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Planets of the Solar System: Inner Planets and Outer Planets

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  • A celestial body moving in an elliptical orbit around a star is known as a planet. The planets of our solar system are divisible in two groups:
  1. the planets of the inner circle (as they lie between the sun and the belt of asteroids) or the inner planets or the ‘terrestrial planets’ (meaning earth-like as they are made up of rock and metals, and have relatively high densities) and
  2. the planets of the outer circle or outer planets or the ‘gas giant planets’ or the Jovian planets – meaning Jupiter-like or more like the sun.
  • The inner circle consists of four planets (Mercury, Venus, Earth and Mars) having smaller and denser bodies. The outer circle comprises four planets (Jupiter, Saturn, Uranus, and Neptune) having a larger size and less dense materials and a thick atmosphere, mostly of helium and hydrogen.
  • All eight planets in the Solar System orbit (revolve) the Sun in the direction of the Sun’s rotation, which is counter-clockwise when viewed from above the Sun’s north pole.
  • Six of the eight planets also rotate about their axis in this same direction (counter-clockwise). Venus and Uranus have a strange retrograde rotation (clockwise), i.e., opposite of the sun’s rotation.


Surface Temp in ֯C

Period of Rotation

Period of Revolution

Distance in AU

Diameter (km)




Density (gm/cm3)

Specific gravity (m/s2)

1. Mercury +427

58 days

87 days





0 5.4 3.7 0.38
2. Venus


243 days

224 days




6 0 5.2 8.9 0.9
3. Earth +22

23:56 hrs

365 days








4. Mars -23

1.025 days

687 days




7 2 3.9 3.7 0.38
5. Jupiter -150

9.9 hrs

11.9 years





79 1.3



6. Saturn -180

10.7 hrs

29 years







10.4 1.06
7. Uranus -214

17 hrs

84 years




3 27 1.3 8.8 0.9
8. Neptune -220

16 hrs

164 years




4 14 1.6 11.1 1.13

Pluto (dwarf)


6.39 days

248 years




9 5 1.9 0.6 0.06

Some of the values given in the table are rounded off.

An Astronomical Unit (AU) is the average distance between Earth and the Sun, which is about 150 million km.

[UPSC Prelims 1998] Match List-I with List-II and select the correct answer by using the codes given below the lists:


(Special characteristic)



A. Smallest planet 1. Earth
B. Largest planet in the solar system 2. Venus
C. Planet second from the sun in the solar system 3. Jupiter
D. Planet nearest to the Sun in the solar system 4. Mercury
  1. A-2; B-3; C-5; D-1
  2. A-3; B-5; C-1; D-2
  3. A-4; B-1; C-2; D-3
  4. A-4; B-3; C-2; D-1

[UPSC Prelims 2002] Which one of the following statements is correct with reference to our solar system?

  1. The earth is the densest of all the planets in our solar system
  2. The predominant element in the composition of earth is silicon
  3. The sun contains 75 percent of the mass of the solar system
  4. The diameter of the sun is 190 times that of the earth

[UPSC Prelims 2003] Among the following which planet takes maximum time for one revolution around the Sun?

  1. Earth
  2. Jupiter
  3. Mars
  4. Venus

[UPSC Prelims 2008] In order of their distances from the Sun, which of the following planets lie between Mars and Uranus?

  1. Earth and Jupiter
  2. Jupiter and Saturn
  3. Saturn and Earth
  4. Saturn and Neptune

[UPSC Prelims 2009] Which one of the following planets has largest number of natural satellites or moons?

  1. Jupiter
  2. Mars
  3. Saturn
  4. Venus

Inner Planets

  • The four inner or terrestrial planets are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals, such as iron and nickel, which form their cores.
  • Three of the four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.

The term inner planet should not be confused with the inferior planet, which designates those planets that are closer to the Sun than Earth (i.e., Mercury & Venus).


  • Mercury’s surface appears heavily cratered and is similar in appearance to the Moon’s, indicating that it has been geologically inactive for billions of years (because there is no atmosphere on Mercury).
  • When viewed from Earth, the planet can only be seen near the western or eastern horizon during the early evening or early morning. It may appear as a bright star-like object but is less bright than Venus.
  • Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System (−173 °C at night to 427 °C during the day).
  • Mercury is smaller than the largest natural satellites in the Solar System, Ganymede (largest moon of Jupiter) & Titan (largest moon of Saturn). However, Mercury is massive (has more mass) than Ganymede & Titan.
  • Images obtained by MESSENGER spacecraft in 2004 have revealed evidence for pyroclastic flows (vulcanicity) and water ice at Mercury’s poles.


