Having a flag for planet Earth is an oxymoron. The very purpose of a flag, and nationalism in general, is to be a symbol of a nation, and the very concept of a nation can only exist in contrast to other nations. One side of the coin can only exist because there is another side. All of Earth, considered as a whole, has nothing to compare itself to. We call the sides of a coin “heads” and “tails”, but we don’t have a word for the “side” of a sphere, since it would be pointless.

(Incidentally, there are already a few symbols for the planet Earth: Unicode 2637;TRIGRAM FOR EARTH¹ and Unicode 2641;EARTH²)

That said, there may be a need for a slightly related symbol: In the world today there is a common assumption of nationalism, so whenever some grand achievement is done, there is the assumption that it is done in the name of some country or other, and everything is done with a place reserved for a symbol for that country – commonly a flag or a heraldric shield. To contrast against this, we may need a symbol against nationalism.

① http://graphemica.com/%E2%98%B7

② http://graphemica.com/%E2%99%81