Astrophile
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Yes. At the time of its discovery, Pluto was thought to be about the same size as the Earth, so it would have fitted the description of a planet, and nothing was known about other objects orbiting beyond Neptune.Was Pluto being our ninth planet scientifically credible at one time?
The discovery in 1978 of Pluto's satellite Charon showed that Pluto has only about 1/500 of the mass of the Earth (it is less massive even than the Moon). The discoveries from 1992 onwards of other trans-Neptunian objects showed that Pluto is only one (and not necessarily even the largest) of a vast number of bodies orbiting beyond Neptune, and that it is a different class of body from the well-established planets. If you regard Pluto, Eris, Haumea, etc. as planets, where do you draw the line between trans-Neptunian objects that are planets and TNOs that are not?
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