The Solar System UPSC Notes | EduRev

Geography for UPSC CSE

UPSC : The Solar System UPSC Notes | EduRev

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  • The solar system consists of the sun, the eight planets and their satellites, and thousands of other smaller heavenly bodies such as asteroids, comets and meteors. The Solar System is at a distance of about 27,000 light years from the centre of the Milky Way galaxy and is about 5 billion years old.
  • The sun is at the centre of the solar system and all these bodies are revolving around it. The gravitational pull of the sun keeps all the solar system and all the planets and other objects revolving round it.
    The Solar System UPSC Notes | EduRevSolar System
  • Thus, the motion of all members of the solar system is governed mainly by the gravitational force of the sun.
  • The solar system is dominated by the sun. The sun accounts for almost 99.9 percent of the matter in the whole solar system. The sun is also the source of all the energy in the solar system.

    Formation of the Solar System: Nuclear Disc Model (neo-Laplacian model)
  • Nebular Theory of Laplace (1796) tried to explain the formation of the solar system. But it had many drawbacks as the theory was based on scientifically erroneous assumptions.
  • But one assumption it got right was that the solar system was born from a giant gas of dust called as nebula.
  • A giant interstellar cloud known as the solar nebula (a vast, swirling cloud of gas and dust) gave birth to our solar system and everything in it.
  • The nebula started its collapse and core formation some 5-5.6 billion years ago and the Sun and the planets were formed about 4.6 billion years ago.

Formation of The Sun

  • The nebula began to collapse (gravitational collapse) in on itself after becoming gravitationally unstable.
  • This was possibly because of a nearby supernova sending shock waves rippling through space.
  • Gravity then caused dust and gas to coalesce to the centre of the nebular cloud.
  • As more matter got pulled in, the centre got denser and hotter, increasing the gravity and pulling even more dust inwards causing a snowball effect.
  • About 99.9% of the material fell into the centre and became the protosun (no sunlight yet).
  • Once the centre of the cloud became hot enough it triggered nuclear fusion, and the Sun was born.

The Formation of Planets

  • The 0.1% of matter that remained orbited around the Sun, causing the randomly shaped gas cloud to form a flat disc shape.
  • This flat disc, called the protoplanetary disc, was where the planets formed.
  • Within the solar nebula, the dust particles in the gas occasionally collided and clumped together.
  • Through this process called accretion, the microscopic particles formed larger bodies that eventually became planetesimals (infant stage of a planet) with sizes up to a few kilometres across.
  • As the disc continued to cool, the planetesimals grew in size through accretion to form protoplanets.
    The Solar System UPSC Notes | EduRevFormation of sun and planets
  • Gradually they got larger and larger, sweeping up all the leftover dust, other protoplanets, planetesimals until they grew into the planets.
  • In the inner, hotter part of the solar nebula, planetesimals were composed mostly of silicates and metals.
  • In the outer, cooler portion of the nebula, water ice was the dominant component.
  • The hot, rocky material near the centre of the solar system gave rise to terrestrial planets with metal cores (mostly composed of iron and nickel): Mercury, Venus, Earth and Mars.
  • And on the cool edges, the gas and ice giants were born: Saturn, Jupiter, Neptune, and Uranus.
  • Rocks that escaped the pull of planets were left as asteroids, scattered through the solar system.
  • Many of these rocks orbit the Sun in an area between Mars and Jupiter known as the asteroid belt.
    The Solar System UPSC Notes | EduRevAsteroid belt between Mars & Jupiter Components of the Solar System
    Our solar system consists of the sun, eight major planets, dwarf planets (Pluto, Ceres, Eris etc.), satellites and countless minor planets, asteroids, meteors, comets and debris.
    The Solar System UPSC Notes | EduRevComponents of Solar System

The Sun

  • The sun is the head of solar family or solar system. Compared with the millions of other stars, the sun is a medium sized star and of average brightness.
  • Though the sun is the nearest star to the earth, even then it is at a distance of 150 × 106 kilometres from the earth and light, travelling at a great speed of 300,000 kilometres per second, takes about 8 minutes and 20 seconds to reach us from the sun. However, light takes about 4.3 years to reach us from the next nearest star called Proxima Centauri.
  • Sun is not a solid body. The sun is a sphere of hot gases. It consists mostly of hydrogen gas.
  • The nuclear fusion reactions taking place in the centre of the sun( in which hydrogen is converted into helium) produce a tremendous amount of energy in the form of heat and light.

