The adaptability of organisms in different varieties of planets is described in the Brahma-saṁhitā as vibhūti-bhinnam, i.e., each and every one of the innumerable planets within the universes is endowed with a particular type of atmosphere, and the living beings there are advanced in science, psychology, etc., according to the superiority or inferiority of the atmosphere. Vibhūti means "specific power," and bhinnam means "variegated." Scientists who are attempting to explore outer space in an attempt to reach other planets by mechanical means must realize that organisms adapted to the atmosphere of the earth cannot exist in the atmospheres of other planets. As such, man's attempts to reach the moon, the sun, or Mars will be completely futile because of the different atmospheres prevailing on those planets. Individually, however, one can attempt to go to any planet he desires, but this is only possible by psychological changes in the mind. Mind is the nucleus of the material body. The gradual evolutionary progress of the material body depends on psychological changes within the mind. The change of the bodily construction of a worm into that of a butterfly and, in modern medical science, the conversion of a man's body into that of a woman (or vice versa) are more or less dependent on psychological changes.
In the Bhagavad-gītā it is said that if a man, at the time of death, concentrates his mind upon the form of the Personality of Godhead, Śrī Kṛṣṇa, and while so doing relinquishes his body, he at once enters the spiritual existence of the antimaterial world. This means that anyone who trains the mind to turn from matter to the spiritual form of the Godhead by performance of the prescribed rules of devotional service can easily attain the kingdom of God, in the antimaterial sky. And of this there is no doubt.
And in the same way, if one desires to enter into any other planet of the material sky, he can go there just after quitting the present body (i.e., after death). Thus if someone wants to go to the moon, the sun or Mars, he can do so simply by performing acts for that purpose. The Bhagavad-gītā confirms this statement in the following words:
That upon which a person meditates at the time of death, quitting his body absorbed in the thought thereof, that particular thing he attains after death.
Mahārāja Bharata, despite a life of severe penances, thought of a stag at the time of his death and thus became a stag after death. However, he did retain a clear consciousness of his past life and realized his mistake. It is important to realize that one's thoughts at the time of death are influenced by the actual deeds which one performs during his life.
In the Śrīmad-Bhāgavatam (Third Canto, Chapter Thirty-two), the process of entering the moon is described as follows:
Materialistic-minded men, who have no information of the kingdom of God, are always mad after material acquisition of wealth, fame and adoration. Such men are interested in the progressive weal of their particular family unit for their own self-satisfaction and so are also interested in the progress of social and national welfare. These men attain their desired objects by material activities. They are mechanically engaged in the ritualistic discharge of prescribed duties and are consequently inclined to satisfy the Pitās, or bygone forefathers, and controlling demigods by performance of sacrifices as prescribed by the revealed scriptures. Addicted to such acts of sacrifices and ceremonial observances, such souls enter into the moon after death. When one is thus promoted to the moon, he receives the capacity to enjoy the drinking of soma-rasa, a celestial beverage. The moon is a place where the demigod Candra is the predominating deity. The atmosphere and amenities of life there are far more comfortable and advantageous than those here on earth. After reaching the moon, if a soul does not utilize the opportunity for promotion to better planets, he is degraded and forced to return to earth or a similar planet. However, materialistic persons, although they may attain to the topmost planetary system, are certainly annihilated at the time of the cosmic manifestation's dissolution.
As far as the planetary system of the spiritual sky is concerned, there are unlimited Vaikuṇṭha planets in the para-vyoma. The Vaikuṇṭhas are spiritual planets which are manifestations of the internal potency of the Lord, and the ratio of these planets to the material planets (external energy) in the material sky is three to one. So the poor materialist is busy making political adjustments on a planet which is most insignificant in God's creation. To say nothing of this planet earth, the whole universe with innumerable planets throughout the galaxies is comparable to a grain of mustard seed in a bag full of mustard seeds. But the poor materialist makes plans to live comfortably here and thus wastes his valuable human energy in something which is doomed to frustration. Instead of wasting his time with business speculations, he might have sought the life of plain living and high spiritual thinking and thus saved himself from perpetual materialistic unrest.