They were named after the characteristics of their gods. Some examples:

• Mercury for its speed.
• Venus for its brightness and beauty.
• Mars for its bloodlike red colour.

There are some interesting factors in the naming of the objects. Jupiter's satellites have naming conventions which consider their orbit - for example ending in e if they are in retrograde orbit.

Source: International Astronomical Union: Working Group for Planetary System Nomenclature

EDIT: 'a' to 'e'

this is somewhat off-topic but i doubt it will see the light of day in any better context.

the Chinese of course have an ancient astrological tradition as well, and named the five planets independently of the Western tradition after the five elements.

• Mercury is the water star(水星)
• Venus is the metal star (金星)
• Mars is the fire star (火星)
• Jupiter is the wooden star (木星)
• Saturn is the soil star (土星)

the precise origins of these names is unknown to me, but the concept of the Five Phases solidified as a framework early in the Han Dynasty (2nd c BC), predating Chinese Buddhism but not Confucianism. early evidence of systematic Chinese astronomy and astrology, however, date back to the Shang Dynasty (17th-11th c BC) -- so surely the planets had names earlier than the Han.

so this is a long way of setting a follow-on question: how did the Chinese come to name their planets, and what names predated the ones in use today?

Follow up question: did the characteristics of the planets they observed influence the name of the god they were name after?

To understand ancient religion, especially ancient Greek religion, you need to understand that their gods were allegorical beings. The question of whether they 'existed' or not is therefore kind of nonsensical.

In Euripides' Bacchae you can see an example of this attitude when Teiresias the seer tries to convince King Pentheus to accept Dionysus as a god.

This new god, whom you ridicule, I am unable to express how great he will be throughout Hellas. For two things, young man, [275] are first among men: the goddess Demeter—she is the earth, but call her whatever name you wish; she nourishes mortals with dry food; but he who came afterwards, the offspring of Semele, discovered a match to it, the liquid drink of the grape, and introduced it [280] to mortals. It releases wretched mortals from grief, whenever they are filled with the stream of the vine, and gives them sleep, a means of forgetting their daily troubles, nor is there another cure for hardships. He who is a god is poured out in offerings to the gods, [285] so that by his means men may have good things.

In other words, by at least one interpretation recognized by Euripides' audience, Dionysus is wine, a gift of the earth which gives men release from grief. The god Dionysus is, in other words, a kind of personification of the intoxicating power of wine. The play is about whether Dionysus should or should not be recognized as a god, but it also makes plain that the Greeks had a very complex and sophisticated notion of what it meant for a god to 'exist.'

By a similar token, the planets Mercury, Venus, Jupiter, Mars, etc. were not so much gods as that they participated in allegorical aspects of their respective gods. Venus is bright and beautiful, Mercury is fast-moving and near the sun, Mars is red like blood, etc. The Greeks believed that all of existence participated in the life of the gods in this way, understanding the qualities and features of all they could observe in the world and within the human heart and mind as aspects of different gods.

Dumb question: how did they differentiate planets from stars?

Edit: Thanks everyone for answering, It seems rather obvious to me now. I'm feeling stupid now, why didn't I think of that earlier. Anyway, it's good to know that the word Planet comes from Greek and means "Wanderer".

hi! you'll find more info in these previous posts

What was the relationship between Ancient Greek religion/gods and the planets named for them? Did they think Venus, Mars, etc were the actual gods?

Did ancient Greeks know that Olympos was the highest mountain in Greece when they placed their gods there? Did they know Jupiter is the largest planet when they named it after the king of gods, Zeus?

Why were the planets named after the gods the ancients named them after? More specifically, why was Saturn given his own planet instead of a more... prominent god?

Another follow up. Any reason why the Ancient Egyptians paid more attention to constellations such as Orion, Leo, and Taurus and not the planets so much? I can understand the relationship between the Nile and the Milky Way for them.

In the same line of thought how did the Pre-Columbians view the planets? It seems they paid more attention to solar/lunar eclipses and precession.

Both civilizations are known for their Astronomy, but you don't hear much about their accuracy of planetary movements. It's generally the longer cycle accuracies that are talked about.

Want to read a very interesting theory about early history that touches on the names of the gods and planets? Read this:

http://abob.libs.uga.edu/bobk/bronze.html

Also did the greeks call the planets the names of their gods (same gods as the romans but with different names) in greece was the planet jupiter known as zeus and mars aries, etc?

We know that the Epicureans believed in the physical reality of the gods, just not in revering them as morally relevant. I've never heard whether or not they associated the planets physically with those embodied superhumans. It's an interesting question.

In the Vassal-Treaties of Esarhaddon, King of Assyria, he invokes Ishtar (Venus) in a curse against violators of his word:

May Ishtar, the brightest among the stars, let your wives lie in the embrace of your enemy before your eyes