At its most basic it's a globe bisected by the equator and a line of principal meridian - the fundaments of our measures of distance and direction. It's also been interpreted as marking the four cardinal directions. But as with interpretation, it's in the eye of the beholder: the symbol is old, thousands of years even, and has been employed by various cultures and religious sects (one of its names, Odin's Cross, references its use in Norse mythology as a stylistic representation of Odin) and this has provided the religiously inclined a symbolic association with their particular deity's reign over the Earth. Some Native American tribes also use the symbol to represent the four elements, which certainly leaves a very short leap to it being used as a symbol for the Earth (the Earth as a sum of its elements). Lastly, it's frequently known as a Solar Cross, and has been used to represent the movement of the sun since, perhaps, the beginning of recorded history, in many cultures throughout the world. It's up to you if you choose its more secular association, but as for its origin, as far as I can tell, if not lost to time it's at least obscured by countless repurposings.

Sources:

http://www.universetoday.com/14512/the-symbol-for-planet-earth/

http://www.warpaths2peacepipes.com/native-american-symbols/circle-symbol.htm

http://symboldictionary.net/?p=784\

Edit: Some added info about the Solar Cross.

I'm fascinated by all of them actually. Neptune's pretty obvious, but all Mercury is suggesting to me is "Devil Woman".