Mean Distance (AU)
1801Ceres2.8
1802Pallas2.8
1804Juno2.7
1807Vesta2.4
1849Hygiea3.1
1851Eunomia2.6
1852Kalliope2.9
1858Europa3.1
1884Ida2.9

A century and a half later, it looks like we've repeated the process a bit further out in the solar system. The evidence seems pretty clear that Pluto is one of the closer and larger Kuiper Belt Objects, just like Ceres was determined to be a member of the Asteroids.

Comparison to the Terrestrial and Jovian Planets

Does this mean we should stop calling Pluto a ``planet?'' Well, maybe we should compare it to the other ``planets:"

PlanetDiameter (km) Inc. to Ecliptic (deg)Eccentricity
Mercury4,8807.000.21
Venus12,1003.390.01
Earth12,7300.000.02
Mars6,7701.850.09
Jupiter138,3501.300.05
Saturn114,6302.490.06
Uranus50,5300.770.05
Neptune49,1051.770.01
Pluto2,39017.160.25

First, Pluto is pretty small. Our own Moon (3475 km) is larger than Pluto. Isaac Asimov was of the opinion that a new term, "mesoplanet," be coined for objects, like Pluto, between the size of Ceres and Mercury.

Second, Pluto has the highest angle of inclination to the ecliptic and it has the most eccentric orbit of the lot (although Mercury pulls a close second).

Both of these data points plus the fact that a grouping of smaller objects has been found around Pluto points pretty strongly to the fact that Pluto is more like the Asteroids than like the Terrestrial or Jovian planets. Were it composed of rock---not ice---and not orbiting out at 40 AU, it would be a shoo-in.

Playing the Moon Card

Now, if there's one thing that's sure to keep Pluto firmly in the "planet" category, it should be the fact that it has its own moon: Charon. Only true card-carrying ``planets" can have moons, right?

Apparently not.

One of the first asteroids to be found with its own moon was Ida. Recall that Ida was discovered way back in 1884. Well, in 1993, Galileo3 took images of this asteroid and, six months later, the data were analyzed to show that, lo and behold, it had a moon, Dactyl, orbiting it. Since then, over sixty other asteroids have been found with natural satellites.

We all learned in school that Pluto is a planet, so we're loathe to change that designation. But we also didn't learn that we had two belts of minor planets.

If the solar system lost a planet but gained an entire Belt of Kuipers would that be so bad?

From a Position of Authority

Even professional astronomers are confused. For some reason, we all feel sorry for a large ball of ice orbiting almost 6 light-hours from the Sun. On 1999 February 3, the IAU released this notice (local copy) stating officially that Pluto was to remain a planet.

Well, on 2006 Aug 24, the IAU came up with a new definition for a planet (local copy):

Pluto is now officially designated as a "dwarf" planet due to the fact that it crosses Neptune's orbit. It is not alone, however. To the "dwarf planet" class we may add at least Ceres, the largest of the asteroids.

An Old Proposal

The problem with the IAU definition is that it is too unwieldly. It does not define how strongly an object can be gravitationally bound to another object before it is deemed a "satellite." It does not define how large a "neighborhood" must be cleared and of what size objects.

It is a shame that they did not follow the biologists' method of taxonomy which was devised by Carolus Linnaeus in 1758. Under his method, objects are classified into large groups such that those groups may be subdivided. The largest grouping is "Kingdom" and the smallest is "Species."

We could do something similar for astronomical objects. We might call all objects which primarily orbit the Sun (or any other star) a "planet." Then we can subdivide planets into classes based on our own Solar system. For example: Mercury, Venus, Earth, and Mars are all rocky objects large enough to be oblate sphereoids. We place them in the class "Terrestrial." The "asteroid" class contains those rocky chunks orbiting between Mars and Jupiter. Jupiter, Saturn, Uranus, and Neptune are all giant gas worlds and are in the class "Jovian." Pluto, Sedna, Xena, etc. are all small ice worlds from which it is believed many short-period comets originate. These objects are in the class "Kuiper." Finally, the long-period comets come from a group of objects stretching out almost a light-year from the Sun. They are part of the class "Oort."

In this fashion, Pluto can remain in the "Phylum" Planet---but it's a Kuiper class planet, not a Terrestrial class planet such as the Earth or an Asteroid class planet such as Ceres.


1Probably not a bank of TRS-80s or TI-99s
2Astronomically speaking.
3The satellite, not the Italian physicist and astronomer.