Terrestrial Planets

Our solar system consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These eight planets are categorized into two groups: the terrestrial planets, which include Mercury, Venus, Earth, and Mars, and the jovian planets, which include Jupiter, Saturn, Uranus, and Neptune.

The list of terrestrial planets also includes Ceres―a terrestrial dwarf planet. While Ceres does find a place in the list of terrestrial planets of our solar system, Pluto doesn't as it is mainly made up of ice.

The term terrestrial planet is used to define a planet which is predominantly composed of silicate rocks. These planets are also referred to as the rocky planets or telluric planets. The term terrestrial is derived from the Latin word terra meaning Earth, owing to the fact that these planets are Earth-like.

In our solar system, the four inner planets are terrestrial, as opposed to the four outer planets, which are jovian planets or gas giants. Basically, these terrestrial planets can be further categorized into two groups: the silicate planets, predominantly composed of silicate compounds, and the carbon planets, predominantly composed of carbon compounds.

These planets typically have a metallic core, most often composed of iron, while the surrounding mantle is made of silicate. They have a range of geographical structures, including mountains, volcanoes, and canyons.

These planets have a secondary atmosphere, which is attributed to internal volcanism and comet impact, as opposed to the jovian planets, which have a primary atmosphere. Among the various terrestrial planets, only the Earth has an active hydrosphere, while it is assumed that Mars may have water in the form of ice.

These planets either have a few moons or no moons at all. Unlike, the jovian planets, they do not have rings around them. Their magnetic field is low as compared to that of the jovian planets.

Mercury: At one-third the size of the Earth, Mercury is the smallest terrestrial planet in the solar system. Its proximity to the Sun ensures that it has maximum surface temperature among the various planets.

Venus: In terms of size, planet Venus is roughly as large as the Earth. However, the fact that this planet is near the Sun makes it impossible for lifeforms to survive there. The temperature of Venus reaches as high as 484°C during the day. The dense atmosphere containing copious amount of carbon dioxide contributes to this high temperature by producing the greenhouse effect.

Earth: This planet is typically characterized by its ability to support lifeforms, and a large part of the credit for this goes to its gaseous atmosphere. The atmosphere of the Earth primarily contains oxygen, which is essential for respiration, and water vapor, which helps in maintaining the optimum levels of temperature.

Mars: At half the size of the Earth, Mars is the only terrestrial planet which has two moons: Phobos and Deimos. Owing to its considerable distance from the Sun, its temperature ranges between -125°C to 22°C.

Even though most of the planets found outside our solar system are gas giants, scientists don't rule out the existence of other terrestrial planets. Efforts to develop telescopes capable of directly imaging these extrasolar planets are underway, so their discovery should not take you by a surprise.