I love this place and we finally get to celebrate the Earth today. The big question I have is why the earth is sometimes capitalized and sometimes not? Google didn't disappoint, although I feel a little disappointed with the answer I found. Here it is, not my words but Google's words, 'Therefore, 'earth' as a planet is not a prototypical proper noun but is only derived from the prototypical common noun 'earth', unlike other planet names such as Mars, Pluto, Venus, etc, which are all prototypical proper nouns'. I'm not going to waste Earth day looking any further than that!