Imam ‘Ali (a.s.) said:
هذِهِ النُّجُومُ الَّتى فِى السَّماءِ مَدائِنٌ مِثْلُ الْمَدائِنِ الَّتى فِى الاَرْضِ مَرْبُوطَةٌ كُلُّ مَدِينَة اِلى عَمُود مِنْ نُور
These stars in the heaven are cities like the cities of earth. Each of the cities is connected (with the other cities) through a column of light.1
It is very selfish to suppose that our earth is the only inhabitable planet and millions of other celestial planets are all underutilized, silent and without any inhabitant.
Today, we know of detailed scientific studies into the conditions required for life to exist on other planets which suggest that millions and millions of these may be inhabited and it is likely that many of them may enjoy more developed civilizations because life had started there thousands or millions of years before it did on the earth. Such scientific knowledge did not exist fourteen centuries ago and hence the above tradition of Imam ‘Ali (a.s.) is nothing less than a miracle.