Sentence:  We learn the energy quotient of a bogus Earthling, note how much it varies from that of a genuine one, and then proceed to carefully single out every <WSD>Neptunian</WSD>. It will take time, no doubt, but not perhaps as long as we think, as the percentage of <WSD>Neptunians</WSD> to Earthlings is small. 
Meaning of the word marked with <WSD> tags:  An (imaginary) inhabitant of the planet Neptune.