The definition of "planet" has for some time been the subject of intense debate.
Although the word dates back thousands of years, no officially decreed scientific definition of "planet" existed before the early 21st century.
Until the beginning of the 1990s, there was little need for one, as astronomers had only a single sample of planets in solar system to study, and one small enough for its many irregularities to be dealt with individually.
However, after 1992 and the discovery of the myriad tiny worlds beyond the orbit of Neptune, the size of the sample rose from nine to at least several dozen.
Following the discovery of the first extrasolar planet beyond our solar system in 1992, the number of samples has now reached hundreds.
These revelations not only increased the number of potential planets, but, in their variety and peculiarity (some nearly large enough to be stars, others smaller than our Moon) challenged long perceived notions of what a planet could be.
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