Planets of the Solar System: Inner Planets and Outer Planets

Planets

• 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.

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

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

• 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

• 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.

Moon

• 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 23.5° 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).

Mars

• 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.

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

• 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

• 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).

Uranus

• 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).

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 contain a higher proportion of “ices” such as water, ammonia, and methane ice giants” to emphasise this distinction.