Origin, Evolution and Structure of the Solar System, Physics tutorial


The development and evolution of the Solar system is estimated to have started in around 4.568 billion years ago having the gravitational collapse of a small portion of a giant molecular cloud. Most of the collapsing mass collected in the centre, making the Sun, whereas the rest flattened to a proto-planetary disk out of which the planets, asteroids, moons and other small Solar System bodies made.

Evolution of the Solar System:

The Solar System has developed considerably as its initial formation. Most of the moons have made from circling discs of gas and dust around their parent planets, whereas other moons are assumed to have made independently and later been captured via their planets.

Still others, as the Moon of Earth, might be the result of huge collisions. Collisions between bodies have occurred constantly up to the present day and have been central to the evolution of the Solar-System.

The planets positions have often shifted and planets have switched places. This planetary migration now is assumed to have been liable for much of the early evolution of the Solar System.

In around 5 billion years, it has been anticipated that the Sun will cool and expand outward to most of the times its current diameter (becoming a red giant), prior to casting off its outer layers as a planetary nebula, and leaving behind the stellar corpse termed as a white dwarf.

In the far far-away future, the gravity of passing stars steadily will whittle away at the Sun's retinue of planets. Several planets will be destroyed, others expelled into interstellar space. Eventually, over the course of trillions of years, it is probable that the Sun will be left with none of the original bodies in orbit around it.

Sun's Interior:

The Stars are made up from clouds of gas and collapse beneath self-gravity. The collapse is stopped via internal pressure in the core of the star. Throughout the collapse, the potential energy of in-falling hydrogen atoms is transformed to kinetic energy, heating the core. As the temperature rises up, the pressure rises up to stop the collapse. The heat from the collapse is ample for the Sun to shine, however merely for a timescale of 15 million years (termed as the Kelvin-Helmholtz time). As the Sun is 5 billion years old, then it should be generating its own energy instead of shining on leftover energy from the stellar formation (such as Jupiter).

The structure of Sun is found out by some relations or physical theories. These comprise:

1) Hydrostatic equilibrium: The fact that pressure balances the self-gravity.

592_Sun In Hydrostatic Equilibrium.jpg

2) Thermal equilibrium: The amount of energy produced equivalents the amount radiated away.

61_Thermal Equilibrium in the Sun.jpg

3) Opacity: The resistance of the solar envelope to the flow of photons that is, how fast the energy is liberated. 

We can notice the chemical composition of the earth's crust, and we recognize from geophysical data that the crust is much less dense than the earth as a whole. The crust is therefore not representative of the mantle and core.

Model of the Solar Interior: The Sun

The composition of the Sun is determined from the spectroscopy studies. It comprises of around 70% hydrogen, 28% helium and approximately 2% everything else. The sun signifies 99.9% of the net mass of the solar system. From orbital computations its average density is 1.4 gm/cm3.

2214_composition of the Sun Interior.jpg

The sun experiences nuclear reaction to generate energy that is received on the surface of earth. The nuclear fusion reaction is termed as a proton-proton or hydrogen-burning reaction - multiple phases as illustrated in the figure shown below.

1) Two hydrogen nuclei (protons) join to form a deuteron - a hydrogen isotope of the atomic weight 2. To do this they release a positron (that is, positive electron) forming one proton into a neutron.

2) A second proton joins by deuterium to form helium-3.

3) Two helium-3 nuclei fuse to helium-4 liberating energy 2 protons.

This reaction releases lots of energy that is the energy we obtain from the sun.

982_Formation of the Sun due to nuclear reaction.jpg

The Origin, Evolution and Structure of the Planets:

The commonly accepted model termed as the nebular hypothesis, was first introduced in the 18th century by Emanuel Swedenborg, Immanuel Kant and Pierre-Simon Laplace. Its successive expansion has interwoven a variety of scientific disciplines comprising astronomy, physics, geology and the planetary science.

