Bear with me, this is a convoluted presentation that I will try to simplify as best as possible.
Discovering intelligent alien life on an inhabitable, Earth-like planet. The stuff of science fiction, rooted in what many would term as plausible or even probable, given enough time.
For the purpose of a smooth discussion, let us assume that we have the technology to reach this Alien Earth within a reasonable time frame. I don't want to get caught up in the technological aspects of this.
It is always inferred in science fiction, be it Hollywood or otherwise, that intelligent alien life would be advanced. This plays on human fears thus the reason for this inference in science fiction entertainment. But not here in this scenario. For the purpose of this exercise, they are well behind us, technologically speaking.
We are parked outside of the orbit of their planet, with our scopes observing them as best as we can. This planet is inhabited by intelligent human lifeforms. They look like us. They walk like us. They are human by all outward appearances. Their technology is comparable to what ours was in the 14th century. As many creatures do, they have borders, they have societies that compete for resources, they have wars, famine, disease....all sorts of infliction and troubles, as we have had and continue to have on our Earth.
The ethical questions are many.
There are no radio waves or transmissions that we could intercept because their lack of technology doesn't allow for this. So we cannot learn their language so we could communicate with them unless we intrude directly. Anything we learn about them would have to be done on a very cursory level.
I'll propose several possibilities/issues in moving forward;
1) Do we have a scientific obligation to observe them?
2) If so, how do we move forward?
3) If they see drones/landing craft, this could be catastrophic for their people. Chaos would likely ensue as word spread of Alien invaders.
4) Given what we know about our civilizations history and how they may have reacted to such news, the religious implications alone are immense. The words "magic" and "gods" are inescapable.
5) The bio-organism threat alone, for both sides, brings many ethical questions.
6) If we could establish first-contact, work out the "bugs" (gratuitous pun), what are the ethics involved in what would bring sweeping socio-evolutionary change to their world.
7) We could undoubtedly cure many of their diseases. We could solve famine issues. Pasteurization. Antibiotics. Our positive contribution brings a seemingly endless list.
8) We could undoubtedly introduce new ways for them to kill each other, new ways to poison their air, their water. The dark ages runs head long into the industrial age, which collides almost instantaneously with the digital age and beyond.
9) The risk of purely clandestine observation is incredible. What if our drones or a manned stealth ship crashes to the foot of their castles and village walls? What terrible thing have we now done for the purpose of interstellar voyeurism? They have no Area 51 to keep it secret. They have not the intellectual ability to contain or understand what they have discovered on their Earth.
Or do we quarantine their sector of the universe from contact? But wait! If they're indeed human, doesn't this provide us with a missing link to our own existence? How do we justify ignoring such a find? How do we justify the intrusion for our own knowledge?
Is it possible that the greatest discovery of our species becomes a taboo pursuit because of the ethics involved?
From an ethical platform; What now?