Which planet is the second smallest?

Mars is the second-smallest planet in the Solar System, after Mercury.

Mars has an equatorial diameter of about 6,792 kilometers, making it substantially smaller than Earth but larger than Mercury. Its position in the size ranking is based on the eight planets recognized by the International Astronomical Union. Neptune, Earth, and Venus are all larger, so none of those alternatives can be the second-smallest planet.

Mars is the fourth planet from the Sun and belongs to the inner group of terrestrial, or rocky, planets. Its reddish appearance comes largely from iron minerals in its surface materials, which oxidize—effectively rust—in the presence of the planet’s ancient and present-day surface chemistry. Mars also has two small moons, Phobos and Deimos.

A common mix-up comes from confusing orbital order with size order. Mars is fourth from the Sun, not second. Another is forgetting Mercury: once Mercury is identified as the smallest planet, Mars follows as the next smallest. Pluto is not included because it is classified as a dwarf planet, not one of the Solar System’s eight planets.