What’s meant by lower heavens that are mentioned in the holy Qur'an?
If we contemplate the galaxies of the universe, we’ll realize that such galaxies consist of billions of stars and such stars adorn the sky, heavens. It could be said, therefore, that all the stars and galaxies that we see around us is within the boundaries of the lower heavens because Allah, the exalted says in the holy Qur’an:
“So He completed them as seven firmaments in two Days, and He assigned to each heaven its duty and command. And We adorned the lower heaven with lights, and (provided it) with guard. Such is the Decree of (Him) the Exalted in Might, Full of Knowledge.”
Fussilat: 41:12
Lights in the above verse means stars and as we know that stars are outside the solar group and the closest star to us in planet earth is 4.3 light years away i.e. all the stars we see adorn the first or lower heavens. As for the other heavens, merely Allah, the omniscient knows their boundaries.