The earliest citation in the OED is in 1854, but that's in English; I'm sure someone must have mentioned it in Latin? Newton?

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It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

It seems impossible to me that Copernicus or Kepler could have espoused their theories without lumping Earth into the same category as Venus, Mars, and Jupiter.

Similarly Aristrachus of Samos may have done the same.

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My guess would be someone slightly after Galileo and Kepler's time. (Unless Galileo did it already -- I don't know, but you can start by looking here.)

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Dictionary dot com says this of the origin of the word “planet”:

Origin: 1250–1300; ME planete (< OF planète) < LL planéta, planétés (found only in pl. planétae) < Gk (astéres) plantai lit., wandering (stars)

Of course the earth was called “the earth” back in those days. According to Dictionary dot com, the origin of that word was:

Origin: bef. 950; ME erthe, OE eorthe; c. G Erde, D aarde, ON jǫrth, Dan jord, Goth airtha

I’ve read old books about astronomy, and books written in or translated into English used different terms, in addition to “planets”.

For example, I have an 1803 science book in my collection (which is also right here in front of me now) that uses the terms “planets”, “orbs”, “worlds”, “globes”, “terrestrial bodies”, and other more general terms such as “heavenly bodies”.

Ok, here I’ve found a quote from Newton dated 1692 in which he refers to the earth as being one of the planets:

“We have shewed before, that if gravity and a projected motion be fitly proportioned, any planet would freely resolve at any assignable distance within the space of the whole system.”

He’s talking here about the earth’s and other planets’ positions in the solar system. This is from my copy of the book “Isaac Newton’s Papers and Letters on Natural Philosophy,” 1958 edition. These pages look like photocopies of the original old English pages, first published in the 18th Century.

we may never know who really first thought that the earth was a planet, since that person probably wasn't famous- but i'm sure we can track down the first name we recognize that said it was.

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