I don't know of any planet with exactly 21 known moons.
The best answer is probably "Uranus" with 27 moons (natural satellites).
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Uranus has thin dark rings and 27 known satellites, including Miranda, Ariel, Umbriel, Titania, and Oberon.
The blue-green planet with 27 satellites is Uranus. It is the seventh planet from the sun in our solar system and has 27 known moons or satellites.
Moons are the heavenly bodies that revolve around a planet. Moons are natural satellites that orbit a planet in a regular and predictable manner, influenced by the planet's gravity. The Earth's moon is an example of a natural satellite that orbits our planet.
The largest planet with no moons is Mercury. It is the smallest planet in our solar system and does not have any natural satellites orbiting around it.
Saturn is the only planet that has 3 rings and 16 satellites .....