Here's why:

* Ancient Roman Mythology: Jupiter was the most powerful of all the Roman gods, controlling the sky, thunder, lightning, and law. He was considered the protector of Rome.

* Ancient Greek Mythology: Jupiter is equivalent to the Greek god Zeus. Zeus, like Jupiter, ruled the sky, thunder, and lightning and was considered the king of the gods.

* Planet's Appearance: The planet Jupiter is the largest in our solar system, and it is bright and prominent in the night sky. Its size and brilliance likely made it seem like a fitting namesake for the king of the gods.

So, the planet Jupiter is named after the Roman god of the sky and king of the gods, Jupiter (who is equivalent to the Greek god Zeus).