As the dawn of the space age in the year 1950 and the discovery of additional solar planets in the year 1990, the models have been both challenged and refined to account for latest observations. There are numerous theories regarding the formation of the solar system. Though two which are most famous shall be illustrated:

Aristarchus's concept

The first step in the direction of the theory of Solar System formation and evolution was the common acceptance of heliocentrism that positioned the Sun at the centre of the system and the Earth in orbit around it. This idea had gestated for millennia, however was broadly accepted only by the end of the 17th century. 

Nebular hypothesis

In this theory, the entire Solar System begins as a large cloud of gas which contract beneath self-gravity. The conservation of angular momentum needs that a rotating disk forms by a large concentration at the centre (that is, the proto-Sun). In the disk, planets form.

The present standard theory for the formation of Solar System, the nebular hypothesis, has fallen into and out of favor as its formulation via Emanuel Swedenborg, Immanuel Kant and Pierre-Simon Laplace in the 18th century. The most important criticism of the hypothesis was its apparent incapability to describe the Sun's relative lack of angular momentum whenever compared to the planets. Though, as in early 1980s studies of young stars have illustrated them to be surrounded by cool discs of dust and gas, precisely as the nebular hypothesis predicts that has led to its re-acceptance.

1489_Formation of Planets round the Sun.jpg

Understanding of how the Sun will carry on evolving needed an understanding of the source of its power. Arthur Stanley Eddington's proof of Albert Einstein's theory of relativity led to his realization that the energy of Sun comes from the nuclear fusion reactions in its core.

In the year 1935, Eddington went further and proposed that other elements as well might form in the stars. Fred Hoyle elaborated on this premise by arguing that evolved stars termed as red giants formed numerous elements heavier than hydrogen and helium in their cores. If a red giant finally casts off its outer layers, such elements would then be recycled to make other star systems.

Pre-Solar Nebula:

The nebular hypothesis maintains that the Solar System made up from the gravitational collapse of a fragment of a huge molecular cloud. The cloud itself had a size of around 20 pc whereas the fragments were around 1 pc (that is, three and a quarter light-years) across.

The extra collapse of the fragments led to the development of dense cores 0.01 - 0.1 pc (2,000 -20,000 AU) in size. One of such collapsing fragments (termed as the pre-solar nebula) would form what became the Solar System. The composition of this area having a mass just over that of the Sun was approximately the same as that of the Sun nowadays, with hydrogen, all along with helium and trace amounts of lithium generated by Big Bang nucleosynthesis, forming around 98% of its mass. The remaining 2% of the mass comprised of heavier elements which were made by nucleosynthesis in earlier generations of stars. Late in the life of such stars, they emitted heavier elements to the interstellar medium.

The study of ancient meteorites discloses traces of stable daughter nuclei of short-lived isotopes, like iron-60, which only form in exploding, short-lived stars. This points out that one or more supernovae occurred close to the Sun while it was forming. Shock wave from a supernova might have triggered the formation of the Sun via creating areas of over-density in the cloud, causing such areas to collapse. As only massive, short-lived stars generate supernovae, the Sun should have formed in a large star-forming area which generates massive stars, possibly identical to the Orion Nebula. The study of the structure of the Kuiper belt and of anomalous materials in it propose that the Sun formed in a cluster of stars having a diameter between 6.5 and 19.5 light-years and a collective mass equal to 3,000 Suns. Some simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produce anomalous orbits noticed in the outer Solar System, like detached objects.

Due to the conservation of angular momentum, the nebula spun faster as it collapsed. As the material in the nebula condensed, the atoms in it started to collide with increasing frequency, transforming their kinetic energy to heat. The centre, where most of the mass collected, became rousingly hotter than the surrounding disc. Over around 100,000 years, the competing forces of gravity, gas pressure, magnetic fields and rotation caused the contracting nebula to flatten to a spinning protoplanetary disc having a diameter of around 200 AU and form a hot, dense protostar (that is, a star in which the hydrogen fusion has not yet started) at the centre.

