The Origin of the Earth

The Stages of Planetary Formation

Our knowledge of the process of planetary formation comes from a number of diverse sources which include:

Broadly, four stages can be identified in the process of planetary formation.

  1. The gravitational collapse of a star leads to the formation of a core to the gas cloud and the formation of a huge rotating disc of gas and dust, which develops around the gas core. A star such as Beta Pictoris shows a central core of this type, with a disc of matter rotating around the core. Beta Pictoris is thought to be a young star showing the early stages of planetary formation. (See http://www.ifa.hawaii.edu/faculty/jewitt/Origins-bpic.html.)
  2. The condensation of the gas cloud and the formation of chondrules. Chondrules are small rounded objects found in some meteorites. For some general background on meteorites see http://www.ex.ac.uk/Mirrors/nineplanets/meteorites.html. The presence of chondrules gives rise to a special class of meteorites known as chondrites. (For images of chondrites see one or more of the following sites:
  1. The accretion of gas and dust to form small bodies between 1-10 km in diameter. These bodies are known as planetesimals. They form initially from small fragments of solar dust and chondrules by the processes of cohesion (sticking together by weak electrostatic forces) and by gravitational instability. Cohesion forms fragments up to about 1 cm in diameter. Larger bodies form by collisions at low speed which cause the material to stick together by gravitational attraction.

Planetary accretion processes.

  1. More violent and rapid impact accretion. The final stage of accretion has been described as 'runaway accretion'. Planetesimals are swept up into well defined zones around the sun which approximate to the present orbits of the terrestrial planets. The process leads eventually to a small number of large planetary bodies. Evidence for this impacting process can be seen in the early impact craters found on planetary surfaces (http://www.hawastsoc.org/solar/eng/mercury.htm#views, http://www.seds.org/billa/pics/Luna2.jpg).

An explanation of the type given above for the origin of the planets in the solar system is supported by mathematical simulations which show how accretion works by the progressive gathering together of smaller particles into large. It also provides an explanation of the differences between planetary bodies in the solar system and explains the differences between the heavier terrestrial planets close to the sun, and the lighter, more gaseous planets situated at a greater distance.