Astronomers are now refining their concept of “planet”, based on new discoveries. In particular, given their recently expanded context of knowledge, they are actively working on a revised definition of the term “planet” as well as creating new subcategories.

Why is this important? According to the article,

The effort has been ongoing for several years, as new sorts of objects in the solar system and beyond have rendered the traditional idea of planet useless to astronomers and confusing for the public.

The announcement in July of a world larger than Pluto orbiting our Sun out beyond Neptune brought discussions to a head. A 19-member working group within the International Astronomical Union has been scrambling ever since to reach consensus, but to no avail.

The main sticking point: If Pluto is a planet, then so is 2003 UB313, the object discovered in July. But by that logic, there are several other round objects nearly as big as Pluto that should be considered planets, some astronomers say.

The compromise currently being floated by the working group is to add an adjective in front of the term planet for each different type of non-stellar round object.

Pluto and 2003 UB313 could be called Trans-Neptunian objects. Earth would be called either a terrestrial planet or perhaps a “cisjovian” planet, meaning it’s inside Jupiter.

Further complicating the matter are extrasolar planets much more massive then Jupiter, planet-like objects orbiting dead stars called pulsars, and possibly even free-floating worlds that don’t orbit stars.

Much of the debate centers on whether some of these new subcategories based on essential vs. nonessential differences. For instance, here’s one point of debate:

Even the sorts of adjectives that might be used is not totally agreed upon. The terms Trans-Neptunian and cisjovian are based on location, not composition.

“I don’t believe we should classify planetary objects by location,” committee member Alan Stern of the Southwest Research Institute told Nature. “We should use properties of the objects as a guide.”

One problem is that we don’t (yet) have a well-established theory of planetary formation, so we’re not yet able to understand the causal factors that give rise to the physical objects under consideration. And without this understanding, the concepts astronomers create today may not be based on the most fundamental attributes of the entities, and hence will eventually require additional refinement as the relevant scientific understanding continues to advance. The properties that we can currently measure may not be the most fundamental.

(For instance, we might be in a position similar to that of primitive people who can see that birds and bats both can fly, but that turns out not to be the most fundamental property as far as biological classification.)

In any case, I find it fascinating to see the process of concept-formation in action, especially in non-trivial scientific settings such as this.