At this point in its evolution, the Sun is assumed to have been a T Tauri star. The study of T Tauri stars illustrate that they are frequently accompanied by discs of pre-planetary matter having masses of 0.001 - 0.1 solar masses. Such discs extend to some hundred AU. The Hubble Space Telescope has noticed protoplanetary discs of up to 1000 AU in diameter in star-forming areas like the Orion Nebula and is instead cool, reaching just one thousand Kelvin at their hottest.

In 50 million years, the pressure and temperature at the core of the Sun became so great that its hydrogen start to fuse, forming an internal source of energy which countered gravitational contraction till hydrostatic equilibrium was accomplished. This marked the Sun's entry to the prime stage of its life, termed as the main sequence. Major sequence stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a major sequence star nowadays.

Formation of Planets:

The different planets are thought to have made from the solar nebula, the disc-shaped cloud of gas and dust left over from the formation of Sun. The presently accepted process by which the planets made is termed as accretion, in which the planets started as dust grains in the orbit around the central protostar. Via direct contact, such grains made up into clumps up to 200 meters in diameter, which in turn collided to make bigger bodies (that is, planetesimals) of around 10 km in size. Such gradually increased via further collisions, growing at the rate of centimeters per year over the course of the next some million years.

Terrestrial Planets:

The inner Solar System, the area of the Solar System within 4 AU, was too warm for the volatile molecules such as water and methane  to condense, therefore the planetesimals which formed there could only form from compounds having high melting points, like metals (such as iron, nickel and aluminum) and rocky silicates. Such rocky bodies would become the terrestrial planets (such as Mercury, Venus, Earth and Mars). These compounds are pretty rare in the universe, consisting of only 0.6% of the mass of the nebula; therefore the terrestrial planets could not grow very large.

If the terrestrial planets were forming, they remained immersed in a disk of gas and dust. The gas was partly supported through pressure and so didn't orbit the Sun as fast as the planets. The resultant drag caused a transfer of angular momentum, and as an outcome the planets steadily migrated to new orbits. Models illustrate that temperature variations in the disk regulated this rate of migration, however the total trend was for the inner planets to migrate  inward as the disk dissipated, leaving the planets in their present orbits.

Jovian Planets:

The gas giants (that is, Jupiter, Saturn, Uranus and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool adequate for volatile icy compounds to remain solid. The ices which made up the Jovian planets were more plentiful than the metals and silicates which made the terrestrial planets, allowing the Jovian planets to grow massive adequate to capture the hydrogen and helium, the lightest and most rich elements. Planetesimals beyond the frost line accumulated up to four Earth masses in around 3 million years.

Nowadays, the four gas giants include just fewer than 99% of all the mass orbiting the Sun. Theorists assume that it is no accident that Jupiter lays just beyond the frost line. As the frost line accumulated huge amounts of water through evaporation from in-falling icy material, it created an area of lower pressure which raised the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier which caused material to accumulate fast at around 5 AU from the Sun. This surplus material coalesced to a large embryo of around 10 Earth masses, which then started to grow rapidly via swallowing hydrogen from the surrounding disc, reaching 150 Earth masses in just other 1000 years and lastly topping out at 318 Earth masses. Saturn might owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was fewer gas available to consume.

T Tauri stars such as the young Sun have far stronger stellar winds than more stable, older stars. Uranus and Neptune are assumed to have made up after Jupiter and Saturn did, whenever the strong solar wind had blown away much of the disc material. As an outcome, the planets accumulated little hydrogen and helium - not more than 1 Earth mass each. Uranus and Neptune are at times termed to as failed cores. 


The Sun: The central star in the Solar-System.


Mercury: The very first planet in the Solar System that is as well the smallest planet in the Solar System. Mercury takes just 88 days to complete an orbit around the Sun.

Venus: The second planet from the Sun. In numerous ways, Venus is a twin to our own Earth. It has almost similar size and mass as Earth, however the thick atmosphere on Venus makes surface temperatures hot adequate to melt lead. Venus is as well unusual as it rotates in reverse to all the other planets.

