Planets that are defined as “terrestrial planets” share a number of characteristics. Also called “telluric planets or rocky planets,” they must be made up of mostly metals or silicate rocks.
In our Solar System, the terrestrial planets are the ones that are closest to our Sun or the “inner planets.” These planets are Mercury, Venus, Earth, and Mars.
The name for terrestrial and telluric is derived from the Latin words for Earth: Terra and Tellus. They are named so due to the similar composition of the planets to that of Earth. In other words, they are the most Earth-like.
Conditions on the two closest planets to the Sun, Mercury and Venus, are too harsh and extreme to harbor any form of life. However, it is thought that Mars may have once had life and there is a possibility that life may still be on Mars today.
Terrestrial planets have surfaces that are rocky and may also have atmospheric gases.
Beyond just the hard surface and possible atmosphere, there are a number of other characteristics that define terrestrial planets.
One of the most important is that there is a heavy core that is made of mainly iron, with part of the core in a molten state that creates electric currents during the rotation of the planet. Core size may vary in different planets.
If you take a look at earth’s surface we see all kinds of terrain. Earth’s surface has changed over the millennia due to erosion and weather. The other inner planets have surfaces that may have changed as well, but none so drastic as Earth’s.
We often see volcanoes, canyons, craters, and mountains on the terrestrial planets. Earth has tectonic plates as well as erupting volcanoes that change how the surface appears.
Each planet’s atmosphere also plays a role in the surface appearance. The thin atmosphere of Mercury allows it to constantly leave the gravitational field and it has to be constantly renewed. The dense atmosphere of Venus is due to trapped greenhouse gases.
Earth has a dense atmosphere but it is not as dense as that of Venus. Mars has a thin atmosphere and it was once thought to have more atmosphere that escaped into space. The thinner the atmosphere, the more likely the planet will display craters on the surface from impacts.
While Mercury is the closest distance to the Sun, Venus carries the title of being the hottest of any planets in the Solar System. Mercury’s thin atmosphere allows heat to escape, whereas Venus’ thick atmosphere traps the heat.