The Solar System is the stellar system formed by the Sun and the group of celestial objects gravitationally bound to it:
- the eight planets and their 162 known moons,
- five dwarf planets and their six known moons, and
- thousands of small solar system bodies (SSSB).
The Sun is the main component of the Solar System, a star that contains 99.9% of the Solar System's mass. The Sun releases enormous amounts of energy in the form of electromagnetic radiation, which includes visible radiation (light), ultraviolet radiation and infrared radiation.
The planets are the biggest objects orbiting the Sun. Their orbit is almost circular. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky planets; the four outer planets are gaseous giant planets with a small rocky core. All planets but the two first are orbited by natural satellites (usually called "moons"). The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology. The following table shows some major magnitudes measured relative to the Earth:
| ||Diameter (relative to the Earth)||Mass (relative to the Earth)||Distance to the Sun (in AU)||Revolution period (in Earth's years)||Rotation period (in Earth's days)||Moons|
The dwarf planets are also rocky objects orbiting the Sun, smaller than the planets, but bigger than asteroids. There are currently (20-sep-2008) five dwarf planets in the Solar System; the two best known of which are:
- Ceres, formerly considered the Solar System's biggest asteroid;
- Pluto, a trans-neptunian object formerly considered the Solar System's ninth planet;
The SSSBs comprise several types of celestial bodies, the best known of which are:
- Asteroids = planetoids = minor planets. They're the smallest rocky bodies orbiting the Sun. Unlike planets and dwarf planets, they are not spherical, but irregularly shaped. Most of them occupy orbits between the ones of Mars and Jupiter, and make up the asteroid belt. The biggest asteroid is called Vesta.
- Comets, very small icy trans-neptunian objects that orbit the Sun in very eccentric orbits. When a comet approaches the Sun, its icy surface begins to boil away, creating two long tails, one of gas and another of dust, which are often visible with the naked eye. Two well known comets are Halley and Hale-Bopp.