Solar Nebula Theory
Read These Notes: The solar nebula hypothesis is in one form or another, the most widely
accepted theory of how our solar system formed. Although details may vary, the general theory
is widely accepted since it can explain the properties of the solar system previously described.
Originally a large cloud of dust and gas (75% H and 24% He) became unstable. The
most dense part of the cloud started to collapse under the force of gravity. Gravity pulled
the dust and gas toward the center of the cloud, causing the cloud to take on a spherical
As the size (radius) of the cloud decreased, the collapsing cloud increased its rate of
rotation. The principle on which this conclusion is based is a law of physics called "the
conservation of angular momentum". The effect is similar to that of an ice skater who
must pull in his or her arms in order to increase his or her rate of spin.
The increased rotation of the cloud caused the cloud to change shape. It became
flattened at the rotational poles. Thus the once spherical cloud became disk shaped. This
rotating disk of dust and gas became the "solar nebula". Most of the matter in the
collapsing cloud ended up in a central bulge
As the cloud collapsed, gravitational energy was released, heating the central portion of
the nebula where a protosun formed.
Meanwhile, condensation was occurring within the disk surrounding the protosun.
Because temperatures within the disk varied with distance from the center of the nebula,
different materials condensed at different locations within the disk. Closer to the center,
where temperatures were high, high temperature condensates such as iron and silicates
formed. Farther from the center, where temperatures were low, hydrogen, water and
other low temperature condensates formed.
Collisions between the newly condensed particles caused larger bodies called
planetesimals to accrete. This accretion process continued eventually forming the planets
and moons. These violent and cataclysmic process of planetary formation is today
evidenced by the cratered surfaces of Mercury and our Moon. The recent collision
between comet Shoemaker-Levy and Jupiter also gave us a glimpse at what probably
happened on a much grander scale when the solar system was very young.
The heat generated by these impacts and by radioactive decay of elements resulted in
molten planets which subsequently became differentiated.
The evolving star at the center of the solar nebula passed through a T-Tauri star at which
point it released bursts of energy. These bursts swept light elements such as hydrogen
out of the inner solar system and into the outer solar system where it is swept up by the
distant jovian planets.
The young protosun got hot enough to ignite the hydrogen its core. Thermonuclear
reactions in the core is what distinguishes a "sun" from a protosun.
The terrestrial planets evolved their secondary (Venus and Mars) and tertiary (Earth)