Inner Planet
Inner planets, also called terrestrial planets, are the four rocky planets of the Solar System — Mercury, Venus, Earth, and Mars — that orbit within the asteroid belt at distances less than about 1.5 AU from the Sun. They are characterised by solid, rocky surfaces with metallic iron cores, relatively small sizes and masses compared to the gas giants, and slower rotation in some cases due to tidal interactions with the Sun. Their proximity to the Sun means they experience intense solar radiation, shorter orbital periods, and in most cases lack large moons, making them distinct in composition and environment from the outer planets.
Physical and orbital properties of the inner planets
| Planet | Mass (× 10²⁴ kg) | Radius (km) | Distance from Sun (AU) | Orbital Period (days) |
|---|---|---|---|---|
| Mercury | 0.330 | 2,440 | 0.387 | 88.0 |
| Venus | 4.867 | 6,052 | 0.723 | 224.7 |
| Earth | 5.972 | 6,371 | 1.000 | 365.25 |
| Mars | 0.642 | 3,390 | 1.524 | 686.97 |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Outer Planet
Outer planets are the four large planets — Jupiter, Saturn, Uranus, and Neptune — that orbit beyond the asteroid belt at distances greater than 5 AU from the Sun. They are divided into gas giants (Jupiter and Saturn, composed mainly of hydrogen and helium) and ice giants (Uranus and Neptune, containing a higher proportion of icy materials like water, ammonia, and methane). Outer planets are characterised by massive sizes, strong magnetic fields, ring systems, numerous moons, and rapid rotation, and their gravitational influence shaped the architecture of the entire Solar System during its formation.
Solar Wind
Solar wind is a continuous stream of charged particles — primarily electrons and protons — that flows outward from the Sun's corona in all directions at speeds of 400–800 km/s. This plasma flow carries the Sun's magnetic field into interplanetary space, forming the heliosphere, and interacts with planetary magnetospheres to produce phenomena such as aurorae and geomagnetic storms. Solar wind intensity varies with solar activity, and during coronal mass ejections it can temporarily intensify to disrupt satellite communications, power grids, and GPS systems on Earth.
Gravitational Force (Astronomy)
Gravitational force in astronomy is the attractive force between any two masses, governed by Newton's Law of Universal Gravitation, which states that the force is proportional to the product of the masses and inversely proportional to the square of the distance between them. This force is responsible for holding planets in orbit around the Sun, governing the motion of moons, shaping the structure of galaxies, and dictating the trajectories of spacecraft. It is the dominant long-range force at astronomical scales and underlies phenomena from tidal locking to the formation of planetary systems.
The adjective "inner" comes from Old English innera, comparative of inne ("within"). The term "terrestrial" (used synonymously) derives from Latin terrestris, from terra ("earth, land"), reflecting their Earth-like rocky composition.