Classifications of Planets

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GEOGRAPHY

Introduction

  • A celestial body moving in an elliptical orbit around a star is known as a planet.
  • The eight planets have been divided into two groups. All the planets of a particular group have some common features. ‘Terrestrial planets’ or ‘Rocky planets’ and ‘Jovian planets’ or ‘Gaseous planets’ (Gas giants) are the two groups of planets.
  • The four planets nearest to the sun are Mercury, Venus, Earth, and Mars are called terrestrial planets, because their structure is similar to the earth.
  • The inner circle consists of four planets (Mercury, Venus, Earth and Mars) having smaller and denser bodies while the outer circle comprises four planets (Jupiter, Saturn, Uranus, and Neptune) having a larger size and less dense materials and have a thick atmosphere, mostly of helium and hydrogen
  • Other four planets are Jupiter, Saturn, Uranus and Neptune is called Jovian planets.
  • If we take Jupiter, the biggest planet, as the centre of the planets of our solar system, the size of the planets becomes smaller as we go away from either side of Jupiter (Mars being the exception).
  • The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits.
  • Planets are classified into the following two groups inner and outer planets. These are separated by asteroids belt.

Some Notable facts about Various Planets

Inner Planets

  • The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.
  • They are composed mainly of silicates and metals.
  • The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems.
  • They 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 is (i.e. Mercury and Venus).
  • The term superior planet designates planets outside Earth’s orbit and thus includes both the outer planets and Mars.

Mercury

  • 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) and Titan (largest moon of Saturn).
  • However, Mercury is massive (has more mass) than Ganymede and Titan.

 Venus

  • Venus is the second planet in distance from the sun.
  • This planet is nearest to the earth and is also the brightest planet.
  • Venus is known as the ‘evening star’ as well as ‘morning star’.
  • Venus is surrounded by a thick cloud cover, hence known as the ‘veiled planet’ (veil means unclear/cover).
  • 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 sulphuric 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 (3,000 ft) underwater on Earth.
  • Venus is like the earth in size and mass, and hence also known as the ‘Earth’s twin’.
  •  It also rotates clockwise like Uranus.
  • Venus is the hottest planet (even hotter than mercury) of our solar system, due to its veil of cloud.
  • Venus has no water on it. There is no sufficient oxygen on the Venus.
  • A day on Venus is equivalent to 243 earth days and lasts longer than its year (224 days).

Earth

  • Our home planet Earth is a rocky, terrestrial planet. It has a solid and active surface with mountains, valleys, canyons, plains and so much more.
  • Earth is the third planet from the Sun and the only astronomical object known to harbour and support life.
  • About 29.2% of Earth’s surface is land consisting of continents and islands. The remaining 70.8% is covered with water, mostly by oceans, seas, gulfs, and other salt-water bodies, but also by lakes, rivers, and other freshwater, which together constitute the hydrosphere.
  • Earth is special because it is an ocean planet. Our atmosphere is made mostly of nitrogen and has plenty of oxygen for us to breathe.
  • Earth formed over 4.5 billion years ago
  • The atmosphere also protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.

Mars

  • Mars is often referred to as the “Red Planet” because of the reddish iron oxide prevalent on its surface.
  • Mars has a thin atmosphere and has surface features ranging from impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.
  • Mars 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 has two irregularly shaped moons, Phobos and Deimos, which are thought to be captured asteroids.
  • Liquid water cannot exist on the surface of Mars due to low atmospheric pressure (less than 1% of the Earth’s).
  • The two polar ice caps appear to be made largely of water.
  • Mars can easily be seen from Earth with the naked eye.
  • Mars is less dense than Earth, having about 15% of Earth’s volume and 11% of Earth’s mass.
  • Landforms visible on Mars strongly suggest that liquid water has existed on the planet’s surface.
  • 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 and water.
  • Methane has been detected in the Martian atmosphere (may indicate the existence of life).
  • Methane can exist in the Martian atmosphere for only a limited period before it is destroyed — estimates of its lifetime range from 0.6-4 years.
  • Its presence despite this short lifetime indicates that an active source of the gas must be present.
  • Geological means such as serpentinization, volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources.

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).

Jupiter

  • Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun.
  • Jupiter is the third-brightest natural object in the Earth’s night sky after the Moon and Venus. It has been observed since pre-historic times and is named after the Roman god Jupiter, the king of the gods, because of its observed size.
  • Jupiter is primarily composed of hydrogen, but helium constitutes one quarter of its mass and one tenth of its volume. It likely has a rocky core of heavier elements,] but like the other giant planets, Jupiter lacks a well-defined solid surface.
  • The on-going contraction of its interior generates heat greater than the amount received from the Sun. Because of its rapid rotation, the planet’s shape is that of an oblate spheroid; it has a slight but noticeable bulge around the equator.
  • The outer atmosphere is visibly segregated into several bands at different latitudes, with turbulence and storms along their interacting boundaries.
  • A prominent result of this is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century, when it was first seen by telescope.
  • Jupiter’s familiar stripes and swirls are actually cold, windy clouds of ammonia and water, floating in an atmosphere of hydrogen and helium. Jupiter’s iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.

Saturn

  • Saturn isn’t the only planet to have rings, but it definitely has the most beautiful ones. The rings we see are made of groups of tiny ringlets that surround Saturn. They’re made of chunks of ice and rock. Like Jupiter, Saturn is mostly a ball of hydrogen and helium.
  • When Galileo Galileo saw Saturn through a telescope in the 1600s, he wasn’t sure what he was seeing. At first he thought he was looking at three planets, or a planet with handles. Now we know those “handles” turned out to be the rings of Saturn
  • Saturn is a gas giant like Jupiter. It is made mostly of hydrogen and helium.
  • Saturn has a thick atmosphere.
  • Saturn has a lovely set of seven main rings with spaces between them.

Uranus

  • In contrast to all other planets, it is tipped and spin on its sides that is 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)
  • 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 sun’s rotation.

Neptune

  • 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 contains a higher proportion of “ices” such as water, ammonia, and methane ice giants” to emphasise this distinction.
  • Neptune has the strongest sustained winds (2,100 km/h) of any planet in the Solar System.

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