Inner Planet
The inner planets are Mercury, Venus, Earth, and Mars: the four small, rocky, terrestrial planets that orbit closest to the Sun.
Definition
The inner planets are the four closest to the Sun, Mercury, Venus, Earth, and Mars: small worlds of rock and metal with solid surfaces you could stand on, also called the terrestrial planets. They have rocky crusts, metal-rich cores, high densities, few or no moons, and no rings, and they orbit between $0.4$ and $1.5$ AU, inside the asteroid belt, with periods from $88$ days for Mercury to $687$ days for Mars. They formed inside the frost line of the protoplanetary disk, where it was too hot for water and other ices to condense, so they accreted from silicates and metals, giving densities of $3.9$ to $5.5\ \text{g/cm}^3$ and differentiated interiors with iron-nickel cores. Their thin atmospheres are secondary, released by volcanism and impacts after the primordial gas was lost, and their small masses let light gases like hydrogen escape.
Example
Mars is an inner planet, and rovers like Perseverance drive across its rocky, dusty surface, something impossible on a gas planet like Jupiter. Venus is nearly Earth's twin in size, but its thick carbon dioxide atmosphere traps heat and holds the surface near $465^\circ\text{C}$, while Mercury, closer to the Sun, gets colder at night because it has almost no atmosphere. Earth's mean density is $5.51\ \text{g/cm}^3$ against Jupiter's $1.33\ \text{g/cm}^3$, and escape velocity on Earth is $11.2\ \text{km/s}$ compared with only $5.0\ \text{km/s}$ on Mars, one reason Mars has lost most of its early air.
Key Insight
The inner planets are all close together compared with the outer ones; all four fit inside $1.6$ AU while Neptune sits at $30$ AU. Earth has one moon, Mars has two tiny ones, and Mercury and Venus have none, in contrast to the dozens around each giant. Venus, Earth, and Mars started with similar ingredients and ended up as a runaway greenhouse, a temperate water world, and a frozen desert; distance from the Sun and the ability to hold an atmosphere set their fates.