If we want to survive long term as a species we will have to be able to inhabit other planets. In a couple of billion years our sun will start running out the fuel that powers it at the moment. When this happens the sun will expand making life on Earth impossible. However this is in many billions of years time and we may not exist as a species then (certainly we would probably have evolved to be very different!)
But there are plans afoot at the moment to try and find out whether humans would be able to travel to and survive on Mars. I think, with enough money and with the whole world working together we could get people to Mars in our lifetimes. Who knows maybe that might lead to a city on Mars in the next few hundred years!