SB 5.1.8
sa tatra tatra gagana-tala udu-patir iva vimanavalibhir anupatham amara-parivrdhair abhipujyamanah pathi pathi ca varuthasah siddha-gandharva-sadhya-carana-muni-ganair upagiyamano gandha-madana-dronim avabhasayann upasasarpa.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 within the canopy of the sky to receive Lord Brahma and worship him. As he received respect and adoration from the residents of the various planets, Lord Brahma appeared just like the full moon surrounded by illuminating stars. Lord Brahma’s great swan then arrived at the border of Gandhamadana Hill and approached Prince Priyavrata, who was sitting there.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 Gandhamadana Hill. Three great personalities — Priyavrata, Narada and Svayambhuva Manu — were sitting on this hill. According to Brahma-samhita, 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 Sadhyaloka 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.