Dwarf planet
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.
Outline of Mars
Mars – fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.