Planetary astronomy attempts to study the similarities and differences of the major objects in the solar system. These objects include planets, meteors, comets, and asteroids. All of these objects differ in their origin, composition, and location in the solar system. Planets are of two types; terrestrial and Jovian planets. Both the planets and objects of the solar system constitute the solar system. The sun is the primary source of light for all the objects in the solar system. The essay that follows will discuss the similarities and differences of the various members of the solar system.

Terrestrial Planets

The solar system has four terrestrial planets: Mercury, Venus, Earth, and Mars. Though the Moon bears some similarity to the terrestrial planets, it is not numbered among them because it is structurally different. The four terrestrial planets are also called inner planets because they are found closest to the sun. Additionally, terrestrial planets are also called telluric or rocky planets and they differ markedly from the outer planets in significant ways.

Terrestrial planets share some similarities. All of the terrestrial planets are composed of heavy metal and rocks. All of terrestrial planets comprise a core filled with heavy metals, majority being iron; the metal-filled core is surrounded by a mantle made up of silicate rocks. The terrestrial planets are generally smaller than the outer planets. The rocky planets also have varied terrains that comprise canyons, volcanoes, craters, and mountains. Another common feature shared among the rocky planets is that they contain few or no moons. These planets include Mercury, Venus, Earth, and Mars.

  1. Mercury

Mercury is the first terrestrial planet in the solar system. The planets are numbered according to their proximity to the sun. Therefore, Mercury is the closest planet to the sun. It is also the smallest planet. Mercury lacks any atmosphere that would retain heat; consequently, the planet experiences the greatest temperature variations. Mercury takes 88 days to revolve around the Sun.

Mercury comprises 30% silicate and 70% metallic material. The planet has a density of 5.427g/cm3 and ranks as the second most densest planets. The high density infers to the composition of the Mercury’s core. Mercury has three main surface types; rugged highlands, inter crater plains, and smooth plains. The planet has the thinnest atmosphere of all the planets. The planet has a significant global magnetic field. This magnetic field is similar to that of the earth in the sense that it is dipolar; however, unlike the magnetic field of the earth that is known to be unstable, that of Mercury is relatively stable. The planet has no rings or moons (Seeds, & Backman, 2013).

  1. Venus

Venus is numbered second from the sun and takes 224.7 earth days to orbit the sun. Venus has no natural satellites. Venus has often times been referred to as Earth’s sister planet because of the similarities in bulk composition, gravity and size. However, Venus has been found to differ from Earth in other significant respects. Compared to the other three terrestrial planets, Venus has the densest atmosphere comprising 96% carbon dioxide (Seeds, & Backman, 2013). The close similarities between Venus’ size and density and that of the Earth suggest that their internal structure is also near similar. Like the Earth, Venus’ core comprises some liquid. Venus’ magnetic field is about 0.000015 times that of earth. Similar to Mercury, Venus has no moons or rings.

  1. Earth

The earth is the third planet from the sun. It is the third largest planet in the solar system and the largest of the terrestrial planets. The earth is the only planet that has been found to support life. The earth comprises land, water bodies and an atmosphere that is rich in a mixture of gases. The earth takes 356 days to orbit the sun. The earth’s magnetic field is unstable and fluctuates from between25, 000 nT and 65,000 nT.  The earth’s interior structure is layered according to its density. The heaviest material is in the centre or the core while the lightest is at the crust region. The core houses molten lava while the crust has rocks. The earth bears a similarity with both Mercury and Venus in the sense that it has no rings. However, unlike Mercury and Venus, the Earth has a moon which is about one and a half times bigger than the earth (Seeds, & Backman, 2013).

  1. Mars

Mars is the fourth planet in the solar system and the second smallest among the planets of the solar system. The surface is predominantly covered in iron oxide giving it a reddish appearance. Mars has a thin atmosphere and the surface bears some resemblance with the moon in that it had numerous craters, volcanoes, and valleys. Additionally, Mars bears some similarity with the earth in that it has polar ice caps and deserts similar to those found on Earth. Mars’ magnetic field is very weak and the atmosphere is very thin with about 95% carbon dioxide. Mars has two moons; Phobos and Deimos, and no rings (Seeds, & Backman, 2013).

Jovian Planets

Jovian planets are the outer planets of the solar system. They are also called gas giants because of their composition. The Jovian planets have several things in common; they all lack a solid surface, they have a solid core surrounded by a tightly compacted surface. The surface is made up of gases such as helium, hydrogen, ammonia, and water. All Jovian planets have rings with multiple moons. The lack of solid surfaces and the presence of rings and multiple moons distinguishes the Gas Giants from the Rocky Planets. The Jovian Planets include Jupiter, Uranus, Saturn, and Neptune.

