Download Formation of the Solar System

Analyzing Theories:
Formation of the Solar System
Throughout the centuries, there have been many theories about the formation of the
solar system. Below are the most commonly accepted theories.
Nebular Hypothesis (1796, Immanuel Kant and Pierre Simon La Place)
The protosun (original or first sun) contracted to form a huge rotating gas cloud
(primeval solar nebula). As the gas cloud cooled, it contracted even more and the size
of the Sun became smaller. The decrease in size caused an increase in the speed at
which the Sun was turning and this increased the accelerating forces coming from the
Sun. When the accelerating force from inside the Sun became stronger than the Sun’s
forces of attracting, a ring separated from the main body of the Sun. As the Sun
continued to contract and cool, more rings of matter, like ripples in a pond, moved out
from the Sun. These gaseous rings gradually condensed into spheres that became
planets.
Tidal Theory (1785, George Louis de Button)
A passing star raised a great tide on the Sun and released the gas that later formed the
planets due to forces of acceleration.
Planetesimal Theory (1900, Forrest Moulton and Thomas Chamberlain)
The tidal action of a passing star released matter from the Sun. This matter cooled and
by forces of acceleration large enough to overcome the gravitational pull of the Sun,
formed planetesimals (small planets). These grew in size by picking up scattered
material nearby and forming the planets.
Combination/Accumulation Protoplanet Hypothesis (1950, G.P. Kuiper)
The protosun formed at the center of the protosolar nebula. Rotation supported this
nebula as it collapsed. Dust grains flew through the nebula and collected into a thin
disk of material. Gravitational instability caused the disk to break up into small particles
(planetesimals). Over time, the planetesimals formed the planets. Near the Sun, only
the rocklike materials could survive the high temperatures. Further away from the Sun,
icy and gaseous material could condense. For this reason, the inner planets are small,
dense, and rocky, while the outer planets are large, less dense, and icy or gaseous.
The thin disk of material from the collapsed nebula caused the orbits of all the planets to
line in a flat plane around the equator of the Sun and all the planets except Venus and
Uranus to rotate in the same direction. This theory also predicts that the moons around
a planet should resemble a solar system in miniature, with all the moons lying within the
equatorial plane of the planet, having circular orbits, and revolving and spinning in the
same direction.
Nebular Theory/Accretion Theory
The Sun and the planets formed from a rotating disk of dust and gas. As the speed of
rotation increased, the center of the disk flattened out and matter became more
concentrated in the center of the disk, where the Sun eventually formed. The growth of
the planets began as solid bits of matter began to collide and clump together through a
process called accretion. The colliding matter formed small, irregularly shaped bodies
called planetesimals. AS the collisions occurred, these planetesimals grew larger. As
they grew in mass, they began to exert a gravitational pull on surrounding objects, thus
adding still more mass and becoming planets. In the inner solar system, close to the
Sun, temperatures were so high that only metals and silicate minerals could form solid
grains. It was too hot for the ices of water, carbon dioxide, and methane to form. In the
frigid outer reaches of the solar system, it was cold enough for these ices to form.
Consequently, the inner planets are primarily rocky while the outer planets were able to
increase in size by accumulating metals, silicates, and gases, eventually becoming
large enough to even attract the lightest gases – helium and hydrogen.