What's it gonna take?

Although its easy to say we should colonize a planet, its a whole different story to actually colonize a planet. Depending on the condition of the planet itself, it could take centuries to terraform the environment to the point where it is identical to Earth's. Lets take Mars as an example. The first requirement, that is out of our control, has already been met, it is at a desirable distance from the sun.

The next requirement is water. Mars already has water at the poles and possible underground, but what if Mars did not have water? A planet that does not have water could be considered an automatic turn off, but it is not impossible to import large quantities of water to said planet. To do this, many comets, which are primarily made of water, would need to be crashed into the planet. The downside to this is that if the comets are too large, they could throw up dust clouds all over the planet making it uninhabitable until they settle. This could take a very long time so it is obvious why a lack of water would be too much of a hassle unless absolutely necessary.

Another concern when picking a planet to live on is the atmosphere. For life to exist, the planet needs to have a thick enough atmosphere that can block radiation while also trapping heat on the surface. Earth does it perfectly, but Mars not so much. Mars has a very thin atmosphere that is thought to have decayed over a long period of time, but no one knows for sure why it decayed or where it went. It is possible that the atmospheric particle could have gone into the ground. If that is the case, then the likely solution would be to place algae or some other type of plant that would be able to survive Mars at the poles. The water at the pole will allow then to thrive, while releasing carbon dioxide that is in the ground into the air. This would also take a long time, but it would help in the integration of plants on the planet. If the particle are no in the ground however, they would need to be import through comets or asteroids. The trick for this is to get the right mixture of elements in the air so that they match Earths, around 78% nitrogen, 20% oxygen, and 2% containing various compounds and elements like carbon dioxide.

The final key requirement is that the planet rotates at a desirable rate. A rotating planet means a liquid core which also means the planet will generally be warmer. It also means that the planet will have a strong magnetic field that is vital to repel solar radiation. The rotation would also help[ in an even distribution of heat from the sun. Mars, fortunately, rotates just about 40 minutes slower than Earth which makes it ideal. Planets that do not rotate could be considered to be dead planets, because they are essentially dead inside since it would just be a solid rock flouting in space. In order to get a planet to rotate, it would require massive collisions at specific locations. If it is done right, the comets could do the trick.

If a planet meets all four of these requirements, then the rest will take care of itself. An abundant water supply and a healthy atmosphere will allow for a water cycle. The right distance, rotation, and a healthy atmosphere will keep the planet at an ideal temperature. The only thing missing at this point if for the first seed to be planted.