Origin
of the Solar System
The interstellar gas cloud
Over the ages, many theories have been advanced
as to the origin of the Sun and planets. The current theory for the
origin of the Solar System suggests that the formation of the Solar
System began with the collapse of an interstellar gas and dust cloud,
with a mass slightly larger than the present mass of the Sun. This cloud
included material from earlier supernova explosions of stars, as it
was rich in the heavier elements that make up much of the planetary
material in the Solar System.
The collapse of the cloud caused it to rotate, increasing in speed
as the cloud contracted. As the contraction continued, the cloud flattened
and a disc formed around a central core of condensed matter. This configuration,
known as a solar nebulae, would have resembled the shape of a typical
spiral galaxy but was much smaller.
As the gas and dust was pulled in toward the central condensation, the
potential energy of the gas and dust was converted to kinetic energy
as it was pulled towards the centre of the cloud and the temperature
of the nebulae began to rise. Eventually, the temperature became great
enough in the centre of the cloud for nuclear reactions to begin, thereby
giving birth to a star, our Sun.
Formation
of the planets and moons
The material in the disc continued to collide and gradually formed larger
and larger objects. At this stage, the formation of planets split into
two types. Those close to the newly forming Sun, where the temperature
was too high for water and ice to exist, and those further from the
centre at the distances of Jupiter and beyond, where the colder conditions
allowed ice to form.
The planets forming close to the Sun could not hold onto their lighter
more volatile molecules and elements, leaving the heavier molecules
and elements like iron and the silicates to form the inner planets.
There was much less heavier metals and silicate material than water
available in the gas nebulae so the inner planets had less material
from which to form and as a consequence formed much smaller and denser
planets than their Jovian brothers.
The outer Jovian planets forming in the solar nebulae at lower temperatures
where ice could form
were able to acquire much more mass than planets forming closer to the
Sun. Once a planet achieved a large enough mass it could attract and
retain large amounts of hydrogen and helium from the surrounding space.
Hydrogen and helium are by far the most abundant elements in the Universe,
and so planets forming further out in the nebulae become very massive
indeed.
The formation of the inner planets, including the Earth, was complete
within 100 million years of the collapse of the interstellar gas cloud.
The bombardment of planetary and satellite surfaces by debris left over
from the initial formation of the planets continued for another 600
million years, adding small quantities to the original size of the planets.
Such collisions still continue to take place today, but are much less
frequent.
Rings and moons of the gas planets
In the outer Solar System, the availability of ice led to the formation
of giant planets with large
gravitational fields. These planets were so large that each planet started
with its own nebulae of gas and dust rotating the central planet, eventually
forming the large number of satellites or moons. The formation of planetary
rings around Jupiter, Saturn and Neptune is probably due to the large
gravitational fields exerted by these huge planets preventing the formation
of moons near to the planet, leaving rings of gas, ice and dust.
Asteroids and comets
At some point after most of the planets had formed, a sudden increase
in solar activity and a subsequent increase in the intensity of the
solar wind cleared much of the remaining gas and dust out of the Solar
System. Some larger debris remained to form asteroids and comets. The
rapid growth of Jupiter and its large gravitational field apparently
prevented the formation of a planet in the region between Jupiter and
Mars, leaving instead thousands of asteroids.
The comets are representative of the material found in the outer Solar
System. Most are extremely small, but some have been measured to be
up to 400 kilometres in diameter. It is believed that billions of comets
may exist far beyond Pluto.
Pluto and Charon
Pluto and its moon Charon are enigmas and are probably large comet type
objects or moons displaced from their original orbits, by a close encounter
with a larger planet. There is now much excitement about objects beyond
Pluto. Over the last few years, more and more small bodies have been
found orbiting the Sun at very great distances. The existence of these
icy objects was suggested by astronomer Gerard Kuiper some 50 years
ago; because of this, we call this collection of objects the Kuiper
Belt.
The age of the Solar System
It is generally agreed that the Solar System started to form around
5 billion years ago, with the Earth forming some 4.6 billion years ago.
This is much younger than the Universe, which is now thought to be 12
- 15 billion years old.
|