What is a Planet? Gibor Basri, Astronomy Dept., UC Berkeley Abstract "When I use a word", Humpty-Dumpty said in a rather scornful tone, "it means just what I choose it to mean neither more nor less".
"The question is", said Alice, "whether you can make words mean so many different things".
"The question is", said Humpty-Dumpty, "which is to be master that's all."
Lewis Carroll in Alice Through the Looking Glass I. Introduction The original meaning of the English word "planet" is rooted in Greek and means "wanderer" (it would be interesting to include other cultures and languages). This refers to the apparent positions of the objects in the sky from Earth, and so is not much use as an astrophysical definition. After examining a number of definitions from dictionaries and encyclopedias, a fair summary of the "cultural" definition of "planet" seems to be something like "an object resembling the planets in the Solar System, which is orbiting a star". The problem with this definition is that it is not very specific about how closely an object must resemble one of our planets, or in what ways, and it does not take into account the recent discoveries bearing on this question.
These pronouncements have been accompanied by objections to the terminology employed. It has been retorted that since the free-floating objects are not orbiting stars, they do not conform to the cultural meaning of "planet". It is further suggested they should be given some sort of "stellar" name under the presumption that they formed by themselves in a "star-like" fashion. There has been a suggestion that most of the exoplanets deserve similar treatment, because their eccentric orbits are presumed to suggest that they also didn't form the way Jupiter did. Recently it was proposed that most exoplanets are really brown dwarfs, because the lower limits from Doppler searches consistently and substantially underestimate their true mass. If this were actually the case (which their mass-limit distribution makes highly unlikely), it would be a sound reason for not calling them "exoplanets". | |
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