Why live anywhere else?

 

Earth has a population of more than six billion people. As this number continues to grow, the populated land masses fill up, and it seems that eventually they will all be overcrowded. Certain areas are already showing this overpopulation, such as large American cities including Los Angeles and New York City. Some countries do not suffer from overcrowding due to high population, but to a high ratio of population to resources. As the natural resources of an area are depleted, the quality of life of the people who live there decreases. If the human race is to prosper as a whole, there must be enough resources to support the global population. In order to support our increasing population, we must learn to harness new resources.

http://www.geog.ucsb.edu/~dylan/mtpe/biosphere/topics/pop/population_global_final.jpg

Some scientists are considering the possibilty of learning to inhabit other planets. This plan includes a process called terraforming, which is defined as "transforming a planet into something resembling the earth, especially as regards to human habitation." This is widely thought of as "Science Fiction" - but 50 years ago so was just about everything that is commonplace today.

 

What alternatives do we have? Are there any planets out there that are like enough to earth that they could be so transformed?