Only planet in the solar system not visible to the naked eye

NEPTUNE

This icy giant was the first planet predicted by math before being observed through a telescope.

This clue asks for the planet in our solar system that can't be seen without a telescope. While Mercury, Venus, Mars, Jupiter, and Saturn are all bright enough to spot with the naked eye, Neptune is too distant and dim. The answer is straightforward—no wordplay or tricks—just a direct fact about the farthest major planet from the Sun.