In the past few years, the hypothesis that earth-like habitable planets orbit nearby stars has been tested with some exceptional astronomy. If you haven't heard of the Kepler Spacecraft or about its discoveries, you can start in 2011 when the first results came in. (See Earth-like planets: How will we know if they can sustain life?) In particular, I started writing stories about one of these theoretically habitable planets. There are, however, huge cosmological questions that I ran into as soon as I tried to set up a plot and describe the setting of even one place on a distant world.
Perhaps the biggest question is whether life can exist on other planets, and what that requires. Fundamentally, we think of habitable planets where there is just enough, but not too much, gravity and heat to allow liquid water on the surface. Once this criterion is met, then there are more specific qualities we look for, like plate tectonics which is possible if there are sufficient third and fourth period elements (e.g. silicon, iron) on the planet. Also, a magnetosphere is highly desirable to deflect harmful radiation, and that usually means the planet must spin.
Now, all of this we want to establish before getting there, so remote observation or a sophisticated mechanical observer (like the mars rover, but with a much more powerful communications capability) are the best ways to go with this. In fact, I now find the notion of manned space exploration to be anachronistic, with all due respect to star trek, R is for Rocket, and other classic space exploration narratives. Only when we are ready to colonize or we had no alternative would the energy and time be expended to move our fragile and specific living forms to another planet.
Speaking of time and energy, the ways to travel from star to star are currently quite slow. Even in the fastest recorded man-made spacecraft, which was unmanned and used the sun's gravity to accelerate, the trip to Proxima Centauri, the nearest star to our own, would take over 18000 years. This is why most of our knowledge of the universe is through observing light, which makes that same trip in about 4.35 years. Some authors invent a device that magically fixes this limit, like a star gate, wormhole, or some way of traveling through alternate dimensions (hyperspace). These all require the invention of something we don't even have proper theories about to overcome the plot problem of traveling to the stars. More plausible technologies involve bending or "warping" space to accelerate, which is theorized to be possible with some serious limitations (see Dark Energy Spacecraft). There are also cryogenic methods currently available to us that can sustain a single cell for a long time, which may allow humans to make very long journeys, only to be born when we arrive years in the future. Lastly, there are time-traveling technologies, like the TARDIS or bobbles from Verner Vinge stories, which I personally favor a lot because we have only recently made definitive observations about time dilation and how light (the fastest thing in the observable universe) does not have an absolute speed. In essence, you must have a way for things to get places, and perhaps try to make them at least theoretically possible, or visiting other planets is out of the question.
Finally, there is the problem of the great filter, which is a theoretical choke-point in the evolution of intelligent life in the rest of the universe that prevents us from running into aliens, especially as our sun is relatively young compared to many, many others in our galaxy. If life on other planets is possible and interstellar travel is also possible, then why haven't older (on the order of hundreds of millions of years or more) intelligences traveled the stars and inhabited our world? There are a lot of possibilities, and it is here that often a big idea story gets its teeth. Is intelligence a rare adaptation, or does it lead to assured self-destruction? Are there actually aliens living among us even now, though we do not recognize them as such? Were there aliens here before, but due to an ice age or other catastrophe left? Is life in fact rare everywhere and we just got lucky? Are extinction cycles like those on earth regular enough to destroy most intelligent species before they figure out and engineer interstellar flight? Are there a few utterly powerful and malevolent creatures like Lovecraft's great old ones wrecking the older systems and about which we have little experience, being closer to the edge of the galaxy? Or maybe aliens aren't even made of matter that we can perceive, as we've discovered many types we can only detect indirectly and that might comprise more than half of the mass of the universe.
To summarize, there is a lot to design when you make a story about interstellar travel, and what we know provides many more questions than it answers. I for one find it more interesting to make stories that answer these questions, and I love reading the stories that are informed by our real discoveries about the reality we inhabit, which is in some ways stranger than anyone has imagined.
-F. F. White