The living entities are traveling from one planet to another, but it is not that we can go to any planet we like merely by a mechanical arrangement. If we desire to go to other planets, there is a process for going there. This is also mentioned: yānti deva-vratā devān pitṟn yānti pitṛ-vratāḥ (BG 9.25). No mechanical arrangement is necessary if we want interplanetary travel. The Gītā instructs: yānti deva-vratā devān. The moon, the sun and higher planets are called Svargaloka. There are three different statuses of planets: higher, middle and lower planetary systems. The earth belongs to the middle planetary system. Bhagavad-gītā informs us how to travel to the higher planetary systems (Devaloka) with a very simple formula: yānti deva-vratā devān. One need only worship the particular demigod of that particular planet and in that way go to the moon, the sun or any of the higher planetary systems.
Yet Bhagavad-gītā does not advise us to go to any of the planets in this material world, because even if we go to Brahmaloka, the highest planet, through some sort of mechanical contrivance by maybe traveling for forty thousand years (and who would live that long?), we will still find the material inconveniences of birth, death, disease and old age. But one who wants to approach the supreme planet, Kṛṣṇaloka, or any of the other planets within the spiritual sky, will not meet with these material inconveniences. Amongst all of the planets in the spiritual sky there is one supreme planet called Goloka Vṛndāvana, which is the original planet in the abode of the original Personality of Godhead Śrī Kṛṣṇa. All of this information is given in Bhagavad-gītā, and we are given through its instruction information how to leave the material world and begin a truly blissful life in the spiritual sky.