Terrestrial Planets: The Inner Region of the Solar System

From the Lecture Series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., John Hopkins University

The Solar System is centered around the Sun and extends far beyond Neptune. The inner region of the system hosts four planets with Earth-like characteristics: Mercury, Venus, Earth, and Mars. Other than rocks, metals, moons, and liquid metal cores, they share another important feature: water and ice.

Solar system and space objects.
The Solar System has eight main planets and loads of other things.
(Image: Vadim Sadovski/Shutterstock)

The Solar System consists of eight planets orbiting its core – the Sun. The planets’ distances from the sun range from as close as 40% of Earth’s distance to 30 times as much. Thus, the temperatures range from 800°F to -400°F. The four planets closest to the Sun share similar characteristics and are grouped together as the ‘terrestrial planets’: Mercury, Venus, Earth, and Mars.

Beyond the eight planets, there are dwarf planets like Pluto in the Kuiper belt and Ceres in the asteroid belt. Over 200 moons are also part of the System, orbiting their planets. Comets are other members moving toward the Sun in the dust and energy particles floating in the Solar System. As the name suggests, everything in the ’solar’ System depends on the Sun.

Learn more about Earth: How Plate Tectonics Sets Up Life.

The Sun, the Core of Solar System

The Sun has a diameter of almost 1.4 million kilometers, more than 100 times bigger than Earth. This huge star makes up 99.9% of the total mass of the Solar System! Earth is located 150 million kilometers away from it. The distance of other planets from the Sun creates their similar characteristics and groups them together into two main categories: the inner region and the outer region. As mentioned before, Mercury, Venus, Earth, and Mars are in the inner region.

Bright and hot orange sun on a black space background
The Sun is the main part of the Solar System, with 99.9% of the total mass. (Image: Amanda Carden/Shutterstock)

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Mars, the Most Earth-Like Terrestrial Planet

Mars is the planet with similar diameter and gravity-experience to Earth, i.e., if one walks on it, it will feel almost like walking on Earth. However, it has a lower gravity in general. The atmosphere is different from Earth, and its air density is 1% of Earth’s. The whole planet is covered with iron-oxide, which causes the rusty, reddish surface. The red dust is not significant until the dust devil that happens every Martian year.

On Earth, a dust devil occurs when the surface is warmed. It is a whirlwind of spinning dust, less than a meter in diameter, a few tens of meters high, and with a speed of around 80 kilometers per hour. It lasts a few minutes, usually. On Mars, however, a dust devil gets 50 times wider, up to 10 kilometers high, and can last months with the electricity flowing in it due to the movement. Every few Martian years, the dust devil goes across the whole planet.

Mars - the red planet. Martian surface and dust in the atmosphere. 3D illustration
Mars is covered in iron-oxide dust which gives it the reddish color. (Image: Jurik Peter/Shutterstock)

Mars has the largest mountain in the Solar System: Olympus Mons. It is so high that the summit cannot be seen from the base. Valles Marineris is a canyon four times deeper than the Grand Canyon on Earth, and the highest mountain among the four Tharsis volcanoes is 22km high.

Evidence shows that Mars used to be covered with water and that there is still an under-surface liquid water ocean on the planet. There is also frozen water on the surface, and a frozen north pole around 1000 kilometers across.

Learn more about Water on Mars and Prospects for Life.

Mercury, the closest to the Sun

Mercury is a poster child for collisions in the Solar System. It is constantly blasted with high-energy particles and radiation from the Sun, due to its proximity. Temperatures on Mercury can reach 800°F. Its diameter is 40% of the Earth’s and no atmosphere. Life – as known on Earth – cannot exist on Mercury, despite the planet’s frozen water bases.

Mercury has craters near its poles that never see sunlight or heat. Since the planet lacks atmosphere, these permanently-shadowed craters remain immune from extreme heat. As a result, they are cold enough for the ice to last. Besides all of these, Mercury has a global magnetic field that affects the whole planet.

Learn more about Mercury, the Extreme Little Planet.

Venus

Venus is another terrestrial planet, almost as large as Earth, and has the largest volcano in the entire Solar System. There are more than 500 volcanoes on Venus, greater than 20 kilometers wide. Over 160 of the volcanoes on Venus are similar in size, or bigger, than Mauna Loa on Earth. However, these numerous volcanoes do not erupt so often, and Venus is not covered with explosions from these mountains.

Common Questions about Terrestrial Planets

Q: What are the four terrestrial planets?

Terrestrial planets are the four planets in the inner region of the Solar System, closer to the Sun than the rest of the planets. They are Mercury, Venus, Earth, and Mars.

Q: What are the characteristics of terrestrial planets?

Terrestrial planets are planets similar to Earth. They are made up of rocks or metals with a hard surface. Terrestrial planets also have a liquid heavy-metal core, at least one moon, and topological features such as valleys, volcanoes, and craters.

Q: What is the difference between terrestrial and Jovian planets?

Their main difference is their composition, due to their distance to the Sun. Terrestrial planets are covered with solid surfaces, while Jovian planets normally have gaseous surfaces. Mercury, Venus, Earth, and Mars are the terrestrial planets, while the Jovian planets are Jupiter, Saturn, Uranus, and Neptune.

Q: Is Earth terrestrial or Jovian?

Earth is the third planet in the Solar System in terms of the distance to the Sun. The four innermost planets are referred to as terrestrial planets and have characteristics like liquid heavy-metal core, at least one moon, and valleys, volcanoes, and craters. These are all Earth-like features; thus, Earth is a terrestrial planet.

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