Here I am going to lay out some basic assumptions about the physical universe of our world.
FTL is possible. Not only is it possible, but it works, is fairly cheap and not a lot of trouble. It is based on the work of Miguel Alcubierre and Chris Van Den Broeck and others. The first practical FTL warp drive prototype flew in the mid- to late- 23rd century. FTL drives capable of 1 light year/day pseudo velocity are common with a typical effective range of about 9 light years in a single jump. The jump range of an FTL warp drive is governed by accuracy of astrogation and an increasing tendency to get captured onto the gravitational shadow of stars neighboring the path of travel. A warp drive ship will always drop out of warp somewhere around the 1/10,000th gee isograv of a massive body. If the astrogation is successful and the ship exits at the expected body, there will be no unfortunate side effects. The farther it exits warp from where expected, the greater the chance of a high-energy feedback effect that could wreck the engine, destroy the ship or possible even worse. The upshot of this is that the range of a warp ship can be longer in less densely populated parts of the galaxy and shorter in more densely populated regions. This also makes it difficult to, “thread the needle,” past a star close to the path between two stars. The farther the jump, the harder it is. Could one potentially, then, travel to another galaxy if one were far enough from the clutter of this one? Yes. Possibly. At least that’s the theory. Of course, the density of the stellar population does not effect the speed the ship travels at, so at 1 light year/day, that would take about 6,000 years to travel to the Andromeda Galaxy 2,200,000 light years away. Better pack for a long trip!
The fastest cutting-edge human ships are capable of as much as 5 light years/day, although that requires very expensive equipment and most of the ship’s mass and volume to be dedicated to drive and power systems. Even then, the effective range of a single jump hasn’t increased by much. The most advanced ships might be capable of as much as 9.3 light years if the astrogator is skilled and willing to take risks. These are not necessarily the fastest ships as optimizing for speed is different from optimizing for range. Long jumps are safer, easier and more reliable over more familiar trajectories. For popular trade routes near Sol, full 9 light year jumps can be considered safe and routine. For a ship exploring unknown space, it might be best to keep the individual legs to under 8 light years unless one has extensively travelled among stars near to both termini. The expense of a cutting-edge range-optimized drive can be considered a good investment for an explorer. Even if he has no intention of going beyond the 9 light year limit. or even 8…
Habitable worlds are common. This doesn’t necessarily mean you’ll have Earthlike planets everywhere, but if the planet is orbiting in a pretty wide Goldilocks zone, is old enough to have developed photosynthetic life, and is in a pretty wide size range, it’ll be lifebearing. Also, there’s a whole range of non-Terrestrial forms of life that widens the range considerably, but that doesn’t make a planet habitable by any human definition. It sure adds interest, though. Keep in mind, a lot of those,”habitable,” planets are pretty marginal to hostile to humans. Still, there are a whole lot more Earthlike worlds than expected. There are also hugely more Marginal worlds where humans can get along with a minimum of help, although they might not like the place. There are still many systems with no habitable planets in them, not even for,”Life As We Don’t Know It.” With the limit on the range of the warp drive, people are going to have to stop at a lot of places that aren’t at all hospitable. And just like little towns that sprung up in the desert along Route 66, someone will set up in any frequently trammeled system to try freeing the travelers of the cash in their pockets. Or sometimes even just to avoid contact with the rest of civilization…
Even if every star system had a living planet(which isn’t true), our own Solar system is a good indicator of how much uninhabitable territory you can find in a system with a habitable planet and how much interest that territory holds.
The usual idea was to look for planets around F, G and K stars. Only the smaller, cooler, longer-burning F-class stars, of course! Well G and K are still the winners, here. I’ll usually run the Accrete program about ten times for each of these stars and take the result I like best. The smaller F class stars I’ll run three times, and if they prove to be too young for life, then I guess we’ll end up with the kind of thing the 2300AD RPG referred to as Pre-Garden worlds. Easily-terraformable and if the star doesn’t kill them in the meantime, they may even develop life, but not right now.
In addition, many M class stars seem like they may be likely to host habitable planets, and they certainly have plenty of time to develop life. Possibly, while a lot of the M class stars are flare stars, which complicates things for a potentially habitable world, a majority of habitable worlds orbit M class stars. Pretty much all of them will be face-locked and thus at least a bit strange.
Even with all this, and with my finger on the scale, the majority of the stars will not host habitable planets. Not even places for sulfur-breathing, silicon-based aliens… No habitable planets around O’s or B’s or A’s. No habitable planets around larger F’s. Even most of the smaller F, G, K and M stars won’t have planets. At a guess, I’d say 30% of the G’s and K’s and 5 or 10% of the M’s. Still pretty shockingly common.
Lots of complex life. Most planets that can have life, will have life. Most planets that can have complex multicellular life will have it. And. Most planets that can have interesting megafauna from bigger than your fist to as big as a brontosaurus or Blue Whale, will have such things. This is just an aesthetic choice on my part.
Intelligent life is rare. This is a universe where finding something smarter than a monkey is important. Finding neolithic tribesmen is monumental. And other starring life forms are few and far between. Humanity has contacted perhaps half a dozen starfaring life forms including itself. Most of these were discovered by far-ranging deep space explorers and remain inconveniently far away. We’ve also encountered about twice that number of species in the Paleolithic-to-2050AD range of technologies. We’ve also discovered about a score of “graveworlds” planets that had once been host to intelligent life that had attained a degree of development between Industrial Age and Late Space Age and then been wiped out. Some of these planets are now barren post-Garden. Some of them are desolated, but still alive. Sometimes the sophonts are extinct, sometimes they are merely uncivilized primitives, scratching at the ruins at the ruins of their former greatness. Sometimes the worlds have recovered and grow lush over the ruins of the dead. In one case, the survivors managed to fight way back into space and are now one of the very few starfaring species. These places are grim sobering reminders that humanity in 2448 is still slowly negotiating it’s way over the boundary between scarcity and plenty and we haven’t been following the path of those who successfully made the transition. The good news is, we’ve reached the stars and spread wide, perhaps wider than the conflagration of our future doom might reach.