Were all of the historical Buddhas born on today's planet earth? Were Buddhas always born on planet Earth or they can be born on any habitable planets across the universe? 

How does one Buddha gain knowledge of the existence of the the previous Buddha/Buddhas?
These are things we can only speculate. There later works which might have covered these topics though, but cannot vouch for how accurate they are.

Best cause of action by a Buddhist is to practice the Dhamma to gain realisation and wisdom. Who know, perhaps you might get some insight into this also. (Don't make it your goal to find these type of answers, but practice to awaken wisdom.)

Also the Buddha did not encourage to ponder on unproductive question through I am curious my self.

In my opinion. I don't think it is this planet. Last 4 buddhas were in the current world cycle but it is hard to believe it is this planet, but having said this, the social environment would have been similar to Indian during the times of the current Buddha, where there was much activity in search for the truth. (Also a Buddha to be looks if it is conducive time and county before he is born.)