The Sun and the planets, along with meteorites, asteroids and comets, constitute a community we call the solar system. The space between them is filled with interplanetary dust and gas. Compared to distances on the Earth, the solar system is huge in its dimensions.
The Sun's large family includes nine planets, around 70 moons, countless asteroids and millions of comets. The Sun's mass is almost one thousand times that of all the planets together. It is the center-point of the solar system and also the largest object in it.
The planets can be divided into two distinctly different groups. The planets closest to the Sun, Mercury, Venus, the Earth and Mars are relatively small and consist mainly of rock. Since water exists in liquid form only on Earth, it is the only planet that provides conditions suitable for life.The planets Jupiter, Saturn, Uranus and Neptune are large gas planets, located far from the Sun. Pluto, the furthest planet from the Sun, does not fit into either of the two groups. It seems to have a gas atmosphere, but, unlike the other outer planets, has an interior consisting of rock.
Between Jupiter and Mars there lies a so-called asteroid_belt, in which many thousands of small objects are to be found. However, some of these objects also move outside of the belt.
The mean distance from the Earth to the Sun is approximately 150 million kilometers. This distance is called an Astronomical Unit. Pluto, the most distant planet in our solar system, lies around 39.4 Astronomical Units (AU) from the Sun.
The outer limit of the solar system and the beginning of interstellar space is estimated to be 100 astronomical units out. Furthest out of all are the comets. Their orbits are thought to take them as far as 50,000 astronomical units from the Sun. It is believed that the solar system is not the only such system in space. In 1983, images made by an infrared astronomy satellite revealed that Vega, the brightest star in the northern sky, is surrounded by several objects of unknown size. This makes Vega the only star apart from the Sun that is known to have a system of planets. It lies around 26 light-years from the Earth.
