Srimad Bhagavatam Canto 05, Chapter 20, Text 37

SB 5.20.37

sa loka-trayante parita isvarena vihito yasmat suryadinam dhruvapavarganam jyotir-gananam gabhastayo ’rvacinams triḻ lokan avitanvana na kadacit paracina bhavitum utsahante tavad unnahanayamah.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
By the supreme will of Krsna, the mountain known as Lokaloka has been installed as the outer border of the three worlds — Bhurloka, Bhuvarloka and Svarloka — to control the rays of the sun throughout the universe. All the luminaries, from the sun up to Dhruvaloka, distribute their rays throughout the three worlds, but only within the boundary formed by this mountain. Because it is extremely high, extending even higher than Dhruvaloka, it blocks the rays of the luminaries, which therefore can never extend beyond it.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
When we speak of loka-traya, we refer to the three primary planetary systems — Bhuh, Bhuvah and Svah — into which the universe is divided. Surrounding these planetary systems are the eight directions, namely east, west, north, south, northeast, southeast, northwest and southwest. Lokaloka Mountain has been established as the outer boundary of all the lokas to distribute the rays of the sun and other luminaries equally throughout the universe.
This vivid description of how the rays of the sun are distributed throughout the different planetary systems of the universe is very scientific. Sukadeva Gosvami described these universal affairs to Maharaja Pariksit as he had heard about them from his predecessor. He explained these facts five thousand years ago, but the knowledge existed long, long before because Sukadeva Gosvami received it through disciplic succession. Because this knowledge is accepted through the disciplic succession, it is perfect. The history of modern scientific knowledge, on the contrary, does not go back more than a few hundred years. Therefore, even if modern scientists do not accept the other factual presentations of Srimad-Bhagavatam, how can they deny the perfect astronomical calculations that existed long before they could imagine such things? There is so much information to gather from Srimad-Bhagavatam. Modern scientists, however, have no information of other planetary systems and, indeed, are hardly conversant with the planet on which we are now living.
Srimad Bhagavatam Canto 05, Chapter 20, Text 36
Srimad Bhagavatam Canto 05, Chapter 20, Text 38