The smallest planet in the world is?

If you mean the smallest of the nine planets, it is Pluto.

Pluto

Pluto is the planet with the farthest distance from the sun and the smallest mass among the nine planets in the solar system. Pluto is 5.9 billion kilometers away from the sun. Pluto has a satellite called Chalong. Pluto is about 2400 kilometers in diameter, which is smaller than the moon, while Chalong is 1, 180 kilometers in diameter, and the ratio to Pluto's diameter is 2: 1 0,000.

If you are talking about the universe, then you are a planet in the asteroid belt.