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.