  • Venus is the brightest planet in the solar system and is the third brightest object visible from the earth after the sun and the moon. In ancient literature, Venus was often referred to as the morning & evening star.
  • It is the brightest among planets because it has the highest albedo due to the highly reflective sulfuric acid that covers its atmosphere. It is sometimes visible to the naked eye in broad daylight.
  • Venus is sometimes called Earth’s sister planet or Earth’s twin because of their similar size, mass, proximity to the Sun, bulk composition and presence of similar physical features such as high plateaus, folded mountain belts, and numerous volcanoes, etc.
  • It is radically different from Earth in other respects. The surface of Venus is totally obscured by a thick atmosphere composed of about 96% carbon dioxide, covered with clouds of highly reflective sulfuric acid.
  • It has the densest atmosphere of the four terrestrial planets. The atmospheric pressure at the planet’s surface is 92 times that of Earth, or roughly the pressure found 900 m underwater on Earth.
  • Venus is by far the hottest planet in the Solar System, even though Mercury is closer to the Sun. This is because of the greenhouse effect arising from high concentrations of CO2 and a thick atmosphere.
  • A day on Venus is equivalent to 243 earth days and lasts longer than its year (224 days). It rotates in the opposite direction (clockwise) to most other planets.

[UPSC Prelims 2005] Assertion & Reasoning

Assertion (A): Existence of human life on Venus is highly improbable.
Reason (R): Venus has extremely high level of carbon dioxide in its atmosphere.
  1. Both A and R are true and R is the correct explanation of A
  2. Both A and R are true but R is NOT a correct explanation of A
  3. A is true but R is false
  4. A is false but R is true


  • The diameter of the moon is only one-quarter that of the earth and it is about 3,84,400 km away.
  • The moon is tidally locked (the object’s orbital period matches its rotational period) to the earth, meaning that the moon revolves around the earth in about 27 days which is the same time it takes to complete one spin. As a result of tidal locking, only one side of the moon is visible to us on the earth.
  • The moon is a significant stabiliser of Earth’s orbital axis. Without it, Earth’s tilt could vary as much as 85° (at present the Earth’s axis of rotation is tilted at an angle of 23.relative to the orbital plane).

Formation of the Moon

  • It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of a ‘giant impact’ or what is described as ‘the big splat’.
  • A body of the size of one to three times that of mars collided with 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.
  • Scientists estimate that a day in the life of early Earth was only about 6 hours long and the Moon formed much closer to Earth than it is today.

Effects on Earth

  • As Earth rotates, the Moon’s gravity causes the oceans to seem to rise and fall. There is a little bit of friction between the tides & the turning Earth, causing the earth’s rotation to slow down just a little (1.4 milliseconds in 100 years). As Earth slows, it lets the Moon move away by a little (four cm per year).

Colonizing the Moon

  • Exploration of the lunar surface by spacecraft began in 1959 with the Soviet Union’s Luna program.
  • Luna 2 made a hard landing (impact) on its surface and became the first artificial object on the moon.
  • Crewed exploration of the lunar surface began in 1968 when the Apollo 8 spacecraft orbited the Moon.
  • Neil Armstrong was the first, and Buzz Aldrin was the second to step on the surface of the moon on 29 July 1969 (Apollo 11 mission). To date, only Twelve astronauts walked on the Moon’s surface.
  • Discovery of lunar water at the lunar poles by Chandrayaan-1 in 2009 has renewed interest in the Moon. The Chandrayaan probe discovered that the lunar soil contains 0.1% water by weight.
Advantages of Colonising the Moon
  • A lunar base could be a site for launching rockets with locally manufactured fuel to distant planets.
There are Several Disadvantages to the Moon as a Colony Site
  • The long lunar nights (350+ hours) would impede reliance on solar power. However, the lunar poles avoid the problem of long lunar nights.
  • The Moon is highly depleted in carbon and volatile elements, such as nitrogen and hydrogen.
  • The low gravity on the Moon will have adverse effects on human health in the long term.
  • The lack of a substantial atmosphere results in temperature extremes, harmful radiation reaching the surface and increased chances of the colony’s being hit by meteors.
  • Growing crops on the Moon is difficult due to the long lunar night, extreme variation in surface temperature, exposure to solar flares, nitrogen-poor soil, and lack of insects for pollination.


