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The region of the Solar System that lies closest to the Sun, containing the four rocky inner planets Mercury, Venus, Earth and Mars, and extending out to the Asteroid Belt beyond Mars' orbit. By some definitions, the term also includes the Asteroid Belt, so that the entire inner system extends out to a distance of some 3.2 AU from the Sun. In those terms, the radius of the inner system is a little more than a tenth that of the whole Solar System, measured out to the orbit of Neptune.
The most significant objects in this region are the four terrestrial planets. The smallest of these and closest to the Sun is Mercury, at a distance of 0.39 AU (or less than half Earth's distance from the Sun). Moving outward, beyond cloud-shrouded Venus is the largest and most massive of these four rocky planets, Earth. By definition, the mean distance from Earth to the Sun is one Astronomical Unit. Further out, at a distance of some 1.5 AU from the Sun, is Mars, smaller than either Venus or Earth, and the most distant terrestrial planet from the Sun.
Beyond the orbit of Mars is a belt of relatively small bodies known as the Asteroid Belt. This belt extends across a region between about 2.2 and 3.2 AU from the Sun (that is, from the inner edge to the outer is a distance of about one AU, equivalent to Earth's distance of the Sun). Though the term 'inner Solar System' is not formally defined, the Asteroid Belt is usually considered to mark its outer boundary. The belt contains up to two million asteroids of any significant size, though most of these are only a few kilometres across. Some are much larger, however, and the largest, Ceres, has a diameter of nearly a thousand kilometres and is classified as a dwarf planet.
While most of the Solar System's asteroids are concentrated within the Asteroid Belt, there are many other such bodies closer to the Sun within the inner Solar System. Examples of these are the members of the Apollo Group, such as Sisyphus or Geographos, whose orbits pull them in much closer to the Sun, at times intersecting with the orbits of the inner planets. These are generally relatively small objects (all have diameters smaller than 10 km) representing objects captured from the main Asteroid Belt, or the remains of cometary nuclei. Active comets, too, regularly pass through the inner Solar System, and some short-period comets remain within the inner system for most of their orbits.
The Asteroid Belt marks the transition betwen the inner Solar System and the outer Solar System. Beyond the belt are the four giant planets: the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune. Beyond Neptune's orbit (and often intersecting it) is a wide field of objects known of the Kuiper Belt. Some of these Kuiper Belt objects are large enough and massive enough to be considered dwarf planets, with the most prominent example being Pluto. From Neptune's orbit, this outer belt extends for approximately twenty Astronomical Units (more than six times the radius of the inner Solar System) before reaching the so-called 'Kuiper Cliff', beyond which orbiting objects become much rarer.
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