The solar system is divided into the inner system, containing four small, dense worlds, and the outer system, home to the gas and ice giants. Closest to the Sun, the four inner planets are Mercury, Venus, Earth, and Mars. These worlds form a distinct group based on their location and physical characteristics.
Defining the Terrestrial Planets
These four worlds are collectively known as the terrestrial planets, indicating they possess a solid, rocky surface. They formed in the inner solar system where high temperatures allowed only materials with high melting points, such as silicates and metals, to condense. This composition gives them a relatively high density compared to the gaseous outer planets.
Their internal structure consists of a metallic core, primarily iron and nickel, surrounded by a silicate mantle and crust. All four planets are comparatively small and lack the extensive ring systems or numerous moons characteristic of the outer worlds. Their orbits are also closely spaced, fitting entirely within the distance separating Jupiter and Saturn.
Mercury and Venus
The two innermost planets, Mercury and Venus, represent extremes in planetary environments. Mercury, the smallest planet, has virtually no atmosphere, leading to dramatic temperature fluctuations. The side facing the Sun can reach about 425°C, while the night side plunges to approximately -193°C. Its surface is heavily scarred by impact craters, similar to Earth’s Moon, due to its lack of atmospheric protection.
Venus is shrouded in an extremely thick atmosphere composed almost entirely of carbon dioxide. This dense blanket creates a runaway greenhouse effect, trapping heat and making Venus the hottest planet, with a consistent surface temperature of roughly 462°C, hot enough to melt lead. A unique feature is its extremely slow rotation, where a single day lasts longer than its year, and it rotates backward compared to the other planets.
Earth and Mars
Earth and Mars represent the next pair of inner worlds, both sharing a history that includes the presence of liquid water. Earth is unique for its abundant liquid surface water, covering over 70% of its surface, and for its dynamic geological activity. The planet’s crust is divided into several tectonic plates that move, causing earthquakes, volcanoes, and the reshaping of continents. Earth’s atmosphere, primarily nitrogen and oxygen, supports a protective magnetic field generated by its liquid iron core.
Mars, often called the Red Planet due to the iron oxide dust on its surface, is significantly smaller and colder than Earth. Although it currently has a very thin carbon dioxide atmosphere, evidence suggests liquid water flowed across its surface in the distant past. Today, water ice and frozen carbon dioxide (dry ice) are found in its polar ice caps. Mars is orbited by two small, irregularly shaped moons, Phobos and Deimos, thought to be captured asteroids.
The Asteroid Belt and the Inner System’s Edge
The boundary of the inner solar system is marked by the Asteroid Belt, a vast, torus-shaped region of space between the orbits of Mars and Jupiter. This belt separates the four terrestrial planets from the larger, gaseous outer planets. The region contains millions of irregularly shaped bodies, ranging from dust grains to small worlds, composed of rocky and metallic materials. The gravitational influence of the enormous planet Jupiter is responsible for preventing this material from coalescing into a fifth planet.