All of the planets, except for Earth, were named after Roman gods and goddesses.
Jupiter, Saturn, Mars, Venus and Mercury were given their names thousands of years ago. Those were the planets that the ancient Romans could see in the sky without a telescope.
The other planets in our solar system were not discovered until much later, when telescopes were invented. Even then, the tradition of naming the planets after Roman gods and goddesses continued.
Most of the moons and some asteroids are also named after the critters and creatures and gods and goddesses found in Roman mythology. Some of the constellations in our solar system are named after Roman gods as well.
MERCURY Roman Winged Messenger, winged god of travel because he moves so fast
VENUS Roman Goddess of Love, beautiful
MARS Roman God of War
JUPITER Chief Roman God (Jupiter is King of the Gods, an elected position)
SATURN Former Roman God of Agriculture, retired. Replaced by his daughter, Ceres
URANUS Former Roman God of the Sky, retired. Replaced by his grandson, Jupiter.
NEPTUNE Roman Lord of the Sea
PLUTO Roman Lord of the Underworld (Pluto is no longer considered a planet.)
Remembering the 8 planets:
My Very Excellent Mother Just Served Us Nachos
Once upon a time, remembering the 9 planets:
My Very Excellent Mother Just Served Us Nine Pizzas