Billions of years ago, a cloud of interstellar gas and/or dust (the "solar nebula") was affected by an unknown event, possibly the shock wave from a nearby supernova and started to collapse under its own gravity. As the cloud collapsed, probably taking about 100,000 years, it heated up enough for the dust to vaporize. This collapse compressed the center enough for it to become a protostar. The rest of the dust and gas orbited around it adding to the mass of the forming star. Most of the dust and gas flowed inward, but as the cloud rotates, the centrifugal force from that prevented some of the gas from reaching the forming star. Instead, it formed an "accretion disk" around the star. The disk radiated away its energy and cooled off.

 

The gas cooled off enough to turn some of it back into dust composed of metals, rock and ice. The metals condensed 4.55-4.56 billion years ago while the rock condensed a bit later (between 4.4 and 4.55 billion years ago). The dust and rock particles collided with each other and formed into larger particles. As the particles formed into boulders their gravity (even if it's very small) gave them an edge over smaller particles; it pulled in even more smaller particles and soon the boulders accumulated all of the solid matter close to their own orbit, and are now known as "planetesimals". The planetesimals slowly collided with each other and became more massive. Eventually, after ten to a hundred million years, our solar system was formed with ten or so planets, in stable orbits.

 

The solar system consists of the Sun, the eight official planets, at least five "dwarf planets", more than 130 satellites of the planets and a large number of small bodies (the comets and asteroids).The Sun is at the centre of the Solar System. The inner solar system contains the 'rocky planets' Mercury, Venus, Earth and Mars. The main asteroid belt lies between the orbits of Mars and Jupiter. The 'gas giant planets' are Jupiter, Saturn and the outermost 'ice giants planets' are Uranus and Neptune. Pluto is now classified as a dwarf planet. The solar system is mostly empty space. The planets are very small compared to the space between them.

 

The orbits of the planets are elliptical with the Sun at the centre; all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. Then the Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40,000 km away.

 

Numerous smaller bodies inhabit the solar system: moons of the planets, asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere, comets (small icy bodies) which come and go from the inner parts of the solar system and many small bodies beyond Neptune in the Kuiper Belt. With few exceptions, the planets moons orbit in the same sense as the planets but this is not generally true for comets and asteroids.