The case for planet Pluto

(under construction)
by Wm. Robert Johnston
last updated 17 October 2006

Image credit: JHUAPL/SwRI
Seven years after reaffirming Pluto is a planet, the International Astronomical Union (IAU) reversed itself in September 2006 and declared that Pluto is not a planet. Publishers and educators have obediently begun changing their products and curriculum. Some planetary astronomers have harshly criticized the IAU's new definition of a planet, and they have a good case. I have to weigh in with those who would count Pluto and comparable objects as planets.

The public is understandably confused. They have learned that Pluto is a planet for 76 years, and the background of the IAU decision involves a range of lesser known details:

Any definition of a planet must be recognized as subjective to some degree: distinguishing between small planets and large minor bodies is to some extent arbitrary. Astronomers have not had to deal with this continuum before, but now they must. Further, any planet definition is in some sense just a name. Pluto is the same physical object regardless of which category we put it in.

Despite the limitations on a planet definition, it is still desirable. Such groupings assist us in looking at general characteristics of objects. Astronomers deal with large numbers of objects: currently, astronomers have assigned designations to hundreds of moons, thousands of comets, hundreds of thousands of asteroids, and millions of stars. Astronomers long ago agreed to give the IAU certain roles in regard to nomenclature, such as overseeing the naming of small solar system objects.

Since a planet definition should be useful to the astronomical community, this suggests some tests of a good definition:

With this in mind, we consider the new IAU definition: "A 'planet' is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit." Part (c) is the criterion which disqualified Pluto. This criterion may also fail all of the suggested tests for a good definition:

A criterion of similar spirit but more scientifically robust than the IAU "cleared neighourhood" criterion is that a planet be the dominant object gravitationally in its neighborhood. This would clarifies why Neptune is a planet and Pluto is not: Pluto's orbit is a consequence of resonances driven by perturbations from Neptune. But in some sense the inner planets are dominated by Jupiter; the Earth's "clear" neighborhood is partly due to Jupiter. This definition remains difficult to apply in extrasolar planetary systems, dependent on observations decades away.

The definition adopted by the IAU was hastily offered at the end of the 2006 conference (after most attendees had departed) to replace the controversial definition produced by an IAU-designated committee after lengthy deliberation. In general, this definition used only parts (a) and (b) of the adopted defintion. By counting any Sun-orbiting body nearly in hydrostatic equilibrium, it would tend to include asteroids and TNOs down to diameters of 300-800 km. This would likely have included 2-4 asteroids and 10-40 known TNOs, even apart from yet-to-be-discovered TNOs. The "inclusive" nature of the original proposal was its downfall. The significance is clearly that it did not capture the essence of what astronomers have in mind with the term "planet".

There are significant differences between rocky or icy bodies with sufficient self-gravity to be round, and the larger objects traditionally counted as planets--perhaps including Pluto. Basri and Brown recently reviewed several such differences. Around a size of 3000 km, gravitational energy is sufficient to significantly modify the object's internal chemistry. A bit larger and solid state convection becomes important. Internal pressures significantly compress the internal material of a rocky body larger than 6000 km diameter, or an icy body larger than 1000 km diameter. Gravitational differentiation into a denser core and less dense mantle/crust occurs for objects as small as Pluto, perhaps as small as 400 km. Ice-rock bodies like Pluto and even smaller may have subsurface oceans, according to some models.

Pluto is the best studied of objects that some would lump together as less than planets. These studies show a variety of phenomena more associated with planets than with small solar system bodies. Pluto has an atmosphere, apparently particular to when Pluto is near perihelion, but at which time it drives surface changes: frost deposits form and evaporate over large regions. When Pluto is finally examined close up in 2014, astronomers would not be surprised to find similarities to Neptune's moon Triton, which shows geologic activity including active geysers or cryovolcanoes.

The manner in which the new IAU definition has been implemented is arguably clumsy. Following opposition to the committee proposal, the IAU instead hastily adopted a less than fully robust definition that excluded Pluto. Within weeks, after 76 years as a planet, Pluto was relegated to minor planet number 134,340, sandwiched in the MPC list between two main belt asteroids each smaller than New York's Central Park. The newly created "dwarf planet" category, explicitly not a planet despite the grammatically construction, in practice overlaps with minor planets; since the distinction between dwarf planets and small solar system bodies is difficult to apply and not particularly useful at this time, it is likely this term will not make it into the astronomical lexicon.

This comes to a defect in the IAU's adoption of a planet definition: it is a term, not a name, and was imposed by a minority of the scientific community. The scientific community has established usage of the term "planet", and in fact many in the planetary science community have signed a petition rejection the new IAU definition and promised to produce a better one.

I've been asked what definition I would offer, given my issues with the IAU definition. I concede that no definition is perfect, and that it will likely prove necessary to revisit the issue in the near future. All things considered, I would suggest simply using a size threshold to discriminate between planets and smaller solar system bodies. A diameter cutoff of 2000 km would count Pluto and Eris as planets; a cutoff of 1,300 km would include three more TNOs that already appear distinct from the other 1,122 smaller ones (there is in fact a fundamental argument in favor of this smaller threshold). This threshold is approximately the size range where some of the structural and surface pheonomena we identify with planets start to come into play.

A threshold-based definition has the disadvantage of not having as clear a scientific premise as one based on hydrodynamic equilibruim or cleared orbital neighborhood. It is significantly more straightforward to apply, comes closer to the spirit of what astronomers are trying to capture with the term planet, and is generalizable to other planetary systems. It is also somewhat intended as a stop-gap measure: we are attempting to impose a definition on objects we have yet to examine as more than points of light. The adoption of a definition excluding Pluto is ironic given that a NASA mission is en route to Pluto to provide our first closeup look in 2014; this look might have a bearing on how we think about this object, and we may want to keep an open mind.

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© 2006 by Wm. Robert Johnston.
Last modified 17 October 2006.
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