When we speak of loka-traya, we refer to the three primary planetary systems - Bhuh, Bhuvah and Svah - into which the universe is divided
SB Canto 5
By the supreme will of Kṛṣṇa, the mountain known as Lokāloka has been installed as the outer border of the three worlds—Bhūrloka, 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.
When we speak of loka-traya, we refer to the three primary planetary systems—Bhūḥ, Bhuvaḥ and Svaḥ—into which the universe is divided. Surrounding these planetary systems are the eight directions, namely east, west, north, south, northeast, southeast, northwest and southwest. Lokāloka 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. Śukadeva Gosvāmī described these universal affairs to Mahārāja Parīkṣit 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 Śukadeva Gosvāmī 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 Śrīmad-Bhāgavatam, 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 Śrīmad-Bhāgavatam. Modern scientists, however, have no information of other planetary systems and, indeed, are hardly conversant with the planet on which we are now living.