Viewed from space, the planet earth
appears as a round ball that shines
bright and blue. Aristotle, a Greek
philosopher, believed that the earth was at the
centre of the universe and that the moon, sun,
planets and stars orbited around it. Indian
astronomer Aryabhatta, however, believed in
heliocentric solar system. Today, we know that
the sun is a star and the planets revolving
around it came out of it in the distant past. The
sun, its nine planets and the satellites of the
planets constitute the solar system.
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A D V E R T I S E M E N T
Planets differ in size, constituent matter
and temperature. All these characteristics are
related to their respective distances from the
sun. The earth is one of the small inner
planets, along with Mercury, Venus and Mars.
Inner planets comprise of heavy elements and
are closest to the sun. The earth is, however, the
only planet with conditions favourable for the
sustenance of life.
ORIGIN OF THE PLANETS
Nebular Hypothesis : In 1755, German
philosopher Imanuel Kant hypothesised that
slowly rotating cloud of gas, called Nebula, in
some unspecified fashion condensed into a
number of discrete and globular bodies. The
great French mathematician Laplace also
proposed, more or less, the same theory in
1796. According to Kant and Laplace, the
original mass of gas cooled and began to
contract. The rotational speed increased as a
consequence of the law of conservation of
angular momentum. Thus, successive rings of
gaseous material were spun off from the
central mass by centrifugal force. In the final
stages the rings condensed into planets.
Collision Hypothesis : Sir James Jeans and
Sir Harold Jeffreys, well-known scientists of
England, came forward with the collision
hypothesis. According to this hypothesis,
gaseous material was pulled away from the
pre-existing sun by the gravitational
attraction of a passing star. Giant tongues of
matter came out. These tongues broke
into small chunks or planetesimals, which
went flying as cold bodies into orbits around
the sun in the plane of the passing star.
By collision and gravitational attraction,
the larger planetesimals swept up the
smaller pieces, and thus, were formed the
planets.
THE BIRTH OF THE SOLAR SYSTEM
The earth was formed at the same time as the
sun and the other planets of the solar system.
The entire solar system, consisting of the sun,
nine planets, and their satellites form a very
small part of the galaxy that consists of many
stars. Innumerable such galaxies
form the universe. Our galaxy is popularly,
called the Milky Way. The pressure and
temperature at the centre of the Nebula that
produced the solar system became so great,
that it triggered a nuclear reaction. Some of the
hydrogen in the cloud fused into helium,
releasing great amount of energy.
The gaseous cloud exploded to form a
supernova. The explosion caused shock waves
that pushed the denser portions of cloud to
collapse under their own gravity. The dense
core grew larger and hotter as its gravity
attracted more material. In the process, the
hot core developed into a protostar. Finally, the
protostar became the infant sun.
Homogeneous Accretion Hypothesis : The
homogeneous accretion hypothesis maintains
that the earth accreted from an intimate mixture
of silicate particles and metal particles. The
material was assumed to have been formed in
the solar nebula by a complex series of chemical
and physical processes which had occurred
prior to the accretion of planets. According to
hypothesis, accretion of the earth occurred over
a sufficiently long period (107 � 108 years) so that
its gravitational potential energy was efficiently
radiated away and it formed in an initially �cool�
and unmelted condition.
Subsequently, heating by long-lived
radioactive elements occurred, leading to
melting of the metal parts and its segregation
into the core. The major element composition
of the earth and its depletions in volatile
elements can be explained through this
hypothesis. It also explains the approximate
chemical uniformity of the mantle formed after
the formation of the core.
Heterogeneous Accretion Hypothesis :
According to heterogeneous accretion
hypothesis, the composition of the material
from which the earth accreted changed with
time giving rise to layered structure of the
earth. According to this model, the earth
formed �inside out�, with a cool, oxidised, and
volatile rich nucleus and more metal-rich and
devolatilised outer rings.
ORIGIN OF THE SATELLITES (MOON)
A discussion on the origin of the earth will be
incomplete without discussing the origin of the
moon. In this case also there are many
hypotheses. Radiometric dating of the rocks
from the moon show that it was born along
with the earth. It is not younger. Apparently
then, there are two possibilities. It either came
out of the sun in a gaseous form but being too
small was attracted by the earth, or it flung out
of the earth due to a huge meteorite falling on
the earth. The area where the meteorite fell,
a huge hollow was created, which is now filled
up by the ocean and the landmass flung to the
outer space created the moon.
In all fairness, several possible courses for
our planet�s evolution are presented here. In
general, there is fair agreement in the course
of events. Looking for the plausible
explanation to the earth�s evolution is like
making a house out of a variety of blocks. The
constraint is that each block has to fit with the
ones under it and over it and that the whole
structure has to stand up, but there may be
more than one way to build the house.
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AKHAND PRATAP SINGH
