I see that the search is case insensitive, but this is not true for 'Earth'.
Search for 'Earth' returns 2 places, whereas search for 'earth' returns no places.
Thanks for bringing this to our attention. GeoPlanet performs case-sensitive searches for places, since some place names may have different meanings when they are not capitalized. "Earth" more often refers to a place than "earth", which could mean the same as "dirt" or "ground". In this particular case, "earth" has a low enough probability (according to our algorithms) to be ignored by GeoPlanet. We'll see if we can improve this in a future release.