Which planet in our solar system is the only one not named after a Greek or Roman god?

Answer

Earth's name comes from Old English and Germanic words for ground. The other seven planets carry names from Roman mythology.

Context

The English word Earth descends from Old English eorthe and related Germanic words that meant ground or soil. It was a word for the dirt underfoot long before people understood that the ground was a planet, which is why it never took a deity's name the way the other planets did. The Latin name Terra and the Greek Gaia do belong to goddesses, but they are not the source of the English name.

The other seven planets follow a naming habit that began with the five bright wanderers seen without a telescope. Mercury, Venus, Mars, Jupiter and Saturn carry Roman names that replaced earlier Greek ones, such as Hermes and Aphrodite. When William Herschel found a new planet in 1781, astronomers eventually settled on Uranus, the Latinized form of the Greek sky god Ouranos, after a proposal from Johann Bode. Neptune, found in 1846, took the name of the Roman sea god, continuing the pattern.

Pluto, discovered in 1930 and reclassified as a dwarf planet in 2006, also has a Roman underworld name, so the convention extends beyond the eight planets. The statement in the question is therefore a point about naming history rather than a rule of astronomy. Earth's name simply predates the idea that our home is one planet among others, and nobody renamed it afterward.