What’s in a name? Pluto

Let’s turn back to 1801, when Italian astronomer Guiseppe Piazzi discovered an object between Mars and Jupiter. The object was thought to be the “missing planet” as predicted by the Titius Bode Law 2

, orbiting at 2.76 AU. 

Piazzi called his new object Ceres Ferdinandea, the 8th planet. However, the subsequent discovery of other “small planets” by German Heinrich Olbers immediately cast doubts about Piazzi’s planet, which became known simply as Ceres. By 1849, 10 such objects were known, and the 100th minor planet or asteroid was announced in 1868. By 1923, 1000asteroids were catalogued, and today the number exceeds 12,000.

With the discovery of the first four minor planets - Ceres, Pallas, Juno and Vesta - in the early 1800s, it was obvious that these objects formed a new class of objects in the Solar System. They all shared very similar orbits, were eccentric, had large inclinations and were all small. As a group, we now call small objects orbiting between Mars and Jupiter asteroids or minor planets.

We also know now that there are several different types (or groups) of asteroids:

• Near Earth Asteroids (of which there are 3 subclasses);
• Main Belt Asteroids;
• the Trojans; and
• the Centaurs.


And that’s just their dynamical categories. Asteroids are also classified by their albedo and spectral
characteristics.


So what’s with all these names? Are we just going to classify objects and subdivide groups of objects ad infinitum? Having suitable names for the various classes of objects is useful for people working in the field so that they have a common language when discussing these different objects. It’s also important in helping us understand their origin.

Clearly the Solar System is a complex object, and so if we can break it up into smaller parts and try to understand each individual component, we can then go back and try to build a more complete model of the  Solar System. So we start by subdividing groups of objects and learning about each group, and then try to see how each group is related.

It often turns out that an individual object can fall into two classes, and this is where things start to get messy...


The current classification debate in planetary science is about the exact nature of the KBOs or TNOs 3.

They’re listed by the IAU’s Minor Planets Centre 4 as minor planets (so that means asteroids). They actually fall into two groups: the TNOs (1010 listed) and the Centaurs/scattered disk objects (173 listed). But this, remember, is a dynamical listing rather than a compositional categorisation. 3 See how important names can be! For now, “KBO” and “TNO” are used interchangeably. 4 To visit the Minor Planet Centre on the web

It is difficult to tell the difference between Centaurs and scattered disk KBOs. For example, 1995 SN55 is a Centaur and yet its eccentric orbit carries it past the orbit of Neptune. It could just as easily be classified as a scattered disk KBO.

Is it OK for one object to have two classifications? Taking this idea one step further, is it also OK for Pluto to have two classifications?

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The new Solar System

Our knowledge of the Solar System and planets in general has changed dramatically over recent years. The field of planetary science is currently going through a revolution!

You might think of the Solar System as 4 terrestrial planets, 4 Jovians, Pluto, dozens of satellites, thousands of asteroids, and possibly millions of comets. But lately things have been getting a bit confusing. For example, planetary scientists will tell you there are three clear groups of planets: the terrestrial, gas giants (Jupiter and Saturn), and ice giants (Uranus and Neptune) – though ice giants are a sub-class of gas giants. Some asteroids have comet-like coma (like 2060 Chiron) and as we’ve seen hundreds of new objects are being found orbiting the Sun out past Neptune.

Added to this, over 350 extrasolar planets have been found, none of which look much at all like our Solar System. Many stars seem to host “hot Jupiters” – Jupiter mass planets on extremely short periods (ten times closer to their host star than Mercury is to the Sun), and most planets seem to be on eccentric orbits.

And if you thought all this wasn’t bad enough, free floating “planets” have been found in the Orion star forming region – surely a planet has to at least orbit a star? So with all this new information, there are strong arguments for defining exactly what a planet is. Let’s have a look at the debate about defining planets in more detail.

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