When writing fiction set in the future you are going to need to do a lot of research into what kinds of technology is realistic. Even after you do this, there will be large holes you will have to fill in with what are little more then educated guesses. This is because there are things that we simply don’t know yet.
This is not bad, this is one of the things that make Science Fiction interesting.
While we may have been sending people to earth orbit for decades there are still two big questions that we don’t know the answers to when it comes to how less gravity effects the human body.
First there has never been a serious injury in orbit. We have no idea how a broken bone will set in orbit, let alone something more complicated.
Secondly while we do know what how humans respond under zero gravity, and obviously under one G. what we don’t know is anything in between. The only time humans have been in anything between zero and one G for any length of time is the Apollo astronauts. Realistically there is undoubtedly some amount of gravity less then one G that we could survive at with enough exercise without any negative side effects of zero G. However we don’t know that number and won’t until we start going out into space on a larger scale then we are now.
The second big one is life.
We don’t know if life is something that is all but inevitable under the right conditions or if we are it for the universe. Right now we can’t even make a good guess at it because we lack data. We also have exactly one example of how life works and we have no idea if life on earth is a common example or not. For all we know earth life is atypical.
Before the first bathysphere it was assumed that life ended where the light stopped. All life on the surface of the earth derives its energy from the sun, either directly or indirectly. It was assumed that sea life worked the same way therefor life could not persist past sunlight. Turns out that assumption was wrong. Several times sense we have found extreme conditions that we assume life can’t survive, then are proved wrong (see extremophiles). We have even found plankton on the outside of the international space station (Found it’s way there from the surface of the ocean).
We are currently in a golden age of planet finding, it seams practically a day does not go by that a new exoplanet is found. However most the planets that we are finding are quite large. If you plotting out the layout of a solar system then you will have to decide how many earth and smaller planets there are and what smaller bodies there are. There are some facts to go off of but you will have to make a large portion of it up.
One last thing, don’t forget the difference between science that has been proven and theories. Just because science accepts something as a given does not by itself mean its true unless its proven.