Is Pluto a planet?

The IAU 2006 General Assembly was held in Prague in August 2006. One topic of interest was the status of Pluto and as a result a number of proposals related to the definition of the word "planet" were presented and voted on. The result of the IAU Resolution votes on the 24th August 2006 are examined and the consequences explained.

A brief summary of this page is available at the bottom.

The resolutions presented for voting went through a number of changes during preceding discussions but the final versions were as follows:

RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites,be defined into three distinct categories in the following way:

(1) A "planet"¹ 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.

(2) A "dwarf 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) shape²,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

¹The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 5B
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:

(1) A classical "planet^"1 is a celestial body . . .

and

¹The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

IAU Resolution: Pluto

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

RESOLUTION 6B
The following sentence is added to Resolution 6A:

This category is to be called "plutonian objects."

The key to understanding whether Pluto is or is not a planet lies in the primary criteria listed as (a) through (c) in part (1) and part (2) of Resolution 5A but importantly also Resolution 5B. Resolution 6B has no impact on our question and will be ignored from here on.

Consider first how the text would read if Resolution 5B were included. The definitions would then look like this:

RESOLUTION 5A including 5B
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites,be defined into three distinct categories in the following way:

(1) A "classical planet"¹ 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.

(2) A "dwarf 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) shape², and
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

¹The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

It is fairly easy to see how this definition works. Two categories of body are defined described as "classical planets" and "dwarf planets". Both groups are therefore planets with the adjectives "classical" and "dwarf" separating them into distinct sub-categories. I have italicised these words to emphasise this in the preceding text.

Excluding criterion (2)(d) which removes artificial satellites from consideration, three criteria must be met if an object is to be either a "classical planet" or a "dwarf planet". Two of these criteria are common; the body must:

(a) be in orbit around the Sun, and
(b) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

These two then define a planet in the general sense. The third criterion in each case relates to whether the body has "cleared the neighbourhood around its orbit" and it is key to note that these criteria are mutually exclusive. That is either a body has "cleared its neighbourhood" or it hasn't so a planet can either be "classical" or "dwarf" but not both. While that may appear obvious, is is critical to understanding the status of Pluto. If Resolution 5B had been passed then Pluto would have been a planet in the sub-category "dwarf".

However, Resolution 5A was passed while Resolution 5B was not passed. Consequently the original Resolution 5A text stands unmodified. The relevant part is repeated here:

(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.

(2) A "dwarf 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) shape²,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Note that criterion (c) of definition (1) and criterion (c) of definition (2) remain mutually exclusive and any body can meet either definition (1) or definition (2) but not both. Thus a body which is a "dwarf planet" cannot also be a "planet". In this situation, the term "dwarf planet" is not a sub-category within the group of planets and the word "dwarf" is no longer used as an adjective but has become part of an indivisible phrase, "dwarf planet". Since Resolution 6A was also adopted without modification, Pluto is a member of the category of "dwarf planets" and cannot therefore be a member of the category of "planets". This is confirmed by its exclusion from footnote 1 which lists all planets.

To summarise, Resolution 5A was proposed which had the effect of removing Pluto from the list of planets and creating a new non-overlapping group called "dwarf planets". An amendment was proposed as Resolution 5B which would have had the effect of reinstating the status of Pluto as a planet of sub-type "dwarf". Resolution 5A was adopted but Resolution 5B was not, hence Pluto is no longer a planet according to the new IAU definition.

p.s. What does mean to say that an object has "cleared its neighbourhood"? This paper might give you an idea or just look at figure 1 for mass versus semi-major axis or figure 3 showing object mass versus mass remaining in the neighbourhood.

George Dishman
4^th September 2006