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Last week, I was reading the works of Johannes Kepler from the 1600s and stumbled on his definition of a planet. It was actually this! He said the defining character of planets is gravitational rounding (not what or how they orbit).
2/ Correction: that was Huygens in 1698, not Kepler. I was just now having a hard time finding it online. No wonder: I was searching the wrong name. Ugh.
3/ Huygens also included in his description of planets that they are the places where there exist vegetation, animals, and civilizations possessing culture, art, and technology. He said these would be similar what we see on Earth, but different!
