What is a planet: problems with the current definition

Yesterday, while eating lunch, I got around to reading the article in SCIENTIFIC AMERICAN “What Is A Planet”, by Steven Soter. The article tries to address the rationale for the new definition of a planet, and why Pluto no longer meets that definition.

I can’t say that I disagree with the definition as presented. The definition is based more or less on the orbital characteristics of a body, how much it sweeps clean of it’s surroundings, and the ratio of the body’s mass as compared to the total mass of all other objects in its orbital zone. These are fairly measurable things and seem to support the categorization as it has been made.

I do have some problems with the definition, however, one scientific, the other semantic.


For reference, the new definition of a planet is a body that meets all of the following criteria:

  • It orbits a star
  • It is large enough to be round
  • It has cleared other bodies out of its neighborhood

This definition eliminates all satellites, which orbit planets (although, strictly speaking, satellites also orbit stars, since they follow their primaries around the sun). It seems to me that this definition eliminates any body whose orbit stays more or less within the zone of the asteroid belt, as well as any body whose orbits stays within the zone of the Kupier belt. The asteroid belt is akin the to the Roche limit of the sun. It is not the Roche limit of the Sun, but it behaves as though it is, in large part because of Jupiter’s gravitation and its distance from the Sun. Objects in the Kupier belt are too far away from the sun to for the Sun’s gravity to assist in clearing up orbits, and the objects are too small gravitationally, to do the job themselves. Therefore, I think it would be very surprising to ever find a body that meets the new definition of a planet in either of these regions, and if such a body was found, we may need a new definition.

Why do I have a problem with this scientifically? In terms of the new definition, I have difficulty seeing how Mercury and Pluto are different. Certainly this could be a shortcoming on my part. But look at the two bodies in terms of the new definition . Both orbit a star (the Sun). Pluto’s orbit is more eccentric and it is tilted more to the plane of the ecliptic than Mercury, but it meets the first criteria, nonetheless. Both are large enough to be round. Mercury is 4,879.4 km in diameter with a mass of 3.302 x 1023 kg, and Pluto is about half the size at 2,390 km in diameter with a mass of 1.305 x 10 22 kg. They both meet the second criteria.

It is the third criteria that makes them different. Mercury has cleared other bodies from its neighborhood, while Pluto has not. But has Mercury really done this? Mercury is far closer to the Sun than Pluto (or any other planet, for that matter) and I wonder how much of the fact that Mercury has cleared its orbit is due to some assistance from the gravitational forces of the Sun. Put another way, if we swapped Mercury and Pluto’s orbits, would Pluto clear its orbit, being as close to the sun as Mercury? I suspect that it would, and this is why I have a problem with the definition from a scientific standpoint. The definition is not limited to the body itself, but to the location of the body in space, and that doesn’t make sense to me.

I also have a problem with the definitions semantically. Toward the end of the article, Soter writes

The historical definition of nine planets no doubt retains a strong sentimental attraction… Some argue that culture and tradition are sufficient grounds to leave it that way. But science cannot remain bound to the misconceptions of the past. To be useful, a scientific definition should be derived from, and draw attention to, the structure of the natural world.

I do have a sentimental attraction to the nine planets. The very first astronomy book I ever read back in 1977-78 was called The Nine Planets. I also agree that we need a good definition and that it should derive from the the structure of the natural world. But I also think we need to be consistent about this throughout science and scientists are not consistent about this. One inconsistency in naming that comes to mind is the electron. An electron carries a negative charge and its antiparticle is called a “positron”, which carries a positive charge. While we are reclassifying planets, shouldn’t be also be doing the same with their sub-atomic kin? A “positron” accurately describes the particle, which carries a positive charge. Shouldn’t an electron be renamed “negatron”? This would much more accurately describe the particle, and in doing so, “be derived from, and draw attention to, the structure of the natural world”. But do you think such a usage would ever catch on? I doubt it. “Electron” retains a strong sentimental attraction. In fact, much of our world today is based on it–in the form of “electronics”. So we grandfather in the electron, but we don’t grandfather in Pluto. It seems a bit too arbitrary to me.

It seems to me that the best solution would be similar to what is done with “absolute magnitude” when referring to a star’s brightness. We can use the same new criteria that defines a planet, with one exception: the criteria has to be applies to the body, as if the body were at a standard distance from its star. What that standard distance should be is unclear–but perhaps the distance of Mars from the Sun would be a good starting point. A body at this distance from the Sun that met all three criteria would be called a planet.

I don’t know if Pluto would be included in this definition, but I think it is better than what has presently been agreed to.