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What is a Planet

Gibor Basri, Astronomy Dept., UC Berkeley A spate of new discoveries of objects in the mass range 0.5-15 Jupiter masses (jupiters), along with the Pluto controversy, has caused increasing debate over the meaning of the word "planet" (as differentiated from "stellar or substellar" objects). A variety of uses for this word have occurred, each with a somewhat different intent. No formal definition has been officially adopted. I examine the problem in the light of current astrophysical observations and theory. The debate can be framed in terms of three distinct arenas that drive it: characteristics, circumstance, and cosmogony. By "characteristics" I mean the physical properties of an object - primarily its means of support against gravity and its source of luminosity. By "circumstance" I mean the environment in which it is found, most importantly whether it is in orbit, what primary object and other companions are present, and their orbital characteristics. By "cosmogony" I mean the origin or mode of formation of the object. The current lack of consensus derives from differing weights being applied to these three arenas in forming the definition of "planet". Each arena contains dividing lines between "planets" and "stars" which are partly matters of opinion, and end up sorting objects differently. The main purpose of this paper is to bring order to the discussion, so that we may eventually find a broadly accepted consensus definition. I cannot resist, however, offering a possible solution that steers a middle path through the various quandaries that come up in attempting a formal definition.

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