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?
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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.)
In Digha Nikáya Sutta 14 Mahapadana Sutta & DN 32 The Āṭānāṭiya Discourse, the Supreme Buddha stated that six Supreme Buddhas appeared over 91 world-cycles. The seven Buddhas, including "our" Supreme Buddha, mentioned in DN 14 & DN 32: Vipassi, Sikhi, Vessabhu, Kakusandha, Konagamana, Kassapa, and Gotama.
They were all born in this earth, in the land of the Rose Apple (Jambudipa), in north-central India in the area known then as the Middle Land (Majjhima Desa). This is been said by the Supreme Buddha as He can remember the past up to infinite time such as many Sanvatta Kalpa, Vivatta Kalpa, and Sanvatta Vivatta Kalpa. Buddha not only stated the aeon in which the past six Buddhas arose, but also their social rank, family, life-span, tree under which they became Enlightened, the names of the two chief disciples, the number of the Arahants present at assemblies, the name of the attendant Bhikkhu and the name of the then Buddha’s mother and father and birth place. In accordance with this Sutta, such natural happenings and laws are very common to all Bodhisattas. The main event of all seven Buddhas are the same. Their mothers die within seven days of birth. They all go forth after seeing four signs (an aged man, sick man, corpse and recluse) to attain Enlightenment etc.
Though all seven Buddhas were born in this earth, there are other planes of existence. Various planes of existence of beings are present even though we cannot see most of them. The Supreme Buddha taught that there are nine such different planes of existence. These are known as ‘Sattāvāsa’. Some of these worlds belong to the type called ‘Ēkatta Kāyā’ ‘Ēkatta Sagnyā’. The ‘Ēkatta’ denotes a meaning of ‘single’. This means, all the beings in this type of worlds are similar to each other with respect to their bodies and minds. These worlds also belong to one world which is known as ‘Ābhassara Brahma’. Beings in this world feed on a great deal of happiness. The main reason for such happiness is the absence of variety in these worlds that could induce hate.
On the other hand, the human world is completely different. Humans belong to the type known as ‘Nānātta Kāyā’ ‘Nānātta Sagnyā’, which means different bodies and minds. In human world, we have to face feelings mixed with suffering and happiness. This difference leads them to divide into different groups even due to a small incident and fight with each other. Human world is the lowest plane in which one can realize the four noble truths, that is why all of the Buddhas are born here.