It's unclaimed space. Anyone can plop down a new colony on any ball of rock of they want to. If the colony is established by a government, that planet is now effectively part of that state. The only thing that keeps other people from colonizing that planet as well are the people already on the planet and the people who have funded the colony.
If people want to start a fight over a planet, they absolutely could. But I see little point in it. I established that calculating new routes to new system is very expensive so that players will have to restrict their journeys to the lines on the map. But compared to the overall costs of establishing a new colony, that's a pretty small factor. And it completely pales compared to the costs of armed conflict. Fighting over dirt is just not economical.
If people do fight over planets, and as said earlier, that's assumed to be a very rare thing, then it's about infrastructure and possibly populations. The dirt is the same as in thousands of other systems. It's what people have build on that dirt that's valuable.

Quote Originally Posted by Mechalich View Post
Well, it's huge and not huge at the same time. Austria is geographically tiny. Planets are big. 10 million people spread evenly across a planet would be super isolated. The Earth has ~150 million square kilometers of land area. Even if people utilize only 10% of the landmass, so 15 million sq km, 10 million people would be a population density of 0.66 per sq km. That's less than half the population density of Mongolia, which at 2.07 people/sq km is the least densely populated nation of any consequence on the planet. Mongolia is also an interesting point of comparison, because half of Mongolia's population lives in the capital city of Ulaanbaatar. Many colonies may be organized in similar fashion - a single large metro area with roughly half the population surrounded by cultivation/extraction zones containing the other half. However, I doubt they would be anything like that level of spread out. Instead, you probably have one major city centered around the primary spaceport and then high concentrations of people in the best extraction sites on the planet nearby. The average colony probably has a presence on well below 1% of the average planetary landmass. Austria, for example, occupies 0.55% of the Earth's land area.
Yes, that's my perspective on this as well.

If the world population were spread out evenly, we had 53 people per square kilometer. Not completely desolate, but still lover than three quarters of countries that currently exist.

I think I'll probably be changing the numbers, though. More to 90% of people living on the homeworlds and 99% living in the sectors of the homeworlds. That would still be 1 billion people in frontier sectors and provide average frontier sectors with a 5, 4, 3, and 2 million people colony, plus dozens of outposts with hundreds of thousands of people each.