Creation
After the Big Bang, the universe consisted entirely of two elements: hydrogen and helium. Due to collision and gravitational attraction, stars were formed in nuclear fusion reactions between these particles. These stars, which eventually formed galaxies, could explode into supernovas. Approximately 5 billion years ago a supernova exploded, ejecting heavier particles created during the fusion reactions into a cloud of hydrogen and interstellar dust. Compressed under its own gravity, the mixture was heated. At the center our sun was formed, still surrounded by the nebula formed from the remains of the supernova and existing material. [1]
Solid Planet Formation
Over millions of years, the new star at the center of the nebula grew and the chain of fusion and fission reactions restarted. Meanwhile, other components of the disc-shaped cloud began to freeze. Towards the centre, closest to the sun, are compounds of iron, silicon, magnesium, aluminium and oxygen. Over time, these tiny particles coalesced in to larger grains, and eventually small chunks large enough to exert their own gravitational forces. These chunks, called planetisimals, grew via collisions with other bodies in the nebula in a process known as "accretion".
Heat from the collision and gravitational pressure kept these proto-planets molten. As they grew, gravitational forces 'sorted' the particles based on their atomic masses. Heavier elements, such as iron metal, settled in the middle of the growing planet while less dense materials, such as light rock, surrounded them. Already we can see a primitive planet forming; a center of liquid metal with a crust of molten rock. This process of separation is called "differentiation".
Further Reading: Accretion.
This article, by Philip Mathew (2003) and hosted on Physics Post, explains the process of accretion in more detail.
At some point during the accretion process, the young sun ignited. This was energetic enough to remove most of the remaining gas from the protoplanetary disc. The protoplanets and other solid matter remained, separating themselves into stable orbits around the sun.The accumulation process continued, eventually forming the solid planets Mercury, Venus, Earth and Mars. [2]
Further Reading:The Moon.
There are several theories explaining the formation of Earth's only natural satelite, the Moon. A favourite among scientists at the moment is the collision theory. This page introduces the collision theory.
Gas Giant Formation
Gas Giants are large planets formed mainly of non-solid matter. They often have a small solid core, but are primarily composed of a thick atmosphere.
There are two major theories for the formation of gas giants, such as Jupiter, Saturn, Uranus and Neptune. The first is similar to the accretion theory behind the formation of solid planets. Water and other particles which existed as volatile gasses or liquids were frozen into ices and underwent an accretion process. Once these ice-based planets reached a critical mass, their gravity was sufficient to pull in large amounts of gas from the protoplanetary disc, forming massive gas giants.
The second theory is called the "disk instability" model. In a few orbital periods, concentrated areas of hydrogen and helium can pull in surrounding gas through gravitational attraction. This may form spiral arms of gas around the nebula, as illustrated in the image to the right. These arms may colide with eachother, leading to clumps of gas. If dense enough, these clouds of gas can collapse into a gas giant protoplanet, which then attracts dust from the nebula into its planetary core.
Hybrid models also exist; these combine the accretion and disc instability models. [3]
Bibliography.
- [1]: Planet Formation, Stardate Online. Accessed 21 April 2010.
- [2]: Earth's Formation in a Nutshell, about.com. Accessed 21 April 2010.
- [3]: Gas Giants, Solstation.com. Accessed 21 April 2010.