The inner planets are the planets in our Solar System that orbit closest to the sun – Mercury, Venus, Earth, and Mars. Beyond location, there are a number of other differences that separate them from the outer planets. For starters, the inner planets are composed mostly of rock, while the outer planets are gas giants. Generally, inner planets are smaller and denser than their counterparts, and few or no moons and no rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.
Of the inner planets, Mercury is the closest to our Sun. This small planet looks very much like the Earth’s Moon and is even a similar grayish color. It has no moons of its own and, comprised mostly of iron and nickel, is one of the densest planets in the Solar System. Mercury has a very thin atmosphere, which means that it is burning during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out.
Venus is the hottest planet in the solar system. Its atmosphere is a thick cloud composed of various gases, including carbon dioxide, which traps the heat and transforms the planet into a raging storm. Venus is often called the morning star because, with the exception of Earth’s moon, it’s the brightest object in the sky. Like Mercury, it also has no moon of its own.
Earth is the third inner planet and the one we know best. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight in, allowing life to survive. Like Mercury, the Earth has an internal magnetic field. An our moon, the only one we have, is comprised of a mixture of various rocks and minerals.
Mars is the final inner planet, also known as the Red Planet due to the rust of iron-rich materials that form the planet’s surface. Mars’ thin atmosphere has led some astronomers to believe that water that once existed there might have actually leaked into space. The planet has two small moons called Phobos and Deimos. One of Mars’ best-known features is its network of canals and canyons that led many to believe there was even a civilization on this planet.
Beyond Mars are the four outer planets: Jupiter, Saturn, Uranus, and Neptune.
Universe Today has numerous articles on the inner planets including the planets of the inner solar system and some on the outer solar system
Astronomy Cast also has episodes on all of the inner planets including this one about Mercury.