Like all other animals we are closely attuned to our environment, more so than many of us realize anymore. In these days of electricity we can heat or cool our homes, spend half the night watching TV or reading books, source food from all over the world so nothing is ever out of season, cross distances that took years in days. Yet we cannot escape the environment which shaped us.
I think there are five vital factors we will not easily overcome.
TimeTime is relative. When it's 8am on a Thursday where I live, it's 6pm the previous day in New York. Yet it's the same 'time'. However, the elements that we use to measure time have a profound impact on our bodies. By that I mean the rotation of our planet, and its orbit around the sun. Whether we think the sun is rising where we are, or setting, our bodies are built to expect a 'day' of twenty-four hours or so, because that's how long it takes the Earth to revolve on its axis. What's more, if we are suddenly wrenched from one time of day to another, as happens with long distance air travel, it takes time for our bodies to adjust. (It's called jet lag.)
GravityWe have evolved to suit the amount of force the planet exerts upon is. The advent of space travel and weightlessness has proved how important gravity is to our ability to function. Without gravity our bones lose density and muscles atrophy. Returning astronauts struggle to rebuild their bodies. In science fiction space ships have ways of providing gravity, either by centrifugal force, like the rotating space station in 2001: a Space Odyssey, or via an unspecified means of creating artificial gravity, found in Star Wars, Star Trek and the like. Long periods on a planet with low gravity is sure to have similar effects. Mars and the Moon are obvious examples. Sure, you can bounce around. But what happens if you come back to Earth? Or move on to another planet with much stronger gravity than Earth?
AtmosphereMost of our atmosphere is nitrogen, with twenty-three percent oxygen and a bunch of other gases in smaller quantities, including carbon dioxide. It also has a level of density. There's more of it at lower altitude (see gravity). See what happens to mountain climbers if they climb before becoming acclimatised. Their bodies can't cope. And if that mixture of gases changes past a certain level of tolerance, then what? Sure, we can wear space suits. But that's not ideal if you're colonizing a new planet.
TemperatureHumans exist in an apparently wide range of climates, providing they can find protection from the elements. But the range is actually not that wide in the scheme of things. This article in New Scientist speculates that global warming of only about 11° would render many places on our own planet 'unlivable'.
LightEarth orbits a G class star which emits light towards the red end of the spectrum. We're used to seeing colors in that light. If we lived on a world orbiting a cooler star with redder light, or a brighter star with more bluish light, we'd see colors differently. For example, with light from a red star, blues and greens would be brighter, and reds and oranges more subdued.
Humans are adaptable. That's why the species has been so successful. But even so, we've only ever had to adapt to the extremes of one planet. If humans are to venture to other planets I believe we will have two choices; terraform the planet into another Earth or modify the settlers to cope with the conditions. That would mean physically very different races of humanity occupying different planets.
And here again, SF can offer plenty of examples. Elizabeth Moon's Serrano series has terraformed planets, as does Jack McDevitt's universe in Slow Lightning. A different approach is taken in Moon and McCaffrey's joint effort, Sassinak, where members of the Star Fleet have different body characteristics, depending on which planet they come from.
I admit I don't take that route in my own writing. Like the Star Treks and Star Wars of this world, I simply assume all planets are earthlike, with only small variations in light, heat, time and gravity. I reckon I'm in pretty good company. Come on SF fans and writers, what do you do, what do you prefer? Can you think of other limiting factors? Or do you think I'm pessimistic?