Another way to create a habitable environment is to find a planet that can be terraformed rather "easily," using resources brought to the planet on an armada of ships [source: University of Maryland].
In our solar system, Mars has often been mentioned as the best possible candidate for terraforming. Some people estimate that it would cost about $2 to $3 trillion and take 100-200 years to make Mars' atmosphere dense enough, and the planet's temperature hot enough, to get water melting in the Martian poles and in the soil, thereby creating seas. Once that happened, an additional 200 to 600 years might pass before microbes and algae greened up the planet [source: Lilico].
One possible way to transform its desert-like landscape would be to pump the Martian atmosphere full of greenhouse gases. These gases, such as carbon dioxide, would trap heat from the sun, which would raise the planet's temperature, eventually allowing fauna and flora to thrive [source: NASA]. To do this, Earth would have to send solar-powered factories to Mars to pump the heat-trapping gases into the atmosphere. Once Mars heats up, its polar ice caps, which are composed of carbon dioxide, would begin to melt, warming the planet to a scorching 158 degrees F (70 C). It would be so hot that ice trapped on the planet would melt, providing water and oxygen (one of the molecules in water), the main ingredients for life as we know it.
Another way to heat up the red planet would be to direct gigantic mirrors to reflect the sun's radiation onto the polar ice caps. That would melt the carbon dioxide in the caps and start the greening process, too [source: NASA].