So you want to write hard science fiction. You want to write stories that are consistent with science as we know it today, and perhaps you also want to locate your stories away from the earth—perhaps far from the earth. If you know what that story is, and you know the science you need to write it, close your browser window and do it now. You owe it to your readers, and you owe it to yourself.
If, however, you’re stuck in a rut, you may need to try something different to get inspired. If, like me, you haven’t finished a story in four months (with or without a health concern to justify that situation), you may need to do some research. In that frame of mind, let’s talk about what your off-world setting requires to support life.
The chemistry of life requires a liquid medium to transport chemicals within the living body. In our neck of the woods, that means water. The need for liquid water, however, puts a hard limit on the locations where water-based life could develop.
At any pressure lower than 6.117 millibars (the triple point of water), liquid water can’t exist. Instead, it sublimates directly from a solid state to a gaseous one. For comparison, one earth atmosphere is 1013 millibars. Mars, with its surface pressure of 6.36 millibars, has just barely enough atmosphere to sustain liquid water. The tiny Jovian moon Europa can sustain liquid water because its icy crust holds things down. Most small planets, however, especially small rocky ones, cannot support liquid water, and you’ll have to work hard to justify the presence of living organisms there.
What this means for world building is that you probably need a world with either underground seas or a mass large enough to keep your atmospheric pressure up above the triple point of water. Size isn’t the only factor—Venus, which is smaller than Earth, has a surface pressure 92 times ours—but it’s something to consider.
A related question is the temperature range required for life. Assuming the need for liquid water, biological processes need a local (internal) temperature between 0°C and 100°C at a “typical” earth atmospheric pressure. Traditionally, this is interpreted to mean that your planet needs to be in the “Goldilocks” Habitability Zone, neither too hot nor too cold. Earth is in this zone, mostly because it’s the right distance from our sun, but there are other factors, including geological heating or atmospheric collection and reflection of heat, which can modify this range.
If you’re designing a brand new world for your SF story, you probably want to give it goldilocks habitability. If the star is red or orange, your planet will be close to its sun, and may even be tidally locked. If you have a blue-white supergiant, the planet will be farther away, and the sun may perhaps appear smaller. If other factors affect your world’s temperature, like insulation from thick clouds or tidal heating from the gas giant it happens to be orbiting, these factors will affect the descriptions in your story, and you’d best think them through in advance. There is a lot of room for creativity here, but it’s a lot of work, too.
Pressure and temperature, however, are really just expressions of a bigger need for all living things, and that is energy. Life requires energy to overcome the limits imposed by the second law of thermodynamics, which states that entropy (disorder) in a system will always increase. Living things are massively more ordered than the universe at large, so we can only survive by creating disorder somewhere else. Generally this implies a transfer of energy from a state in which it is concentrated to one where it is dispersed.
Nearly all of the energy available to living things on earth comes from stars, and I don’t simply mean solar energy. Coal, gas, and oil come from the bodies of plants and animals, which themselves can trace their source of energy back to the sun. Wind energy comes from the sun, and even geothermal energy comes from the decay of radioactive isotopes forged in the core of a long-past supernova. The only energy source I can think of that isn’t indirectly solar is tidal energy, and tides get their energy from the same force of gravity that drives fusion in our sun.
When writing SF about life on other planets, it may be useful to ask where the energy comes from, and how it travels through your world to enable the processes of life. It isn’t enough to have a static, warm world for life to exist; we need a dynamic one with an external source of energy that life can tap to survive. This may sound hard, but we’re talking about hard SF here, and I think the most interesting story ideas can be born when a creative mind tries to wrap itself around a difficult issue.
 There may be life based on liquids other than water, but its chemistry would be far different from ours. As a polar molecule, water readily dissolves ions that non-polar solvents like liquid methane or nitrogen could not.
 Yes, there are bacteria at the South Pole, where the temperature peaks around -17°C. Even if these bacteria are biologically active (and the American Society for Microbiology asserts that they can’t be), these bacteria were imports from warmer climes.
 At higher pressures, water stays liquid longer, so a hot super-earth might conceivably have liquid oceans. Unfortunately, the energy that makes it hotter might also cause water vapor to escape, leading to a water-deprived atmosphere like the one on Venus.
Photo credits: Triple point diagram derived from a lecture at the University of Arizona chemistry department. Thermometer by User:Gringer [Public domain], via Wikimedia Commons. False-color image of the sun from NASA via Wikimedia Commons.