Since inter-stellar space travel is unlikely to be developed any time soon, writers of mundane science fiction are struck with our own Solar System. Consequently possible locations are limited to Earth, the Moon, Mars, the giant planets and their moons, and artificial space habitats.
Save from our own planet all other celestial bodies in our Solar System are hardly habitable. For instance in order to live on Venus we either need massive terraforming or floating cities (fortunately breathable air is a lifting gas in Venus’ atmosphere). Other bodies suffer from the fact that their gravity is either too low or to great (Jupiter, Saturn) for humans.
Artificial rotating space habitats seem to be the most realistic option for off-world human settlements. These have the advantage that they can be located anywhere. It would make sense to locate the first human space colonies close to Earth, since in case of an emergency the settlers could evacuated to Earth in a matter of days. In comparison it takes six to eight months to go to Mars and a round-trip mission would take about two years.
And if we got more experience with space colonization, we could try for more distant location such as Mars or even floating cities in the atmospheres of Uranus or Neptune. But planets in other stellar systems are still too far.
There is, however, reason to take seriously the possible existence of Earth-sized planets in our own Solar System. Computer calculations performed by astronomer Rodney Gomes suggest that the orbits of certain Kuiper belt objects could be explained through the existence of a planet-sized object.
Gomes’ calculations do not tell much about this planet. His planet could be a planet with the size of Neptune located at 1,500 astronomical units from the Sun or a Mars-sized object at 53 AU. An Earth-sized planet would be located between these extremes. (An astronomical unit is equal to the (average) distance between the Sun and Earth; one light-year is approximately 60,000 AU.)
Of course, this planet proposed by Gomes is very hypothetical, but currently it could not be ruled out completely. Hence such planet could be an intriguing subject for writers of mundane science fiction. Though it would be quite remote, it could be reached in reasonable time.
See also: New Planet Found in Our Solar System? at National Geographic