The Universe in the Classroom

A Good Definition of the Word "Planet": Mission Impossible?

5) Runts and Runaways

Indeed, one alteration of orbits can be the outright ejection of a planet. When several big planets are orbiting a star under each other's influence, the system is inherently unstable. The almost inevitable result is that the smallest body will be ejected from the system. We would see it as an isolated object smaller (less massive) than a brown dwarf. Such objects have indeed been found in some young star clusters. The problem is, we are not sure whether such small objects can also just form by themselves, without ever being in orbit around a star. Originally theorists claimed this was not possible, but as more details were added, they retreated. We know of no good reason why the smallest objects with fusion should have the same mass as the smallest objects that can form by themselves. At the moment it seems more likely that the discoveries are not true ejected planets (but it is very hard to be sure).

A controversy therefore arose when they were dubbed "free-floating planets" by some of the discoverers. The question is: if the object was never in orbit around a star, can we call it a planet? Some astronomers say that if it never had fusion then it clearly isn't a star (failed or otherwise). Since the free-floating objects have the same mass as some of the accepted extrasolar planets, they should be called planets too. Others say that planets can only form and be found around stars (leaving aside the problem of ejection), so if the new objects formed in isolation they should be called "sub-brown dwarfs" or "grey dwarfs", but certainly not "planets".

The latest computer simulations of star formation only further confuse the issue. They show that in the formation of a cluster of stars, fetal objects are often interacting with each other, forming loose alliances, then being ejected from the group while formation is still in progress. Brown dwarfs sometimes form by themselves. Sometimes they are part of a multiple star system in the process of formation, when they are suddenly tossed out (robbing them of their "rightful" supply of gas). The same could be true of objects in the planetary mass range, in which case it is hard to say whether they formed in orbit around a star or not. (For more information on these computer models, try this link:

6) Mission Definition

By now you should be at least as confused as professional astronomers are. What seemed like an easy question ("what is a planet?") has become a morass. Still, the word is in very common use, and it would be nice to know what we are talking about. A final definition should be acceptable to both scientists and the lay public. Your mission, should you choose to accept it, is to concoct such a definition. There are three arenas from which the definition could spring. They are not necessarily compatible with each other, and one or more may not be necessary. These are 1) the characteristics of the objects themselves; 2) the circumstances in which they are found; and 3) their cosmogony (how they form).

I here suggest a few properties that the definition perhaps should satisfy, but you are free to add to and subtract from this list at will. A good way to start is probably to settle on the list for yourself. My suggestions (in no particular order) are that a definition should 1) be physical: give some fundamental properties of the object; 2) be observable: depend on measurements that are feasible to accomplish; 3) be succinct and clear, with little ambiguity; 4) be general beyond current observations: allow room for new discoveries; 5) have well-defined limits: except right at these limits it should be easy to place an object inside or outside the category; and 6) be easily understood by the public but satisfactory to scientists.

The International Astronomical Union (the only body empowered to make an "official" definition), has found the task problematic so far. Nature, of course, cares nothing about classification, and the truth is that there is a continuum of characteristics, circumstances, and probably cosmogonies to the objects out there. This article contains the beginnings of many of the relevant issues. Have the students extract them (and perhaps augment them with their research), and organize them so to aid in carrying out the mission. Can they bring clarity to the classification and cut through the confusion? Can they make a definition for "planet" that other students will like and understand, and that teachers and astronomers will also find compelling? It should at least be fun and informative to try.

I don't think this mission is impossible, and will suggest a definition in a future article in Mercury. Or you can get a preview at my website ( after you have finished your own thought process. This document will not self-destruct anytime soon, and you are free to spread it around. Good luck!

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