The Sun’s Internal Structure and Atmosphere

  • The solar interior, from the inside out, is made up of the core, radiative zone and the convective zone.
  • The solar atmosphere above that consists of the photosphere, chromosphere, and the corona (solar wind is an outflow of gas from the corona).
    The Solar System UPSC Notes | EduRevSun's Internal structurePhotosphere
  • The photosphere is the bright outer layer of the Sun that emits most of the radiation.
  • The photosphere is an extremely uneven surface.
  • The effective temperature on the outer side of the photosphere is 6000°C.


  • Just above the photosphere is the chromosphere.
  • It is relatively a thin layer of burning gases.
  • The chromosphere is a bit cooler — 4,320°C.


  • A dark patch on the surface of the Sun is known as a sunspot.
  • Sunspots appear as dark areas because they are about 500-1500°C cooler than the surrounding chromosphere.
  • The individual sunspot has a lifetime ranging from a few days to a few months.
  • Each spot has a black centre or umbra, and a lighter region or penumbra, surrounding it.
  • It has been suggested that the Sun is 1% cooler when it has no sunspot and that this variation in solar radiation might affect the climates of the Earth.

Solar Wind

  • The solar wind is a stream of energised, charged particles, primarily electrons and protons, flowing outward from the Sun at speeds as high as 900 km/s and at a temperature of 1 million degrees (Celsius).
  • It is made of plasma (ionised atoms)
    The Solar System UPSC Notes | EduRevSolar wind

Effects of solar wind – Aurora

  • An aurora is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctic) regions. (This is due to magnetic field lines of earth and solar wind)
  • Auroras are caused by charged particles, mainly electrons and protons, entering the atmosphere from above causing ionisation and excitation of atmospheric constituents, and consequent optical emissions.
    The Solar System UPSC Notes | EduRevAuroraEffects of solar wind – Some planets have atmosphere whereas others don’t
  • As the solar wind approaches a planet that has a well-developed magnetic field (such as Earth, Jupiter and Saturn), the particles are deflected.
  • This region, known as the magnetosphere, causes the particles to travel around the planet rather than bombarding the atmosphere or surface.
  • The magnetosphere is roughly shaped like a hemisphere on the side facing the Sun, then is drawn out in a long trail on the opposite side.
  • The boundary of this region is called the magnetopause, and some of the particles are able to penetrate the magnetosphere through this region by partial reconnection of the magnetic field lines.
  • The solar wind is responsible for the overall shape of Earth’s magnetosphere.
  • Moreover, planets with a weak or non-existent magnetosphere are subject to atmospheric stripping by the solar wind.
  • Venus, the nearest and most similar planet to Earth in the Solar System, has an atmosphere 100 times denser than our own, with little or no geomagnetic field. This is a strange exception.

Solar flares

  • Solar flares are produced on the sun’s surface due to magnetic anomalies.
  • They are magnetic storms which appear to be very bright spots with a gaseous surface eruption.
  • As solar flares are pushed through the corona, they heat its gas to anywhere from 10 to 20 million °C.
    The Solar System UPSC Notes | EduRevSolar flare

Solar prominence

  • An arc of gas that erupts from the surface of the Sun is called solar prominence.
  • Prominences can loop hundreds of thousands of miles into space.
  • Prominences are held above the Sun’s surface by strong magnetic fields and can last for many months.
  • At some time in their existence, most prominences will erupt, spewing enormous amounts of solar material into space.
    The Solar System UPSC Notes | EduRevSolar prominenceCorona
  • A corona is a distinctive atmosphere of plasma that surrounds the Sun and other celestial bodies.
  • The Sun’s corona extends millions of kilometres into space and is most easily seen during a total solar eclipse.
  • Sun’s Corona visible during Total Solar Eclipse
    The Solar System UPSC Notes | EduRevSun's CoronaPlanets
    Planets are solid heavenly bodies which revolve round a star (e.g. the sun) in closed elliptical paths. A planet is made of rock and metal. It has no light of its own. A planet shines because it reflects the light of the sun. since the planets are much nearer than the stars, they appear to be big and do not twinkle at night. The planets move round the sun from west to east, so the relative positions of the planets keep changing day by day. The planets are very small as compared to the sun or other stars. There are 8 major planets including the earth.
    These planets in the order of increasing distances from the sun are given below-
    1. Mercury (Budha): It is nearest to the sun.
    2. Venus (Shukra)
    3. Earth (Prithvi)
    4. Mars (Mangal)
    5. Jupiter (Brihaspati): Biggest Planet
    6. Saturn (Shani)
    7. Uranus (Arun)
    8. Neptune (Varun)
    The Solar System UPSC Notes | EduRevPlanetsThe 8 planets have been divided into two groups. All the planets of a particular group have some common features. The two groups of planets are:
    1. Terrestrial Planets
    2. Jovian Planets
    The four nearest planets to the Sun, Mercury, Venus, Earth and Mars, are called terrestrial planets because their structure is similar to earth.

