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:
- The physical observation of stars
- The physical properties of the planets of the solar system
- The study of meteorites
- Theoretical calculations
Broadly, four stages can be identified in the process of planetary formation.
- 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.)
- 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
- 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
- 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,
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.