Just as on Earth we protect some areas of land, we will need to protect areas in future. There might be planets where terraforming and Industrial colonization will be halted. Those will be known as reservations or reservation planets.
Virgin planets[]
Every human activity pollutes the cosmos in a way or another. Airless worlds are the most exposed. The footsteps and rover tracks we left on the Moon will last for millennia. Even on Mars, the trails of our rovers are still visible. Imagine what a large scale mining project will do. Terraforming will dramatically alter the landscapes on any planet.
Just as today, many ecologist movements will rise and will try the best to stop terraforming and industrial colonization. This implies that there will be a fight between terraformers and industrial corporations on one side and ecologists on the other side. Nobody will win the battle, but concessions will be made.
Some asteroids, some moons and some planets, with rare features (like geological activity and unique environments) will not be terraformed and will not be targeted for mining. These worlds will become tourist destinations.
Another category is formed of planets and moons that have little economic value, like planets Where terraforming is impossible and with without important resources.
It is possible that at some point these worlds will become terraformed or mined. It all depends on the feasibility of such an action and on how strong ecological movements will be.
Alien life planet[]
It is said that the holy grail of Astronomy is finding Alien life. Some argued that life or sub-life can exist on a high variety of celestial bodies, including rocky planets, planets with subsurface oceans, giant planets, even asteroids and nebulae. There is no way to know until we find real life.
Given the huge importance of such a discovery and the fact that from all miracles in the Universe, the rarest seems to be life, once we find alien life, the area must be protected. We should take care not to contaminate the environment. Only research missions will be allowed to visit.
If the planet has non-intelligent life or has sub-life, we should protect it and its surrounding space. We must make sure that no runaway ship will fall to the planet and will contaminate it with Earth germs. The protected area will include the planet, its moons and surrounding space (its Hill sphere).
If the planet has intelligent life but the intelligent creatures have not reached space age, we should protect the whole solar system. We can send research ships to explore, but we must not interfere with the local civilizations. Research activity will look like spy missions.
However, if we find a civilization that has reached space age, we should try to establish diplomatic relations with it. We should recognize their solar system as their property and consider them as a sovereign state.
On Earth, we have evidence of maybe millions of species of life, but only one of intelligent life in the way that humans can understand. This shows us that we have a significant chance of finding life somewhere, but the chance of finding intelligent life is very small, if it exists at all.
Earth life reservation planet[]
As humans will get far away from Earth, maybe in other galaxies, they will want to preserve the Earth life somewhere. There are many planets throughout the cosmos, that can become habitable after terraforming, but no planet that is just like Earth. However, there will be many places suitable for various Earth species. We can easily find a planet suitable for some species, another for other species and so on, to save the Earth biodiversity.
The importance of these reservations is huge. They will provide us with life needed to terraform other worlds. If one day a terraformed planet is destroyed, we still have where to find life to make it habitable again.
Indigenous reservations[]
This will happen later, during Galactic Colonization. Let's suppose that a group of people moved far away, into a remote galaxy. They terraformed a planet or just established a few colonies. At some point, the bloom colonization from the Milky Way will reach them. The galaxy will be transformed.
In order to avoid any conflict, given the huge size of a galaxy, places that were colonized before galactic colonization will have a special status of a reservation. It will be like the reservations made in America for the Amerindians. Local population will have internal autonomy, they will have their own government and the galactic federation will not be allowed to interfere inside the reservation. However, reservations will not be independent states. They will not have external autonomy.
No settlers from the outside world can move to a reservation without accept from the indigenous government. No industrial corporation from the outside can enter a reservation without approval from the indigenous government.
Museums[]
This is a special kind of reservation. A museum will keep evidence of important events during terraforming and colonization. Also, museums will remember future people of important events like declaration of independence of a new cosmic state, a cosmic disaster(like a nuclear war or a supernova) and not only.
When should a planet become a museum? In case of an intergalactic federation, the first terraformed planet could become protected, to preserve historical buildings of the first settlers. In a dictatorship, the planet containing remnants of the founder of the dynasty could also become a museum. And after a galactic war, if a planet was devastated far beyond rehabilitation, it should also become a museum, so that people will see what happened and will never do the same things again.
Smaller reservations[]
The same types of reservations could form on limited area of a single planet. They will cover up to a few square km or almost all the surface of a planet.
Larger reservations[]
As shown above, in case of alien life, the protected area will be large, even up to the size of a solar system. We might see reservations covering multiple solar systems and even galaxies.
However, as human population increases, reservations might shrink or vanish, after a period when limited human activity will be permitted. The number and fate of reservations depends on a delicate equilibrium between those who want to colonize and those who want to protect. There is not a formula or a clear line to be drawn to what planet should be declared reservation and what planet should be inhabited.