As Lord Brahma descended on his carrier, the great swan, all the residents of the planets named Siddhaloka, Gandharvaloka, Sadhyaloka and Caranaloka, as well as great sages and demigods flying in their different airplanes, assembled
SB Canto 5
As Lord Brahmā descended on his carrier, the great swan, all the residents of the planets named Siddhaloka, Gandharvaloka, Sādhyaloka and Cāraṇaloka, as well as great sages and demigods flying in their different airplanes, assembled within the canopy of the sky to receive Lord Brahmā and worship him. As he received respect and adoration from the residents of the various planets, Lord Brahmā appeared just like the full moon surrounded by illuminating stars. Lord Brahmā's great swan then arrived at the border of Gandhamādana Hill and approached Prince Priyavrata, who was sitting there.
It appears from this description that there is regular interplanetary travel between the planets of the demigods. Another significant point is that there is a planet covered mostly by great mountains, one of which is Gandhamādana Hill. Three great personalities—Priyavrata, Nārada and Svāyambhuva Manu—were sitting on this hill. According to Brahmā-saṁhitā, each universe is filled with different planetary systems, and every system has a unique opulence. For example, on Siddhaloka, all the residents are very advanced in the powers of mystic yoga. They can fly from one planet to another without airplanes or other flying machines. Similarly, the residents of Gandharvaloka are expert in musical science, and those on Sādhyaloka are all great saints. The interplanetary system undoubtedly exists, and residents of different planets may go from one to another. On this earth, however, we have not invented any machine that can go directly from one planet to another, although an unsuccessful attempt has been made to go directly to the moon.