If I send a colonizing ship to an unused planet, and another ship from another player gets there first, what happens to mine?
Does it gets destroyed? Atacks or simply comes back?
This one is pretty simple. If someone colonizes the planet before you get to it, then your colony ship returns. The only thing you lose is the Deuterium to fly your ship to and from the target location.
If someone colonizes before you, then destroys their planet before you get there, your ship still returns home. The Destroyed Planet stays there for approximately 48 hours.
Your colony ship will not attack unless you sent it on an attack mission.