  1. Jupiter

Jupiter is the largest planet in the solar system. It is the fifth planet from the sun. The planet has about 63 moons, with faint rings. Jupiter has a low density with a magnetic field that is ten times that of the earth. The planet lacks a solid surface; its internal surface comprises gases with a solid core. The atmospheric gases include helium and hydrogen.

  1. Saturn

This is the sixth planet from the sun. Saturn is the second largest planet in the solar system. Saturn is about ten times larger than the Earth with a magnetic gravity that is about 1.08 times that of the Earth. It is the only planet that is less dense than water. Similar to Jupiter, Saturn is made up of helium and hydrogen gases. The planet has thin and bright rings with about 33 moons that include Phoebe, Hyperion, Mimas, Janus, Pandora, Dione, Titan among others.

  1. Uranus

Uranus is located seventh in the solar system relative from the Sun and the third largest planet in the solar system. Uranus is an ice giant covered with thick clouds. The bluish color of the planet stems from the methane that is in its atmosphere. The atmosphere consists of helium, hydrogen, and methane gases with a molten core. Uranus is the second least dense planet after Saturn. The planet is about 4 times the diameter of the earth. The magnetic gravity on Uranus is about 90% that experienced on earth. Uranus has 11 rings and 22 known moons.

  1. Neptune

Neptune is numbered 8th away from the sun in the solar system. Neptune’s diameter is 3.88 times that of the Earth with a density of 1.639 g/cm. Supposing Neptune was hollow, it would hold up to 60 Earths. Neptune had a hazy atmosphere with rampant strong winds. The planet is orbited by 8 moons and its rings are arranged in clumps. The gravitational pull in Neptune is about 1.20 times that experience on the Earth.

Minor Members of the Solar System

Comets, meteors, and asteroids are considered part of the solar system. However, they are not planets; they are small pieces of ice and rock.

  1. Comets

A comet is considered a small, icy celestial body found orbiting around the Sun. Comets are believed to be formed at low temperature by the combination of volatile carbon monoxide and water. Every Comet has a nucleus, a gaseous coma, and two tails. The ion tail is developed when the comet is closer to the sun. This tail develops way from the sun and can span up to 250 million kilometers in length. The dust tail is long and comprises microscopic dust particles. This tail is buffeted by photons that are emitted by the Sun and it curves as the comet moves. Away from the sun both the two tails fade. A tall hydrogen envelope surrounds the coma (Vogt, 2003).

  1. Meteors

Meteors are considered small bodies that have entered the Earth’s atmosphere. Meteors are often referred to as falling stars or shooting stars. Meteors are associated with comets in that the meteors are formed from the dust left behind by comets when they leave the Sun. This means that meteors originate from comets and they comprise dust particles that have been compacted to form fiery rocks. The size of the meteors varies from small to huge pieces. Meteors and meteorites originate from meteoroid. Meteoroids are formed from the dust particles left behind by the comets. The meteoroids become meteors when the burning rocks enter the earth surface. A meteorite is a meteor that has fallen on the Earth (Vogt, 2003).

  1. Asteroids

Asteroids are metallic or rocky objects which orbit the Sun. They are located in the asteroid belt that is found between Mars and Jupiter. Asteroids are often times referred to as planetoids. Asteroids range in size from pebbles to gigantic bodies with orbiting moons. Asteroids are thought to have originated from a planet that failed to form or from a planet that exploded. Asteroids are classified according to their geologic features. Some are considered carbonaceous, silaceous or metallic (Vogt, 2003).

The Drake Equation

This equation is probabilistic in nature and is used to estimate the number of active and communicative extraterrestrial life forms in the Milky Way galaxy. This equation was devised in 1961 by Frank Drake for the purposes of stimulating scientific dialogue in the first Search for Extraterrestrial Intelligence (SETI) meeting (Maran, 2012). The equation outlines the factors that must be considered when determining whether there are other radio-communicative lives outside the Earth. Drake Equation is given by: N= R*. fp .ne .fl .fi .fc .L

Where: N= this is the number of civilizations in the planet with which it is possible to maintain a radio-communication;

R* = the average rate by which starts are formed in our galaxy

fp =  the fraction of starts that have planets

ne = the average number of planets in the stars that can sustain life

fl = the fraction of the planets that can support life at some point in time

fi= the faction of the planets that actually go on to develop intelligent life

fc= the fraction of intelligent life that develop technologies that make them detectable in space.

L= the duration of the signals released in space

The equation is not useful because the four terms fl .fi .fc .L are not known and they are equally hard to estimate.



Seeds, M. A., & Backman, D. E. (2013). The solar system. Boston, MA: Brooks/Cole, Cengage Learning.

Vogt, G. (2003). The solar system. Mankato, Minn: Bridgestone Books.

Maran, S. P. (2012). Astronomy for dummies. Hoboken, N.J: Wiley.

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