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fa-qaḍāhunna sabʿa samāwātin fī yawmayni wa-ʾawḥā fī kulli samāʾin ʾamrahā wa-zayyannā s-samāʾa d-dunyā bi-maṣābīḥa wa-ḥifẓan dhālika taqdīru l-ʿazīzi l-ʿalīmi
Then He set them up as seven heavens in two days, and revealed [to the angels] in each heaven its ordinance. We have adorned the lowest heaven with lamps, and guarded them. That is the ordaining of the All-mighty, the All-knowing.
— Ali Quli Qarai
Surah Fussilat - Verse 12
12. Then He completed and finished [it was in the form of smoke] from their creation seven heavens in two days and He made in each heaven its affair. And We adorned the nearest heaven with lamps to be an adornment as well as to guard [heavens]. Such is the Decree of Him, the Omnipotent, the Omniscient.
Although God Almighty was Able to create everything at a single instance, but it was because of His Omniscience that He created the heavens in two stages. In terms of Divine Omnipotence, there is no difference between the Creation of the earth or that of the seven heavens as He created them in two stages. Whatever lies within our knowledge concerning celestial bodies and whatever which will be discovered later on are adornments of the lower heaven:
The blessed Verse in question is saying that Divine Decree was acted upon throughout the seven heavens in two days and the Decree of each heaven was Divinely revealed unto it. It is a reference to the fact that each heaven is inclined by nature to act upon its existential Command as per Divine Omniscience and the order of the world. The Arabic word sama’ deriving from sammu designates “high” and it is applied to the atmosphere encompassing celestial bodies. The lights of the heaven are the sun, moon, and all the stars and planets.
They are likened to lights owing to their brilliance and giving off light onto the earth day and night. Specification of lights to the sky of the world indicates that all the planets and stars visible in the sky are those belonging to the sky of our own world which is lower than all the planets since the word:
derives from dunuw (“proximity”) above which lie other heavens whose number is solely known by God Almighty as their innumerability is attested in a number of traditions. It is narrated from Imam Ja’far Sadiq (as) in al-Khisal that the noble Imam said:
“To God Almighty belong twelve thousand worlds each of which is greater than the seven heavens and the seven earths none of which is sentient of other worlds.”
Many other traditions indicate that the world is not limited to the seven heavens and the seven earths.