Dante says that the “shoulders” of the hill “glowed” with the planet’s rays. The only celestial bodies bright enough to light up the shoulders of a hill are the Sun and the Moon. The next tercet continues:
and the shining strengthened me against the fright
whose agony had wracked the lake of my heart
through all the terrors of that piteous night.
meaning that the shining of the planet marks the end of the night, and a few tercets further on the narrator says:
This fell at the first widening of the dawn
as the sun was climbing Aries with those stars
that rode with him to light the new creation.
Hence the planet is the Sun. In Ptolemaic cosmology, the Earth is at the centre of the universe, and the seven celestial bodies that appear to move relative to the fixed stars (the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn) are known as “planets”, from ancient Greek πλάνητες meaning “wanderers”.
Dante gave an outline of his cosmology in the Convivio:
And the order of their position is this: The first in the enumeration is that wherein is the moon; the second is that wherein is Mercury; the third is that wherein is Venus; the fourth is that wherein is the sun; the fifth is that wherein is Mars; the sixth is that wherein is Jupiter; the seventh is that wherein is Saturn; the eighth is that of the fixed stars; the ninth is that which is not perceived by the senses save by that movement which was spoken of above; and it is called by many the crystalline heaven, that is the diaphanous, or all transparent. But beyond all these the Catholics assert the empyrean heaven, which is as much as to say the heaven of flame, or the luminous heaven; and they assert it to be immovable, because it hath in itself with respect to every part that which its matter demandeth.
Dante Alighieri (c. 1307). Convivio, book 2, chapter 4. Translated by Philip H. Wicksteed (1903). London: J. M. Dent.
In the Convivio, Dante also ascribed symbolic meanings to the Sun:
Here you are to know that just as it is suitable to treat of an object of sense by means of a thing which is not an object of sense, so it is suitable to treat of an object of the intellect by means of a thing which is not an object of the intellect. And so, since in the literal exposition the discourse opened with the corporeal sun, accessible to sense, we are now to discourse of the spiritual sun, accessible to the intellect, that is God. No object of sense in all the universe is more worthy to be made the symbol of God than the sun, which enlightens, with the light of sense, itself first, and then all the celestial and elemental bodies; and in like manner God illuminates first himself with intellectual light and then the celestial and other creatures accessible to the intellect. The sun quickens all things with his heat, and if he destroys certain things thereby that is not of the intention of the cause, but is an incidental effect; and in like manner God quickens all things in goodness, and if any of them be evil, it is not of the divine intention but must needs be in some way incidental to the progress of the effect intended.
Convivio, book 3, chapter 10.
This explains why the planet from Inferno canto 1 “leads men straight on every road”: because the Sun lights up the road for men to follow, which symbolizes the way that God enlightens men with intellect to enable them to follow the path of goodness.
The identification of Dante’s “planet” with the sun, goes back to the earliest commentators on the poem. Dante’s son Jacopo wrote:
Essendosi raveduto dell’essere istato nella bassezza della detta ignoranza, la quale figurativamente quì valle si chiama, l’animo suo al pie’ d’un colle incontanente pervenne, per lo quale l’altezza dell’umana felicità si considera, la quale coll’intelletto de’ raggi del sole coperta la vide, cioè della chiarezza dell’intellettuale verità, con la quale dirittamente si guida chi co’ lei si rimira.
Having repented of being in the depths of ignorance, which is figuratively called the valley here, his soul reached the foot of a great hill, representing the height of human happiness, which with he saw with the intellect covered with the rays of the sun, that is, with the clarity of intellectual truth, which directly guides whoever contemplates it.
Jacopo Alighieri (1322). Chiose alla Cantica dell’Inferno di Dante Alighieri, comment on Inferno 1.13–18. Firenze: R. Bemporad e figlio, 1915. Via Dartmouth Dante Project.
dissi era da vedere quello che l’autore vuole intendere per lo sole, che sopra il monte vide, e per lo monte. Per li monti intende la Scrittura di Dio spesse fiate gli apostoli: e questo, per ciò che, come i monti son quegli che prima ricevono i raggi del sole materiale surgente, così gli apostoli furono i primi che ricevettero i raggi, cioè la dottrina del vero sole, cioè di Gesù Cristo, il quale è veramente sole di giustizia e luce, la quale illumina ciascuno che viene in questo mondo: e che esso sia vero sole per molte ragioni si dimostrerebbe, le quali al presente per brevità ometto.
I said it was to be seen what the author intends by the sun, which he saw above the mountain, and by the mountain. By the mountains the Scripture of God often means the apostles: and this, because, as the mountains are those who first receive the rays of the rising material sun, so the apostles were the first to receive the rays, that is, the doctrine of the true sun, that is, of Jesus Christ, who is truly the sun of justice and light, which illuminates everyone who comes into this world: and that it is a true sun for many reasons, which at present for brevity I omit.
Giovanni Boccaccio (1375). Esposizioni sopra la Comedia di Dante, comment on Inferno 1.13–18. Milan: Mondadori, 1965. Via Dartmouth Dante Project.