Earth: Our home planet, the third planet from the Sun. Earth is the single planet in the Solar System identified to support life. This is due to the reason that we are at just the right distance from the Sun so that our planet does not get too hot or too cold. We as well comprise one moon - the Moon.

Mars: Mars is the 4th planet from the Sun and is much smaller and colder than the Earth. Temperatures on Mars can go up to 20-degrees C, however dip down to -140-degrees C in the northern winters. Mars is assumed to be the best candidate for life elsewhere in the Solar System. Mars consist of two small, asteroid-shaped moons:  Phobos and Deimos.

Ceres: Ceres is the very first dwarf planet in the Solar System, and the biggest member of the asteroid belt. 

Jupiter: Jupiter is the fifth planet from the Sun, and the biggest planet in the Solar System. Jupiter has as much mass as 2.5 times all the rest of the planets joined - nearly all of this mass is hydrogen and helium; however, scientists think it consists of a solid core. Jupiter has at least 63 moons.

Saturn: Saturn is the sixth planet from the Sun, and is famous for its beautiful system of icy rings. Saturn is nearly as big as Jupiter, however it consists of a fraction of Jupiter's mass, so it consists of a very low density.

Saturn would float if you could find out a tub of water large adequate. Saturn consists of 60 moons at last count.

Uranus: Uranus is the seventh planet from the Sun, and the first planet discovered in the modern times; however, it is now possible to see with the unaided eye. Uranus consists of a total of 27 named moons.

Neptune: It is the eighth and final planet in the Solar System. Neptune was only discovered in the year 1846. It consists of a net of 13 known moons.

Pluto: Pluto is not a planet any more. Now it is just a dwarf planet. Pluto consists of one large moon, termed as Charon and then two smaller moons.

Eris: The subsequent dwarf planet in the Solar System is Eris that was only discovered back in the year 2003. However, it was because of Eris that astronomers decided to re-categorize Pluto as a dwarf planet. 


Moons have come to exist around most of the planets and many other Solar-System bodies. Such natural satellites originated by one of three possible methods:

1) Co-formation from the circum-planetary disc (that is, only in the cases of the gas giants)

2) Formation from impact debris (given a large adequate impact at a shallow angle)

3) Capture of a passing object.

Jupiter and Saturn include a number of large moons, like Io, Europa, Ganymede and Titan, that might have originated from discs around each and every giant planet in much similar way that the planets made up from the disc around the Sun. This origin is pointed out by the large sizes of the moons and their proximity to the planet. Such characteristics are not possible to achieve by capture, whereas the gaseous nature of the primaries make formation from collision debris impossibility. The outer moons of the gas giants tend to be small and contain eccentric orbits having arbitrary inclinations. These are the features expected of captured bodies.

Moon ringing system:

The evolution of moon systems is determined by the tidal forces. A moon will lift up a tidal bulge in the object it orbits (that is, the primary) due to the differential gravitational force across diameter of the primary. Whenever a moon is revolving in the similar direction as the rotation of planet and the planet is rotating faster than the orbital period of the moon, the bulge will continuously be pulled ahead of the moon. In this condition, angular momentum is transferred from the rotation of the primary to the revolution of the satellite. The moon gets energy and steadily spirals outward, whereas the primary rotates more slowly over time.

The Earth and its Moon are one illustration of this configuration. Nowadays, the Moon is tidally locked to the Earth; one of its revolutions around the Earth (presently approx 29 days) is equivalent to one of its rotations about its axis, therefore it always illustrates one face to the Earth. The Moon will carry on receding from Earth, and Earth's spin will carry on to slow gradually. In around 50 billion years, if they survive the Sun's growth, the Earth and Moon will become tidally locked to one other each will be caught up in what is termed as a 'spin-orbit resonance' in which the Moon will circle the Earth in around 47 days and both the Moon and Earth will rotate around their axes in similar time, each and every just visible from one hemisphere of the other.

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