Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets.Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.Dynamically and physically, objects orbiting the Sun are officially classed into three categories: planets, dwarf planets and small Solar System bodies.
A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighbourhood of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto does not fit this definition, as it has not cleared its orbit of surrounding Kuiper belt objects. A dwarf planet is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite.
By this definition, the Solar System has five known dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Other objects may be classified in the future as dwarf planets, such as Sedna, Orcus, and Quaoar. Dwarf planets that orbit in the trans-Neptunian region are called "plutoids". The remainder of the objects in orbit around the Sun are small Solar System bodies.
The regions (or zones) of the Solar system: the inner solar system, the asteroid belt, the giant planets (Jovians) and the Kuiper belt. Sizes and orbits not to scale, view is tilted.Planetary scientists use the terms gas, ice, and rock to describe the various classes of substances found throughout the Solar System.Rock is used to describe compounds with high melting points that remained solid under almost all conditions in the protoplanetary nebula.[12] Rocky substances typically include silicates and metals such as iron and nickel.
They are prevalent in the inner Solar System, forming most of the terrestrial planets and asteroids. Gases are materials with extremely low melting points and high vapor pressure such as molecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.They dominate the middle region, comprising most of Jupiter and Saturn. Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide,have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.
They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.Together, gases and ices are referred to as volatiles.
