But isn't it going to be a shock for all of us when/if we one day reach the stars and realize that, crap, all this stuff out there isn't ours? We can't even agree about who owns the Moon, so why any of us are under the illusion that somehow we own the Sun, Jupiter, Saturn, all the asteroids and other planets/planetoids, the Kuiper Belt, or even the Milky Way Galaxy and all of its freedom-fighting black holes roaming around eating up planets and other stellar bodies without thinking twice (it's a black hole, after all) is beyond me.
And what if there are aliens out there, all with similar obsessions, or the lack of them, for that matter? Can you imagine us saying "well, actually, Earth is ours, and you can't have it" to an alien species that a) has more firepower than us; and b) has no idea what the hell we're talking about anyway? They'll probably laugh at us, too, when we try to explain to them why the Sun is ours, why we have every right to take that uninhabited planet in Alpha Centauri, and that giant, resource-rich super-Earth in such-and-such star system.
The reality is, the universe isn't owned by anyone. Our claims to ownership over the Earth are the strongest ones we can make, and even those are flimsy at best; there are other beasts on this planet besides us, who share this world, who breathe the same air and eat the same food (technically speaking). The Earth is a place of many creatures and we've already seen what our silly ideas about ownership have done to our particular brand of creature: slavery, violent capitalism, religious wars (physical and psychological), etc. And the purely selfish notion of ownership will produce numerous problems for us in the future. We'll meet aliens who may think like us, or may think differently. Science fiction says we'll have wars, some of them we'll win, and others we'll lose. Will they be worth it?
Maybe we should get over ourselves and think about the bigger picture. Even if we can claim ownership over the Earth, the universe won't care. It's all a pointless gesture, because all it takes is a flick of the metaphorical universal wrist and everything we know to exist will cease to be. Let's do a little growing up. There's nothing wrong with saying that car is yours, but in the grand scheme of things, we have control over very little.