I really needded to know how this is done, so thank you both for your replys, i really apreciate it
I had heard about LODs, in some games it's very easilly noticiable when LODs are changing, like in Oblivion, for example, or any of these space games
I've been thinking alot about this, and with your replys, i think i have an ideia on how to do it: in space i'll just use the Sphere model, and when entering the planet's atmosphere, previously made terrains will start to be generated in their exact positions. So, at a distance but already in the planet's atmosphere, the terrains will be connected to each other by curving their edges, and this curve will start to be less and less noticiable when aproching the ground
I'll keep on researching, now that i actually know the techniques used.
Thanks again for those replys!