In Search of Planet "E"
The "letter" naming convention is based on order of discovery, which is strongly mass-dependent. Although there is some radial selectivity, this may be either negative, as for existing doppler discoveries, or positive, as in astrometric methods soon to be implemented.
Therefore, were another race grokking our star for planets, they would likely find 4 gas giants and name them b, c, d, & e, before finally detecting Earth as planet "f", or perhaps "g" if Venus snuck under the bar first...