Planets & Plutons: An Update

Back in November of last year, I wrote about the International Astronomical Union (IAU) and how planets were named. The IAU is currently meeting and has proposed a definition of planet. (It has not had a formal definition of the term to date.) The organization will vote on the proposal on Thursday. In addition to getting into the lexicographical game by coming up with a definition, the IAU is also proposing a new term, pluton, for Pluto-like objects that orbit the sun beyond Neptune.

The new definition was prompted by the continuing debate over whether or not Pluto should be considered a planet and by the discovery of 2003 UB313, unofficially nicknamed Xena after the warrior princess of television fame, an object much further and much larger than Pluto. But the IAU had some surprises when it announced its proposed definition this week.

Under the proposed definition, a planet is an object that

  • has sufficient mass to assume near-circular shape because of its own gravity, and
  • is in orbit around a star, and
  • is not itself a star, and
  • is not a satellite of a planet in the sense of having an orbit that goes around a center of gravity that is located inside a body that is independently a planet.

If the proposal passes, there will (for the moment) be twelve planets, instead of the current nine. And there are several surprises in the new planetary nominees. One item that is not surprising, but was by no means certain, is that Pluto will retain its planetary status. And given this, Xena will also be a planet–no surprise given that it is larger than Pluto. But the other two new planets are the shockers.

One is Ceres, the largest of the asteroids. Ceres, discovered in 1802, was once considered a planet, but had long since been demoted to asteroid status. It is only 930 km in diameter (Pluto is 2,274 km in diameter; Earth is 12,756 km, and the Earth’s Moon is 3,476 km.) The new IAU definition of planet includes Ceres because it is a circumsolar object (it orbits the sun directly and not another object) and because it has enough mass to form a sphere.

The second surprise is Charon, Pluto’s moon. Even though it orbits Pluto, and not the sun directly, Charon makes the planetary cut because of the fourth criterion. The barycenter (center of gravity) of the Pluto-Charon system is outside Pluto. (By comparison, the barycenter of the Earth-Moon system is 1,700 km beneath the Earth’s surface.) Pluto-Charon is essentially a dual planet.

The number of planets is not fixed at twelve and as more trans-Neptunian objects like Pluto and Xena are discovered many will also be so designated. There are several other known objects that might make the grade at a future IAU meeting, including Sedna and Quaoar, which are both larger than Ceres and Charon. Another is 2003 EL61, which is also larger than Charon and Ceres, but which has an elliptical shape rather than round.

The IAU is proposing a name for these icy trans-Neptunian objects, calling them plutons. This is a useful term, although it is also a geological term for an igneous rock formed below the Earth’s surface–there will probably be little or no confusion between the two. Pluton is superior to trans-Neptunian object as it can also be applied to such objects in other star systems, when and if they are discovered.

But what is of interest to us word mavens is not so much which objects receive planetary designations by the IAU, but the IAU’s attempt to define the word planet. First, they are not using the methodology that lexicographers do. They are not surveying usage and deriving a definition from how people use the term. Instead, as is usually the case for technical definitions, they are establishing a rigid definition intended to categorize objects by criteria that are scientifically useful. While this method is perfectly fine for technical definitions, their results are highly questionable.

The first problem with the definition is that of the barycenter codicil that admits Charon to planetary status. The barycenters of planet-moon systems are not fixed, changing over time. The Earth’s moon, for example, is moving away from the Earth by about 4 cm a year. In 40 million years (which may seem long to us but is only a moment in planetary time) the Moon will become a planet when the barycenter of the Earth-Moon system moves above the Earth’s surface. If objects can move in and out of planetary status in relatively short periods, does the categorization tell us anything meaningful?

Another is the question of how round is round? No planet is perfectly spherical. The Earth is somewhat pear-shaped, a bit fatter in the southern hemisphere than in the north. Saturn is noticeably squashed at the poles. What an object is made of and how fast it rotates are as important as mass in whether or not gravitational forces shape it into a sphere. As I have noted, 2003 EL61 is considerably larger than Ceres, but it is egg-shaped due to its rotation and the fact that it is probably made largely of ice instead of rock. This inserts an subjective and arbitrary judgment into what should be objective and meaningful criteria. The key isn’t roundness, it’s mass; in which case the definition should cut out the middle-man and specify a minimum mass.

But the larger question is whether or not any definition of "planet" serves a useful astronomical purpose. When objects are as different as rocks like the Earth, gas giants like Jupiter, and ice balls like Pluto, is there any sense in trying to lump them into a single category? Astronomy might be better served to classify the objects that orbit the sun into terrestrial bodies (big rocks like Mercury, Venus, Earth, Mars, as well as many moons), asteroids (rocks that are too small to be considered in the first category), gas giants (Jupiter, Saturn, Uranus, and Neptune), and plutons (Pluto, Xena, and comets).

Astronomers should take their cues from another discipline. At one time, biologists considered race an important category in studying humans. But over time, it became clear, that although the characteristics associated with race were biologically determined, the characteristics could not be used to form any scientifically useful categories. Skin color, for example, tells you nothing about the person except their skin color. Today, no reputable biologist attempts to classify people by race. This is not to say that race is not culturally relevant, but it is meaningless as far as the science of biology is concerned. Biologists have gotten out of the race business, leaving the definition of racial categories to social scientists and lexicographers.
Similarly, astronomers should do the same thing with the word planet. They should declare that astronomically, the category is useless. Leave it to the lexicographers to find a definition based on how people (astronomers, schoolchildren, and the general public) use the term. Planets would still exist, but whether an object is defined as a planet should not concern an astronomer. In such a descriptive definition, the planets would consist of the eight classical planets and, depending on who you asked, Pluto and some of the other trans-Neptunian objects.

Little is served by a scientific body giving its imprimatur to a definition that has little or no scientific utility.

See the website Bad Astronomy for an excellent, if opinionated, review of the astronomical issues associated with the IAU’s definition of planet.

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