Why is our planet the third planet from the sun?

Answer

If it had formed 10 million kilometers away, you might have asked ‘why not ten million kilometers closer?’

Elsewhere on this site, the general scenario of planet formation is explained. That includes the idea that terrestrial (fixed) planets form within the snowline of the solar system, the distance from the Sun from which many molecules (such as water, ammonia,…) appear as ice rather than gas. Close to the sun it is warmer, and gas is blown out there. In our solar system, that ice line is about five times farther from the Sun than Earth’s orbit.

In our solar system, all terrestrial planets are indeed within the snowline. But there are four of them, and there must be one the third. But why exactly four, and why exactly at those distances, that is in a sense a coincidence. Not really coincidence in the sense that it is not the effect of a cause, but coincidence to the extent that it depends on the rather arbitrary initial conditions of the system. Your question is a bit like ‘why did it rain hard yesterday in the village next to mine and not here?’

To make it even harder. We have meanwhile learned – through the study of other planetary systems – that after their formation planets can still change place, due to the interaction with the other planets and also with the many small fragments that still roam around in planetary systems. Again, we can only predict the outcome of those processes in statistical terms, but we do notice that the effects can be quite drastic.