Question

What are terrestrial planets? Mention the common characteristics of them.

Answer 1

Recall that terrestrial planets are earth-like planets in the context of their internal structure and composition. In other words, we know that the earth consists of a metallic core and a rocky silicate mantle layer. Think of which other planets in the solar system share this kind of property with Earth. Note that all terrestrial planets in our solar system lie between the Sun and the Asteroid belt that exists just before Jupiter, from where the gas giants begin.

Complete answer:
The term terrestrial is derived from the Latin word “terra”, which means earth. Terrestrial planets can thus be defined as the planets that are earth-like, in terms of their structure and composition. They are also known as telluric or rocky planets since they have earth-like, rocky soil structures.
Terrestrial planets have a solid planetary surface in contrast to the large gaseous planets which are made up of combinations of different physical states of hydrogen, helium and water.
Terrestrial planets are of two types:
1. Solar terrestrial planets: These are the terrestrial planets of our Solar System. They are all the planets that lie ahead of the asteroid belt, namely, Mercury, Venus, Earth and Mars. Earth is the largest solar terrestrial planet and is the only one known to have an active hydrosphere.
2. Extrasolar terrestrial planets: These are the terrestrial planets that lie outside of our Solar System and orbit around a star that isn’t our Sun. Examples: Kepler-10b, Kepler-20b, Kepler 36b and many more.
All Solar terrestrial planets have a common structure that entails a central metallic core composed mostly of iron, with a surrounding mantle made of silicate. They also have canyons, craters, volcanoes and other structures depending on tectonic activity and erosive capability.
In contrast with the outer giant planets that have just a primary atmosphere (that they captured from the solar nebula during planet formation), terrestrial planets have additional secondary atmospheres (generated by debris from comet impact or volcanic gassing).
Terrestrial planets are also known to have few or no moons. Mercury and Venus have no moons, while the Earth has one moon and Mars has two (Deimos and Phobos).

Additional Information:
A parameter called the “uncompressed density” of a terrestrial planet characterizes its metal content. It is measured in $g.cm^{-3}$. A greater uncompressed density indicates a greater metal content. The uncompressed density follows a decreasing trend as the distance of the planet from the Sun increases: Mercury ($5.8\; g.cm^{-3}$), Venus ($4.4\;g.cm^{-3}$), Earth ($4.4\;g.cm^{-3}$), Mars ($3.8\;g.cm^{-3}$). This is the basis on which extrasolar planets are classified if they are potentially terrestrial or not.

Note:
Remember that in addition to the major planets, our Solar System also houses numerous minor or dwarf planets. Of these, dwarf planets such as Ceres, Pluto, and Eris are similar to terrestrial planets in the fact that they have a solid surface but are mostly composed of icy materials.
Also, there are multiple possible classes of terrestrial planets in addition to the one we discussed, based on their structure:
Silicate planets – The standard solar terrestrial planet, made of rocky silicate mantle with a metallic (iron) core.
Carbon (diamond) planets – Planets composed of a large metal core and surrounded by carbon-based minerals.
Iron planets – Planets almost entirely composed of iron, and form close to a star.
Coreless planets – Planets that have a silicate rock layer but no metallic core. They are the opposite of the iron planets, and form farther from the star.