A terrestrial planet is a type of planet distinguished by several characteristics. This type of planet is distinct from the gas giants, which are the other type of planet in the Solar System. The planets are divided evenly between terrestrial planets and gas giants. The terrestrial planets are the four closest to the Sun, which is why they are also known as the inner planets. The gas giants are also called the outer planets.
The term terrestrial planet came from the Latin word for Earth, “Terra.” Thus, terrestrial planets are similar to Earth. The terrestrial planets in our Solar System are Mercury, Venus, Earth, and Mars. Although the Moon is similar to the terrestrial planets, it lacks an iron core. The dwarf planet Ceres is also considered a terrestrial dwarf planet.
As such, a terrestrial planet is mostly solid. Generally, they all have a core made of up of iron or other heavy metals. The terrestrial planets then have a mantle, which is usually composed of silicate rock. Because they are composed of heavy metals and rocks, terrestrial planets are denser than gas giants – which are mostly hydrogen and other gaseous materials – are. Terrestrial planets also have a very diverse landscape with craters, mountains, and valleys.
Terrestrial planets have few or no moons. Venus and Mercury have no moons. Mars has two small moons called Phobos and Deimos after mythological Greek deities. Earth has one satellite, the Moon. Unlike the gas giants, the terrestrial planets have no planetary rings. They are also a fraction of the size of the gas giants. For example, Earth is the largest terrestrial planet, but over 1300 Earths could fit inside Jupiter the largest planet in the Solar System. Scientists have created several categories of terrestrial planets. All of the terrestrial planets in our Solar System are silicate planets, which means they are composed mainly of silicate rocks. A carbide planet is a theoretical terrestrial planet composed of carbon-based rocks and minerals. The term Super-Earths refers to terrestrial planets that are on the high end of the mass range for terrestrial planets.
Although most extrasolar planets are gas giants, astronomers have identified a number of what they believe to be terrestrial planets in other solar systems. Due to how far away extrasolar planets are, it is very difficult for scientists to find terrestrial planets, especially because they are much smaller than gas giants. NASA is in the continuing process of creating imaging technology that will make it easier to identify extrasolar terrestrial planets.
You should also check out comparing the terrestrial planets and the terrestrial planets.
Astronomy Cast has episodes on the terrestrial planets including Mars.