A montage of the planets and the Earth's Moon. These images were taken by a variety of spacecraft. Source: Courtesy NASA/JPL-Caltech
The Sun and the planets formed around 5 billion years ago, from a cloud of gas and dust left behind by dying stars. This was mostly Hydrogen and Helium, but there were traces of the heavier elements which would allow the formation of solid planets and, eventually, life.
The cloud collapsed into a rotating disk, with almost all of the material concentrated in a central bulge, where the Sun would form. At certain distances out from the centre, mass started to accumulate and gravity pulled in more and more material. The planets were beginning to form. They swept the area around them clear of small pieces of dust and rock as they orbited. They also frequently collided with larger rocks, growing in the process. Eventually they reached their current size. The material left became the comets and asteroids.
For another billion years or so stray comets and asteroids continued to bombard the planets. Many planets and moons, such as Mercury and our Moon, still show the scars from this period in the form of craters.
Today we can see the same process taking place around other stars, such as several young stars within the Orion Nebula.
The Solar System Today
The Solar System today consists of the Sun, 8 planets, at least 3 dwarf planets, at least 63 moons and countless other small bodies including asteroids, comets and Kuiper Belt objects. At the centre is the Sun, its gravity dominating the system and locking the others into their particular orbits.
The Solar System Today
Source: IAU/Martin Kornmesser.
The area around the Sun can be divided into several distinct regions:
- In the inner solar system are the four terrestrial, or rocky, planets: Mercury, Venus, Earth and Mars. These are all relatively small and made mostly of rock, so they have solid surfaces.
- Beyond the orbit of Mars, marking the edge of the inner solar system, is the Asteroid Belt, made up of millions of rocks, varying in size from pebbles to rocks the size of Melbourne. The largest of these, Ceres, is now considered a dwarf planet.
- In the outer solar system are the four Gas Giants: Jupiter, Saturn, Uranus and Neptune. These planets are relatively large and are made mostly of gas. They have no solid surfaces, only thick atmospheres.
- Last is Pluto. Until 2006 Pluto was thought of as the ninth and smallest planet but is now considered a dwarf planet. Pluto is neither rocky like the terrestrial planets nor a Gas Giant.
- Beyond the orbit of Neptune is a belt of large icy objects, possibly related to Pluto, called the Kuiper Belt. This region may be the source of short-period comets. There is at least one Kuiper Belt object, 2003 UB313 (nicknamed Xena) that is larger than Pluto and is also considered to be a dwarf planet.
- Far beyond the orbits of the planets, roughly one light-year from the Sun, lies an enormous sphere called the Oort cloud, where long-period comets originate.
As well as the Sun and the Moon, five of the planets (Mercury, Venus, Mars, Jupiter and Saturn) are visible to the unaided eye and have been known since ancient times. The remainder (Uranus and Neptune) and the dwarf planets have only been discovered since the invention of the telescope.