Formation and
History of the Solar System
4600
million years ago, a dark cloud of interstellar gas and dust started to contract under the influence of its own gravity. This contraction shall have been preceded by a supernova that occurred in a region close to that cloud, which
would have been compressed by that explosion. The supernova would have endowed
the early nebula with radioactive elements, which later heated and
activated the planets of the solar system. It should have also reinforced the
cloud contraction process.
A Single
Star
Only one
star formed in the centre of the cloud (not a binary or multiple system),
contrarily to what happens during most of these processes. In a multiple
system, the formation of a planetary system would have been made difficult, given
the orbital instabilities that it would provoke in the eventual planets that
would be formed (unless the stars of the system would be very close or very far
from each other).
The
Transference of Angular Momentum
A big
portion of the angular momentum (the quantity of
rotation of the bodies) was transferred from the centre to the outskirts of the
cloud (the region of the planets), due to the friction between the inner and
outer zones of the nebula or to the "freezing" of the force lines of
a strong magnetic field, which would transfer
the rotation movement from the central source (the Sun) to those outer regions.
The
Formation of the Disk
The solid
(dust) and gaseous components of the nebula, initially mixed in a disordered
way, were gradually segregated.
Given the
rotation of the nebula, this one ended up assuming a flat shape, because this
rotation movement favours a centrifugal force (directed towards the exterior)
that is perpendicular to the rotation axis (counterbalancing the gravitational
force at the equatorial plane), thus not being noticeable along this axis.
It's
possible that this process was initially delayed by the gas turbulence. As the
gas became scarce (absorbed by the giant planets or expelled from the solar
system by the pressure of the intense primordial solar wind) and the bodies
grew up, that friction gradually became less prevailing and the materials were
finally laid at the equatorial plane of the protoplanetary nebula.
The dust disk of the star Beta Pictoris, similar to
what may have been the proto-planetary disk of the Sun.
The
First Planetesimals
It's
thought that the gaseous protoplanetary disk lasted for only 100 000 years.
In the internal regions, the temperature was so high that only the metals
(like the iron and magnesium) and minerals rich in silicates survived and
solidified. In the outer regions, the most volatile materials like the water
ended up forming solid grains too. According to the most popular model for the
formation of the giant planets, it is for that reason that in the outer regions
the solid material availability would be much more expressive and, therefore,
the planetary accreting process would be faster and more efficient there.
How would
that accretion occur? Solid particles with microscopic dimensions would shock
against each other, at low speeds, forming grains under the influence of attraction forces (electrostatic, magnetostatic).
When the dust
layer overtook the critical density, the gravitational attraction between those
particles started to be intense enough for disrupting the disk, which was split
into a sequence of thin rings. These ones were subsequently fragmented into a
big quantity of sub-condensations (dust clusters contracting under the pressure
of their own gravity).
It was
through this way that were born the planetesimals (the first population of
solid bodies that later was aggregated in planets) were born. It's estimated
that only in the internal region there were trillions of those objects
travelling around.
In a primordial stage of the planetary formation,
little grains adhere to each other in order to build accreted objects like this
(D. Richardson, T. Quinn, G. Lake, J. Stadel)
The
First Planets
In a time
range between less than one million and several hundred million years, some large
planets very far away from each other ended up to be formed.
According
to one recently revived model that explains the formation of the giant planets, the collapse of the disk material under its own weight
(as it happens in the case of the stars) is the responsible factor for the
creation of these bodies, which reached the present dimensions some thousand
years later. This model approaches the concept of giant planets to the concept
of brown dwarfs.
However,
the model that still seems to prevail is the one defending that in the region
of the giant planets, the availability of
volatile (icy) material led to the accumulation of both rocky and icy materials
in big bodies, which hold a composition that is moderately similar to the
actual comets'.
When a
critical value equivalent to some (10 to 15) Earth masses was overtaken, these
bodies started to absorb gaseous material too. They started to grow faster and
the process would be completed in 10 million to some hundred million years
after the genesis of the Solar System. The formation of Jupiter suspended the formation of planets in its adjacent
region, which is nowadays called the zone of the asteroids.
Around the
giant planets themselves mini-planetary disks were formed, and they
generated the satellite systems that presently surround them.
The
Future
In about 5
billion years, when the fuel of the nuclear fusion starts to be exhausted in
the Sun's core, our star will begin to expand and to be transformed into a red giant. Mercury and Venus will vanish during this phase, being swallowed and
vapourized in the interior of the Sun. Later, the Sun will expand even further
until it reaches the orbit of Mars.
Finally,
the Sun will expel its outer layers, while the core is transformed into a white dwarf, which will dissipate fossil energy until it reaches
the condition of a black dwarf. It's probable that in this
distant epoch, what are left from the solar system will be just frozen and
inactive corpses of the giant planets.
_