  • Mars is often referred to as the “Red Planet” because of the reddish iron oxide prevalent on its surface. It can easily be seen from Earth with the naked eye.
  • Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density.
  • The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen, methane, and water.
  • Methane can exist in the Martian atmosphere for only a limited period before it is destroyed by the solar wind. Its presence despite its short lifetime indicates that an active gas source must be present. Geological means such as serpentinization (reactions in rocks), volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources.
  • Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets’ rotational axes.
  • The lack of a magnetosphere and the extremely thin atmosphere of Mars is a challenge: the planet has little heat transfer across its surface and poor insulation against the bombardment of the solar wind.
  • Landforms visible on Mars strongly suggest that the atmosphere was once thick and dense and liquid water existed on the planet’s surface. However, now, liquid water can no longer exist on the surface of Mars due to low atmospheric pressure (less than 1% of the Earth’s). Whatever water is left is locked in the two polar ice caps.
  • Mars has surface features like impact craters, valleys, deserts, and polar ice caps. It is the site of Olympus Mons (shield volcano), the largest volcano and the highest known mountain (24 km) in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System.
  • Mars is nearly geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.
  • Mars has two irregularly shaped moons, Phobos and Deimos, which are thought to be captured asteroids.

Mars Compared to Earth

  • 53% of the diameter of Earth
  • 10% of the mass of Earth
  • Surface gravity on Mars is only 38% of Earth’s gravity.
  • A day on Mars lasts 1.03 Earth days.
  • Axial tilt on Mars is 25.19° (close to Earth’s 23.5° tilt).
  • A year on Mars lasts about twice as long as an Earth year; the seasons are twice as long.
  • The atmosphere of Mars (95% carbon dioxide) is less than 1% the thickness of Earth’s atmosphere.

[UPSC Prelims 1997] Which one of the following conditions is most relevant for the presence of life on Mars?

  1. Atmospheric composition
  2. Thermal conditions
  3. Occurrence of ice caps and frozen water
  4. Occurrence of ozone

Hint: on earth, prokaryotes (their cells lack a nucleus) were the earliest life forms. They fed on carbon compounds that were accumulating in Earth’s early oceans.

[UPSC Prelims 2006] Assertion & Reasoning

Assertion (A): To orbit around the Sun the planet Mars takes lesser time than the time taken by the earth.
Reason (R): The diameter of the planet Mars is less than that of earth.
  1. Both ‘A’ and ‘R’ are individually true and ‘R’ is the correct explanation of ‘A’.
  2. Both ‘A’ and ‘R’ are individually true but ‘R’ is not the correct explanation of ‘A’.
  3. ‘A’ is true but ‘R’ is false.
  4. ‘A’ is false but ‘R’ is true.

Outer Planets

  • Outer Planets are Jupiter, Saturn, Uranus, Neptune, and the dwarf planet – Pluto. The four outer planets, called the gas giants, collectively make up 99% of the mass known to orbit the Sun. They are composed mainly of hydrogen & helium & lack a solid surface. Their moons are, however, solid.
  • The two outermost planets, Uranus, and Neptune are composed of substances called ices, such as water, ammonia and methane, and are often referred to separately as ice giants.
  • All four gas giants have rings, although only Saturn’s ring system is easily observed from Earth. The gas giants have a magnetosphere, numerous moons, and significant atmospheric activity.
  • Neptune has the strongest wind speed (2,100 km/h) followed by Saturn (1,800 km/h).

Why are the Inner Planets Rocky while the Outer Planets are Mostly in Gaseous Form?

  • The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases on the surface to condense to solid particles.
  • The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the surface of the terrestrial planets. The terrestrial planets are smaller, and their lower gravity could not hold the escaping gases. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.


  • It is composed mostly of gas & liquid swirling in complex patterns with no solid surface. Because of its rapid rotation (once every 10 hours), the planet’s shape resembles an oblate spheroid (slight bulge at the equator).
  • Jupiter’s four large moons (Io, Europa, Ganymede, and Callisto) are called the Galilean satellites because Galileo discovered them. Ganymede is the largest natural satellite (5,268 km in diameter) in this solar system. It is larger than Mercury, and three times larger than the earth’s Moon (3,474 km in diameter, the fifth largest moon). The latest probe to visit Jupiter is Juno.


  • Saturn’s density is even lesser than water. Its rings are probably made up of billions of ice particles and ice-covered rocks.
  • Titan is the second-largest moon in the Solar System, and it is the only satellite in the Solar System with a substantial atmosphere (nitrogen-rich).


  • In contrast to all other planets, it is tipped and spins on its sides. Its axis of rotation lies in nearly the plane of its orbit (the poles of Uranus lie in a plane where equators of other planets lie).


  • Uranus and Neptune (the ice giants) are called the twins of the outer solar system. They are surrounded by a thick atmosphere of hydrogen and helium and contain a higher proportion of “ices” such as water, ammonia, and methane ice giants” to emphasise this distinction.
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