The solar system is divided into two major groups of planets. The inner planets are the four worlds closest to the Sun, forming a distinct region. This group is formally known as the terrestrial planets, a name derived from the Latin word terra, meaning Earth, because they share many Earth-like properties.
Shared Physical Characteristics
The terrestrial planets share a fundamental structure that sets them apart from the gas giants. Their defining characteristic is a composition dominated by silicate rock and metals, giving them a high density compared to the outer planets. Each planet possesses a differentiated internal structure, consisting of a metallic core, primarily iron, surrounded by a mantle of silicates, and a solid crust. This composition results in solid surfaces that can be landed upon.
The inner planets are significantly smaller in size and mass than the gas and ice giants. They generally possess few or no moons; Mercury and Venus have none, while Earth has one large moon and Mars has two very small ones. None of the inner planets has a ring system, a feature common among the outer planets. Although they all have atmospheres, the density and composition vary widely.
Individual Planetary Profiles
Mercury
Mercury is the smallest of the four terrestrial planets and orbits closest to the Sun. Because of its proximity and very thin atmosphere, it experiences the most extreme temperature swings, with surface temperatures ranging from approximately -170 °C at night to 420 °C during the day. Its surface is heavily cratered, similar to Earth’s Moon, and it has a relatively large, iron-rich core that makes it one of the densest planets.
Venus
Venus, the second planet from the Sun, is often called Earth’s twin due to its similar size and mass. However, its environment is dramatically different, featuring a runaway greenhouse effect caused by a thick atmosphere of carbon dioxide. This traps heat, making Venus the hottest planet in the solar system, with surface temperatures high enough to melt lead. Venus also exhibits a slow, retrograde rotation, meaning it spins backward relative to the other planets.
Earth
Earth is unique among the inner planets for its abundant liquid water on the surface and its capacity to support life. Its atmosphere contains a significant amount of nitrogen and oxygen, allowing for a relatively stable climate. The presence of plate tectonics and a powerful magnetic field further distinguishes Earth, helping to maintain its atmosphere and surface conditions.
Mars
Mars, the fourth and final inner planet, is known as the Red Planet because its surface is covered in iron oxide dust. It is considerably smaller than Earth, with a very thin atmosphere, but it has evidence of past volcanic activity and flowing water. Mars has the largest volcano in the solar system, Olympus Mons, and experiences massive, planet-covering dust storms.
Position in the Solar System
The inner planets occupy a compact region of the solar system, all orbiting relatively close to the Sun. Their region extends from the Sun out to the beginning of the Asteroid Belt, which acts as the boundary separating the inner and outer planets. This belt, located primarily between the orbits of Mars and Jupiter, consists of countless small, irregularly shaped bodies.
The formation of the inner planets relates directly to the solar system’s early temperature gradient, defined by the “frost line.” This theoretical boundary, located around 2.7 to 3.2 astronomical units from the Sun during the formation period, marked where temperatures were cool enough for volatile compounds like water vapor to condense into ice. Inside the frost line, where the inner planets formed, only materials with high boiling points, such as silicates and metals, could condense into solid grains.