There are four terrestrial planets in our Solar System: Mercury, Venus, Earth, and Mars. The Moon is not structurally the same as a terrestrial planet, but it does have some similarities. The terrestrial planets in our Solar System are also known as the inner planets because these planets are the four closest to the Sun. Terrestrial planets are also called rocky planets or telluric planets. They differ from gas giants, the outer planets, in a number of ways.
Terrestrial planets share a number of common features. They are all composed mostly of rock and heavy metals. These planets have a core made of heavy metals that is mostly iron; the core is surrounded by a mantle of silicate rock. Terrestrial planets are much smaller than gas giants. The terrestrial planets also have varied terrain such as volcanoes, canyons, mountains, and craters. Another common feature among the terrestrial planets is that they have few or no moons. Mercury and Venus have none while Earth has one. Mars has two small moons. Also, the terrestrial planets do not have planetary rings like the gas planets do. The atmosphere of planets can vary from Venus’ thick carbon dioxide atmosphere to almost nothing on Mercury.
Mercury is the smallest terrestrial planet in the Solar System. Its atmosphere is very thin, which is why the planet alternates between burning and freezing temperatures. Mercury is also a dense planet and is composed of mostly iron and nickel. Venus is also a terrestrial planet and has a thick toxic atmosphere, which traps the heat making it the hottest planet in the Solar System. It is nearly the same size as Earth.
Earth is the densest of the planets because of its high percentage of iron, even for a terrestrial planet. Its atmosphere is composed mostly of oxygen, nitrogen, and carbon dioxide with traces of other substances. Mars is the fourth terrestrial planet in the Solar System. Its surface is red because of the iron in rocks that has rusted. Although the planet has a thin atmosphere, scientists believe that it used to be thicker. They believe the atmosphere’s thinning caused any liquid water the planet had to evaporate.
Ceres is considered a terrestrial dwarf planet. It has a rocky inner core with an icy mantle with as much as one quarter of the planet water ice. The dwarf planet may also have a thin atmosphere and has varied terrain like the other terrestrial planets. There are also a number of extrasolar terrestrial planets that scientists believe they have identified, although they are much fewer than extrasolar gas giants. Because they are so far away and smaller than gas giants, scientists have been able to find very few extrasolar terrestrial planets.
Universe Today has articles on inner planets and Ceres.
For more information, check out Terrestrial Planet Finder and Ceres.
Astronomy Cast has episodes on all of the terrestrial planets including Earth.