The smallest planet in the solar system is Mercury. Mercury orbits closest to the Sun and has a diameter of about 4,880 kilometers (3,032 miles), making it smaller than both Venus and Mars. It has a thin atmosphere comprised mainly of oxygen, sodium, hydrogen, helium, and potassium, which results in extreme temperature fluctuations between day and night. Due to its proximity to the Sun, Mercury has a short orbital period, completing a revolution in approximately 88 Earth days. Its surface is marked by craters and resembling the Moon, with significant temperature variances due to its lack of substantial atmosphere.