Kraken wrote:28 worlds with higher life forms. How many of those will develop sentient life? It only happened once on earth after billions of years, so the odds seem pretty low. But let's be generous again and say half.
Has sentience only developed once? Are we sure dolphins aren't sentient? They can communicate and use tools. Do all of the other hominid offshoots that have since gone extinct, ones not in our direct evolutionary line such as Neanderthals, count as part of our instance of sentience or are they separate, though extinct, examples of sentience? Not nitpicking, just proposing some notions for discussion.
7 worlds with technological civilizations. One of them is us, so 6 others. How many of those will reach our level of sophistication? Half?
3 civilizations comparable to ours. How many of those will be so far ahead of us that they can travel among the stars or invest time and energy in automated probes that take thousands of years to reach their destinations? Again, we're in the realm of pure speculation. If your answer is 1/3, then there is one. If your answer is under 1/3, there aren't any.
One thing to consider is that another world that's about earth's age, with sentient, technological life forms, could have seen those lifeforms first reach a civilized state when our world was populated by dinosaurs. If that planet hasn't suffered a process-reseting asteroid impact, that could mean that they are hundreds of millions of years more advanced than us, to a degree that we could be looking at their planet, but not recognize the presence of a civilization because we wouldn't recognize what to them would be clear evidence.