Language and Epistomology: The Definition of 'Planet'

The International Astronomical Union (IAU) has voted to approve a new definition for 'planet'. The new definition would recategorize, or "demote", Pluto from a planet to a "dwarf planet".

The controversy has existed for over a decade. Astronomers have identified dozens of celestial bodies that orbit the sun beyond Neptune, and the discoveries amounted to a crisis for our theory of the solar system. If Pluto, whose orbit is very strange compared to the other planets, is termed a planet, why should these other bodies not be so called?

The controversy is far from over, as many expect the outrage over the decision to result in a sort of appeals process. The vote itself was on the last day of a summit, and only 424 astronomers remained. While this redefinition means rewriting textbooks and potential outrage or disappointment from those that have grown up thinking Pluto is the farthest planet, the definition means virtually nothing to astronomical science in the larger sense.

The episode highlights an epistemic theory of mine that description and explanation are different paradigmatically. What really is in a definition? What are they used for? How do they evolve? These are some of the most basic questions of logic. Definitions are the demarcations of one thing from another. In the case at hand, the definition of a planet should distinguish it from other bodies in the solar system. Explaining the nature of the solar system, its origins, is something else entirely.

The evolution of the modern definition of a planet is intriguing. The history follows the Kuhnian model of normal science accumulating evidence until enough anomalies force a shift in paradigm. The anomalies began to appear starting in 1995 when a pair of discoveries forced the issue into the public.

On October 6, 1995 51 Peg b was discovered by Michael Mayor and Didier Queloz. The discovery was confirmed a week later. Bellerophon, as it was named, is about half the size of Jupiter and is in close enough proximity to its star that our theory of planetary formation was questioned.

Also in 1995, the existence of brown dwarf stars was confirmed. These are essentially stars too small to burn their composite gases in fusion reactions. Some brown dwarves may have once burned hydrogen and some of the larger ones may still burn deuterium and lithium. Many are also seen floating about in space.

Since the introduction of such new phenomena to the astronomical catalog, astronomers have attempted to classify hundreds of other bodies found within our own solar system. Exoplanets, brown dwarves, extra moons, permanently orbitting comets - the definition of planet was in crisis.

[Space.com, 2000] Since the first planet was discovered orbiting another star in 1995, more than 50 extrasolar planets, or exoplanets, have been found. These planets are nothing like what we're used to. They are huge -- often many times the mass of Jupiter -- and some are altogether more like another class of object, the brown dwarf.

...

"We are beginning to see this whole series of objects that we were not able to detect before, and it's completely changing our ideas of planetary formation and the mass of the objects we find," said Morris Aizenman, a senior science associate in the Mathematical and Physical Science Directorate of the National Science Foundation.

Aizenman and others say planetary and stellar sciences are undergoing a revolution. The definition of a planet, meanwhile, is crumbling under the weight of discovery.

Where once the distinction between stars and planets was quite simply their immense difference in size, the existence of free-floating brown dwarves acted as a sort of missing link, intermediate between the two. When obvious criteria such as size become irrelevent or indistinguishable, a new criteria is sought.

The gray areas between planets and brown dwarfs boil down to three things: their genesis, their orbit and their size.

A definition favored by the person that conjectured the existence of the brown dwarf in 1975, Jill Tarter, is based on the genesis of the planets from the gas and dust surrounding a newly born star. This definition relies heavily upon our theory of the formation of the solar system, of course. From this simple formulation you can understand that the debate is not an academic one about words but actually goes right to the core of our paradigm.

The orbit of Pluto suggests it may be no more than a large object in the Kuiper Belt, a ring of rocky, frozen debris that occassionally spawns a comet. It has been suggested that Pluto would never have been labelled a planet if the existence of the Kuiper Belt was known at the time. There are likely other bodies in the Kuiper Belt that would qualify as a planet under any simple definition of orbit.

I have already suggested that size is a difficult criteria because of the range other objects have. Also, it is not clear how such a definition would fall from our theories of solar system creation. So where to turn?

In 2003, Gibor Basri proposed a criteria for a definition of planet based on the diameter needed to create gravity, which is about 435 miles (700 km). Under that definition, not only Pluto, but 3 other orbital bodies would be christened planets: two more from the Kuiper Belt, and the asteroid Ceres which orbits between Mars and Jupiter.

Basri's corresponding criteria for distinguishing planets from stars would be at the mass necessary to start hydrogen fusion. Colleagues of Basri disagree that any such firm definition is even called for at this early stage in the hunt for planets. Considering that we have found only 100 of the likely billions of exoplanets in even our own galaxy, we may just be too early in the paradigm shift to see a good definition.

[Space.com, 2003] Debra Fischer, also at Berkeley, agrees that there is more to learn before firm definitions are set. She worries that a quick resolution to the debate could set astronomers' up for a repeat of the definition problems they face today.

"It's a little arrogant, I think, for us to imagine that we understand what the full spectrum is going to shake out to be," Fischer said. "Are we really in the ultimate position right now where we should redefine things, because it freezes it in again?"

Fischer said drawing sharp lines could cause trouble in a decade or two. "Let's admit that at either end, the high-mass end and low-mass end, this has been completely arbitrary, and that some things don't fit with our classification scheme."

So does today's announcement of a formal definition that delists Pluto constitute a step forward? Let's look at the definitions set out by the IAU:

[Space.com, 2006] The decision establishes three main categories of objects in our solar system.

• Planets: The eight worlds from Mercury to Neptune.
• Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
• Small Solar System Bodies: All other objects orbiting the Sun.

Alan Stern, leader of NASA's New Horizon's mission to Pluto, said he was "embarassed for astronomy". The vote was unfair as it represented such a small portion of the world's astronomers. He went further to suggest that the definition itself is seriously flawed.

Stern ... said the language of the resolution is flawed. It requires that a planet "has cleared the neighborhood around its orbit." But Earth, Mars, Jupiter and Neptune all have asteroids as neighbors.

"It's patently clear that Earth's zone is not cleared," Stern told SPACE.com. "Jupiter has 50,000 trojan asteroids," which orbit in lockstep with the planet.

As inadequate as it may be, a definition is a means of proceeding, as Kuhn would say, with "normal science". As an astronomer doing normal science, one would like to be able to describe what it is one is looking at and to give it a name. But for me, the importance here is the evolution from a simple, intuited definition of a planet being whatever our planets are to having a functional definition that actually demarcates one body from another in the solar system. This stepped refinement of what constitutes a planet is nothing but an improvement of our definitions based on new evidence. At the moment there is little explanatory power in the definition. How does a planet "clear its neighborhood"?

The difference between description and explanation is the difference between observation and theory. On the one hand, we can say many celestial bodies exist, but we call these planets. On the other, knowing that Earth and Neptune are both planets, while Pluto and Quaoar are not, does little to explain the origin of the solar system. That theory has not undergone a revolution as yet due to pressures on the description of a planet. It may well in the future, however.