Earth might be our natural home in the solar system, but many scientists propose terraforming nearby planets like Mars and Venus such that they can be made habitable for humankind.
Venus, which is considerably closer to Earth than Mars, sports an insanely-high surface temperature with crushing atmospheric pressures. These two elements make for an inhabitable world, but some scientists think that we could cool the planet down with solar shields or create high-rising cities that reside in the lower-pressure portions of the Venusian atmosphere.
Mars, on the other hand, lacks many of the necessities of life. Its thin atmosphere and lack of flowing surface water mean that it would take significant resources to make the planet habitable. Some scientists have suggested detonating thermonuclear bombs at Mars' poles to release liquid water, but this idea seems a little far-fetched.
In either case, terraforming Mars or Venus would require significant time and resources. So much, in fact, that it may not be feasible in our lifetimes. That aside, scientists are actively researching new ways to move forward with ideas so that they can happen sooner rather than later.