You can see the planet Jupiter on a clear night, but Jupiter is humongous.

Mars and Mercury are smaller than that, and you can see them, too.

Wait a minute, our moon is even smaller, and you can see that.

Could anything smaller than the moon be visible without a telescope?

Actually, yes.

Though it takes a lot of patience and excellent sky conditions, it is possible to see a tiny object called Vesta.

Although it had a molten interior and volcanoes, Vesta isn't a planet or a moon:

it's an asteroid.

Asteroids are stray chunks of rocky or metallic materials that orbit the sun but were never drawn into any of the planets.