The common features of the terrestrial planets are -
1. They have a thin, rocky crust.
2. They have a mantle rich in iron and magnesium.
3. They have a core of heavy metals.
4. They have thin atmosphere.
5. They have very few natural satellites (or moons) or no satellites.
They have varied terrain such as volcanoes, canyons, mountains, and craters. The planets which are outside the orbit of Mars are called Jovian planets because their structure is similar to that of Jupiter. The Jovian planets are: Jupiter, Saturn, Uranus, and Neptune.

The common features of the Jovian planets are -
1. They are all gaseous bodies (made of gases)
2. They have ring system around them.
3. They have a large number of natural satellites (or moons)

Satellites (or Moons)
A satellite (or moon) is a solid heavenly body that revolves round a planet. The moon revolve round the earth, so moon is a satellite of the earth. Except Mercury and Venus, all other planets of solar system have satellites. The satellites have no light of their own.  
They shine because they reflect the light of the sun. It should be noted that though we commonly call earth’s natural satellite as moon, the satellites of all other planets can also be called their moons.

Earth’s moon key points 

  • The moon is a natural satellite of the earth.
  • Moon revolves round the earth on a definite, regular path- the moon’s orbit.
  • Gravitational attraction of the earth holds the moon in its orbits.
  • The moon is about one-fourth the size of the earth in diameter and weight is about one-eighth that of the earth.
  • Moon has no air or water. Its surface is covered with hard and loose dirt, craters and mountains.
  • On the moon, days are extremely hot and nights are very cold.
  • Because the moon is nearer to the earth, it appears to be much bigger than the stars.
  • The moon has no light of its own, it is light from the sun which is reflected by the moon’s surface

Origin of Moon

  • In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body. The whole mass became a dumb-bell-shaped body and eventually, it broke.
  • It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.
    However, the present scientists do not accept either of the explanations.
  • It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.
  • A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

Other Objects in the sky
In addition to the stars, planets and satellite, there are three other objects which we can occasionally see in the sky during night. These are asteroids, comets and meteors. We will discuss all of them one by one.


  • Asteroids are very small planets of rock and metal which revolve round the sun mainly between the orbits of mars and Jupiter.
  • Actually, asteroids are a belt of a kind of debris, which somehow failed to assemble into a planet and keep revolving between the orbits of mars and Jupiter.
  • There may be as many as 100,000 asteroids. The biggest asteroid called ‘ceres’ has a diameter of about 800 kilometres whereas the smallest asteroid is as small as pebble.
    The Solar System UPSC Notes | EduRevAsteroids
  • Some experts believe that asteroids are the pieces of a planet that went close to Jupiter and was broken up by its gravitational pull.
    Others think that they are part of a ring of separate pieces of matter formed at the same time as the planets.
  • Sometimes an asteroid can collide with earth. Though the collision of an asteroid with the earth happens very rarely, even then a careful watch is kept on the motion of asteroids by the astronomers.
  • This is because the collision of an asteroid with the earth can cause a lot of damage to life and property on the earth. In fact, the extinction of dinosaurs the earth which occurred about 65 million years ago, is believed to have been caused by the collisions of some asteroids with the earth.
  • When an asteroid collides with the earth, then a huge crater is formed on the surface of the earth. Many such collisions of the asteroids must have occurred in the past during the entire history of the earth which may have caused craters of different sizes on its surface.
  • However, the natural process of soil erosion like wind and rain, tend to fill up these craters in due course of time. Only a few such craters survived on the surface of the earth so far. The ‘Lonar Lake’ in Maharashtra is one such crater formed by the collision of an asteroid with the earth.


