The second-smallest planet in our Solar System is MARS, which has four letters. MARS is the fourth planet from the Sun and is known as the Red Planet due to its reddish appearance. It is the second smallest planet in the Solar System, with a diameter of 6, 779 km, compared to Earth's diameter of 12, 742 km. MARS is also the most explored planet in the Solar System, with numerous spacecrafts sent to study its surface and atmosphere.