This question is inspired by NASA’s recent TRAPPIST-1 announcement. Imagine two planets orbiting a star with orbits very close together. At the nearest point in their respective orbits they are only 1 million kilometers apart. Both planets lie in the habitable zone and both will evolve sentient life. Much like humans these two distinct species create structures, roads, and farms. They start fires, cut trees (or their flora equivalent), and divert waterways. As they progress technologically they create larger cities and farms and their effect on the world becomes more noticeable. So too does their ability to observe each other. Both species are curious, and naturally they train their telescopes on their clearly habitable neighbor. At what technological level will they most likely become aware of each other? What would be the first signs that would betray one species’ existence to the other?
I’m interested in varying technological disparities between the two species. If one species is neolithic at what point will the other species discover them? Will it require manned/robotic expeditions or could powerful telescopes detect them? What if both species are roughly on par technologically? What is the technological breakthrough that will reveal the other?
Bonus question: are there any plausible scenarios where the technologically inferior species could become aware of the other species’ existence first?
Assume for our purposes the atmospheres of both worlds do not obscure astronomical observation any more than the Earth’s does.
Note: this is distinguished from this earlier question by:
- the separation is 4× greater
- the civilizations may be at different levels