The "Stars" (Planets)
Symbolism, Imagery, Allegory
Like the circles in Hell or the terraces in Purgatory, the stars in Heaven reflect the nature of their inhabitants. For example, the moon is mottled with moon spots, creating an uneven surface. Likewise, the blessed on the moon are inconstant in their vows. Mars is red and its blessed are all warriors who spilled blood for Christ's cause. The Primum Mobile was the first sphere created and it houses the first sentient beings, the angels.
A lot of the planets don't necessarily have physical characteristics that are reflected in their inhabitants, largely because physicality, space, and environment are not major themes in the book. Instead, much of their meaning comes from their names, which are mostly those of Roman gods. Mercury was the messenger god, known for his speed; thus the souls there were hasty in judging what was truly virtuous and went with the easy answer, fame:
[Justinian]: "This little planet is adorned with spirits,
whose acts were righteous, but who acted for
the honor and the fame that they would gain;
and when desires tend toward earthly ends,
then, so deflected, rays of the true love
mount toward the life above with lesser force." (Par. VI, 112-118)
Venus was the goddess of love and her inhabitants were too passionate in life. Jupiter was king of the gods and all his inhabitants were just rulers. You get the idea.