The diameter of the planet Mercury is 4.87 x 10^3 kilometers. The diameter of the planet Venus is 1.21 x 10^4 kilometers. The diameter of Earth is 1.28 x 10^4 kilometers. The diameter of Mars is 6.79 x 10^3 kilometers. Which planet has the smallest diameter?

The planet with the smallest diameter is Mercury, with a diameter of 4.87 x 10^3 kilometers.