I was curious so I SM-ed myself a large invasion force and attacked the planet. Turns out they had a pop of 1.8 billion and absolutely no facilities. They were also curiously passive; I dropped troops and they still didn't go red until I attacked them. They gave me 613k war reparations. This was either their homeworld or a perfectly terraformed (huge) colony, as it fell right in the center of their environmental tolerances.
This seems like it might be a bug in the game, so I'll report it in the bugs thread.
Edit: Now that I think about it, maybe they weren't strangely passive, they just had no idea I was there until I started shooting at them. With no detection facilities or sensors, I guess everyone else is invisible until they make themselves known.
I'm going to have to figure out how to play this without making the game no fun. This is a big, juicy, easy to pick plum. I don't even need drop ships to land troops; they can just walk out the hatch onto the planet with no danger. Or, I could exploit the rest of the system and they would never know, like they were a no-tech race. Feel free to make suggestions.