The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are collectively known as the inner planets. They are also referred to as the terrestrial planets. They share a number of fundamental characteristics that distinctly separate them from the much larger, gaseous outer planets of our solar system. These similarities in their makeup, size, density, and orbital dynamics are a direct result of their formation and location near the Sun.
Shared Composition: The Terrestrial Nature
All four inner planets are primarily composed of silicate rocks and metals, which is the defining feature of a terrestrial planet. Their internal structure is generally differentiated, meaning the heaviest materials have sunk to the center while lighter materials form the outer layers.
The core of each inner planet is predominantly metallic, consisting mainly of iron, often mixed with nickel and sulfur. Surrounding this central core is a thick layer called the mantle, which is made of silicate minerals containing iron and magnesium. The outermost layer is the crust, a hard surface made of lighter silicates like basalt and granite.
This rocky composition means that all four planets have a solid surface upon which one could theoretically stand. Due to their formation in the hotter inner solar system, lighter, volatile compounds were blown away, leaving behind these dense, refractory materials.
Relative Size and High Density
The inner planets are characterized by their relatively small physical dimensions when compared to the outer planets. Earth, the largest of the group, has a diameter of about 12,756 kilometers, while Mercury is the smallest at only 4,878 kilometers. This is dwarfed by Jupiter, the largest planet, which is more than ten times wider than Earth.
Despite their small size, the terrestrial planets possess a high average density, a direct consequence of their heavy metallic cores and compacted rocky material. The elements within the inner planets are packed more closely together, resulting in densities far greater than those of the gas giants. Earth is the densest planet in the solar system, with an average density of 5.51 grams per cubic centimeter.
Mercury is also notably dense because its metallic core makes up a massive 60–70% of its total planetary mass. This combination of relatively low volume and high mass-to-volume ratio sets the inner planets apart from the much larger gaseous worlds. The high density is a key metric showing the abundance of heavy elements like iron and nickel in their bulk composition.
Orbital Position and Speed
The inner planets occupy the region of the solar system closest to the Sun, orbiting inside the asteroid belt. Their orbital paths are relatively close to one another, with the entire group confined to the inner portion of the solar system, extending out to Mars’s orbit. This proximity to the central star governs their movement, resulting in very short orbital periods, or “years”.
According to the laws of orbital mechanics, planets closer to the Sun must travel at higher speeds to counteract the stronger gravitational pull. Mercury, being the closest planet, has the highest average orbital speed of all the planets, zipping around the Sun at nearly 47.9 kilometers per second. This high velocity, combined with a short orbital path, gives Mercury the shortest year at just 88 Earth days.
Moving outward, the orbital speed of the inner planets progressively decreases, with Mars having a much slower average speed and a longer orbital period of 687 Earth days. The inner planets complete their orbits much faster than the outer planets. Their orbits also lie relatively close to the Sun’s equatorial plane, making them nearly co-planar.
Minimal Satellite Systems
Another common characteristic is the general lack of extensive satellite systems and the complete absence of planetary rings. Of the four inner planets, Mercury and Venus have no natural moons orbiting them whatsoever. Earth possesses one large moon, which is thought to have formed from a massive impact early in the planet’s history.
Mars has only two very small, irregularly shaped moons, Phobos and Deimos, which are widely believed to be captured asteroids rather than objects formed alongside the planet. This minimal satellite count is in stark contrast to the outer planets, which collectively possess dozens of moons. Furthermore, none of the terrestrial planets possess any form of planetary ring system.
The complete lack of rings is a defining trait, as all four outer planets are encircled by intricate ring structures. The close proximity of the inner planets to the Sun, and the resulting warmer environment, may contribute to the instability of icy or rocky ring particles. This absence of both rings and numerous moons is a shared feature that highlights the different formative environments of the inner and outer solar system.