I know. It’s sad. Depressing really. Humans will never build a city off our planet Earth.
I say this because of distances. Even if we were to find a planetary candidate, it would take hundreds or thousands or more years to go one way to visit it just to see if the candidate is really like we need, what to speak of building a city.
Distances in space are unimaginably big. Let’s say the nearest star is 4 light years away. There is no guarantee there is life over there, but to travel that 4 light years will not take 4 years, or 40 years. At the fastest speed ever achieved it would take at least 150 years each way.
Suppose we wanted to take a closer look at the Trappist-1 system, 40 light years away. It is the next best candidate for life. That would take a mere 1,500 to 2,000 years to go one way just to see if anything really was habitable, what to speak of building a city.
Sadly, this is indeed science fiction. We want it to happen, so badly, but it never will happen. At most we might leave some foot prints on Mars and send some robotic systems to the moons of Jupiter and Saturn, but that’s it. Long before our sun blows up, life on Earth will be extinguished.