  • According to new definition, Neptune is the outermost planet of the solar system. However, it’s orbit does not mark the boundary of the solar system.
    The solar system extends much beyond at the edge of the solar system, there are billions of very small objects called ‘comets’ these comets were formed very early from the same gas cloud from which other members of the collar system were made.
  • These comets are so far off that normally they cannot be seen. They keep on revolving around the Sun, unknown to the world. Sometimes, however, the normal path of a comet is disturbed and the comet starts moving towards the sun.
    The Solar System UPSC Notes | EduRevComet
  • As the comet approaches the sun, it develops a long, glowing tail and becomes visible only when it approaches the sun because the sun’s rays make its gas glow which spreads out to form a tail million of kilometres long. And it presents a spectacular sight.
  • Thus, a comet is a collection of gas and dust, which appears as a bright ball of light in the sky with a long glowing tail. The tail of a comet always points away from the sun.
  • Comets revolve around the sun like planets. The period of revolution of comets around the sun is, however, very large. For example, Halley’s Comet has a period of about 76 years. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.
  • Just like asteroids, comets are also of great interest to scientists. This is because they are made of the same material from which the whole solar system was made.
  • The study of the tail of the comets has shown the existence of molecules of carbon, oxygen, hydrogen and nitrogen such as CO, CH4 and HCN on it. Since these simple molecules help to form complex molecules necessary for the origin of life, some scientist have suggested that the seeds of life on the earth were brought by comets from the outer space. Comets do not last forever.
  • Each time a comet passes the sun, it loses some of its gas and ultimately only the dust particles are left in space.
  • When these particle enter into the earth’s atmosphere, they burn up due to heat produced by air resistance and produce a shower of meteors or shooting stars.


  • Many times we see a streak of light in the sky during night which disappears within seconds. It is called a meteor or shooting star. Meteors are the heavenly bodies from the sky which we see as a bright streak of light that flashes for a moment across the sky.
  • The meteors are also called shooting stars. Some meteors are the dust particles left behind by comets and others are the pieces of asteroid which have collided. When a meteor enters into the atmosphere of earth with high speed, a lot of heat is produced due to the resistance of air.
  • This heat burns the meteor and the burning meteor is seen in the form of a streak of light shooting down the sky, and it falls on the earth in the form of dust.
    The Solar System UPSC Notes | EduRevMeteor
  • If a meteor is big, a part of it may reach the earth’s surface without being burned up in the air. This fragment is called a meteorite.
  • Thus, a meteor which does not burn completely on entering the earth’s atmosphere and lands on earth is known as a meteorite.
  • Meteorites are a sort of stones from the sky. By studying the composition of meteorites we can get valuable information about the nature of the material from which the solar system was formed.
  • It should be noted that the number of meteorites striking the moon’s surface is quite large whereas very few meteorites reach the earth’s surface. This is due to the fact moon has no atmosphere to burn the falling meteorites by producing the frictional heat.

The Shape of the Earth

  • In ancient times, people believed that the shape of the Earth was flat and it had steep edges. Today we know that the Earth is almost spherical.
  • However, it is not a perfect sphere, rather it an oblate spheroid, bulging slightly at the equator and flattened slightly at the poles.
  • The difference between the equatorial diameter and the polar diameter is less than 44 km. The diameter of the Earth is 12,756 km at the equator, whereas it is 12,712 km between the poles.
  • This is due to the centrifugal force caused by the Earth’s rotation around its axis. This difference is insignificant and thus for all practical purposes the Earth is taken as spherical in shape.
  • The view that the Earth is spherical in shape was first forwarded by the famous Greek philosopher, Pythagoras, in the sixth century BC. But people did not believe him. Later, Aristotle, Varahamihira, Aryabhata and Copernicus also opined that the Earth is spherical in shape.

Evidence of the earth’s Sphericity
There are many ways to prove that the earth is spherical. The following are some of them.

  • The Sun and the other planets in the Solar System are all spherical in shape.
  • If the Earth was flat, then all the places on the Earth would have had sunrise and sunset exactly at the same time.
  • If we watch a ship approaching the land, first we see the smoke of the ship (as the entire ship lies below the line of sight) and gradually the entire ship, as it comes up over the horizon. If the Earth was flat, we would have been able to see the whole ship at a time.
  • A circular shadow observed during the lunar eclipse can only be cast by a spherical body.
  • If you look around from any place, whether a mountain, a level plain, or top of a very tall building, the horizon will always appear circular. This is possible only in case of a spherical body
  • Magellan’s circumnavigation in 1520 proved that the Earth is spherical in shape.
  • Engineers when driving poles of equal length at regular intervals on the ground have found that they do not give a perfect horizontal level. The centre pole normally projects slightly above the poles at either end because of the curvature of the earth.
  • Nowadays, when you can see the Earth in its true perspective from the outer space, the fact that the shape of the Earth is spherical needs no further proof.

Goldilocks zone
A habitable zone, also called a Goldilocks zone, is the region around a star where orbiting planets similar to the Earth can support liquid water.
It is neither too hot nor too cold. Scientists hunting for life in the Solar System and around other stars believe liquid water is an important ingredient necessary for life.
In September 2010 astronomers using the Keck telescope announced they had found an exoplanet, Gliese 581g2, about three times the size of Earth in the habitable zone of its star.

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