Terrestrial planet

A terrestrial planet is a class of planet that is composed primarily of silicate, rocks, or metals. These planets are often called tellurian planets, telluric planets, or rocky planets. In our Solar System, the terrestrial planets are the inner planets closest to the Sun: Mercury, Venus, Earth, and Mars. These planets have solid surfaces, unlike larger gaseous planets, which are made mostly of hydrogen, helium, and water.

Terrestrial planets are important because they are similar to Earth in structure, and scientists study them to learn more about our own planet and the possibility of life elsewhere. Some scientists also consider certain large moons, like Earth's Moon and Io, as possible terrestrial planets because of their rocky composition. The study of these planets involves experts like geologists, astronomers, and geophysicists.

Structure

All terrestrial planets in the Solar System share a similar basic structure. They have a central metallic core, mostly made of iron, surrounded by a silicate mantle.

These planets can also have interesting surface features like canyons, craters, mountains, and volcanoes, depending on whether they have liquids that cause erosion or movements in their surfaces. They have secondary atmospheres created by volcanic activity or impacts from comets, unlike the outer, giant planets which have primary atmospheres captured directly from the solar nebula.

Terrestrial planets within the Solar System

The Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars. These planets are made mostly of rock and metal. Earth is the only one of these planets that has an active surface with water. Some moons, like Jupiter's moons Io and Europa, also have solid surfaces and might be considered similar to terrestrial planets.

During the formation of the Solar System, many small bodies called planetesimals and proto-planets existed. Most of these merged to form the four terrestrial planets we see today. Others, like the asteroids Pallas and Vesta, survived but are smaller and not quite round anymore. Some other small bodies, such as 16 Psyche and 8 Flora, might be the remainders of larger planets that broke apart long ago.

Extrasolar terrestrial planets

See also: Super-Earth, Mega-Earth, and List of nearest terrestrial exoplanet candidates

Most planets found outside our Solar System are big planets, but since 2005, scientists have found hundreds of planets that might be like Earth. Many of these are called super-Earths, meaning they are a bit bigger than our Earth. Some of these planets are made of rock, like Earth, while others might have more gas.

The first planets found around stars were discovered in the early 1990s. In 2005, scientists found the first rocks around a normal star. Since then, many more Earth-like planets have been found, some of which might be able to support life. In 2011, the Kepler space telescope found its first Earth-sized planet around another star. Since then, Kepler and other tools have found hundreds of planets, some as small as our Moon and others a bit bigger than Earth.

Types

Further information: List of planet types

There are several types of solid planets. A silicate planet is made mostly of rocky material with a metal core, like Venus, Earth, or Mars. An iron planet is mostly made of iron, which makes it very dense and small. Mercury is sometimes called an iron planet because it has a big iron core, even though its surface is rocky.

Other types include carbon planets, which are made mostly of carbon and might look like diamonds, and icy planets, which have surfaces covered in ice and can even have oceans inside. While our Solar System doesn’t have examples of all these types, many objects like moons and dwarf planets show similar features.