Tom was somewhat surprised to hear this answer.
"What observation methods does your Bluetook Civilization use?"
Momolans said weakly, "Planetary atmospheric spectral determination technology.
We constructed a giant array of space telescopes to observe the various planets in this star system. We found that the third planet was located precisely within the habitable zone, with temperatures sufficient to support the existence of liquid water.
We determined the elemental composition of this planet's atmosphere and confirmed the existence of life on this planet. We then measured whether the atmosphere contained industrial markers and analyzed the carbon dioxide concentration curve, confirming that there was no intelligent life on this planet, so we chose it as our migration target..."
Tom knew that so-called industrial markers were gases that could not form naturally and could only be produced through the industrial processes of intelligent life.
