–      Just like the Earth, all the planets in the solar system (there are eight of them) rotate around their axes (therefore the planet’s day) and revolve around the Sun (therefore the planet’s year).

–      Planets revolve in a certain path around the Sun called an “orbit.”  An “orbital period” is the time it takes a planet to complete an entire revolution around the Sun.

–      Planets stay in their orbits due to their gravity and the gravity of the Sun.  Anything that has mass has gravity.  The heavier the object, the more gravity it has.

–      The planets in our solar system are separated into three general groups: the “Inner planets,” the “Outer planets,” and the “Dwarf Planets.”


–      The inner planets include Mercury, Venus, Earth, and Mars.

–      The inner planets are also known as the “Terrestrial Planets” because they are mainly made out of rock and are similar in composition to the Earth.

–      Let’s compare the inner planets by looking at their main characteristics:

  • Distance from the Sun:

–      Mercury59 million km

–      Venus108 million km

–      Earth150 million km

–      Mars228 million km

  • Orbital Period (the time the planet takes to revolve around the Sun.  This is also known as the planet’s “year.”)

–      Mercury88 days

–      Venus225 days

–      Earth365 days

–      Mars687 days

  • Rotational Period (the time it takes for the planet to rotate around its axis.  This is also known as the planet’s “day.”)

–      Mercury – 59 days counterclockwise

–      Venus – 243 days clockwise (that’s longer than it’s year!)

– Venus is the only planet in our solar system to rotate clockwise.

–      Earth24 hours counterclockwise

–      Mars24.65 hours counterclockwise

  • Surface Temperatures (how cold and hot it gets on that planet)

–      Mercury – 400oC during the day and -180oC at night.

–      Venus – average temperature is 470oC (it is the hottest planet in our solar system).

–      Earth – Areas on Earth range from -85oC (in the Antarctic) to 65oC (in the deserts)

–      Mars – The temperature ranges from 30oC to -120oC.

  • Moons (Astronomers also call these “satellites”)

–      Both Mercury and Venus do not have any moons.  They are the only planets in our solar system that do not have any moons.

–      Earth has one moon which is about 1/6th of the size of Earth (and therefore has 1/6th of the gravity of Earth which is why astronauts were able to jump higher on the moon).

–      Mars has two very small moons called “Phobos” and “Deimos.”

  • Interesting Features:

–      Mercury – This planet does not have an atmosphere (a layer of gases surrounding the planet) and its surface is full of craters (holes created by pieces of rock colliding with the planet).

–      Venus – Besides Earth’s moon, Venus is the closest object to Earth.  Due to this, Venus is the brightest object in the sky when it’s visible at night.  The surface of Venus has many large volcanoes.

–      Earth – Our planet contains an atmosphere with lots of oxygen and 70% of its land is covered with water.  These two features support life on our planet.

–      Mars – The soil of Mars contains a lot of iron which “rusts” to give the planet a reddish colour.  Since the late 1990’s, various NASA probes have been sent to Mars to analyze the planet and send back data to Earth.


–      The outer planets include Jupiter, Saturn, Uranus, Neptune, and Pluto. [NOTE: Pluto is no longer considered a planet]

–      There is still a lot of debate on whether Pluto should be considered a planet; it is very different from the rest of the outer planets.  In the summer of 2006, a meeting of astronomers decided that Pluto would no longer be called a planet; it would become a “dwarf planet.”

–      The outer planets (not including Pluto) are also called the “Gas Giants” because they are extremely large planets composed mostly of gases.

–      As we did with the inner planets, let’s compare the characteristics of the outer planets:

  • Distance from the Sun

–      Jupiter778 million km

–      Saturn1.5 billion km

–      Uranus2.9 billion km

–      Neptune4.5 billion km

–      Pluto5.9 billion km

  • Orbital Period

–      Jupiter – 11.86 years

–      Saturn – 30 years

–      Uranus – 84.1 years

–      Neptune – 165 years

–      Pluto – 248 years

  • Rotational Period

–      Jupiter – 9.85 hours (the fastest of all the planets!)

–      Saturn – 10.65 hours

–      Uranus – 17.3 hours

–      Neptune – 15.7 hours

–      Pluto – 6.7 days

  • Surface Temperature

–      Jupiter – -160oC

–      Saturn – -180oC

–      Uranus – -210oC

–      Neptune – -220oC

–      Pluto – -220oC

  • Moons

–      Jupiter – 63 known moons.  The first four moons were discovered by an Italian astronomer named Galileo (he is credited with inventing the telescope) in the 1600’s.  These moons are fairly large and are called the “Galilean Moons.”  Their names are Callisto, Io, Ganymede, and Europa.  Many of Jupiter’s moons are very small (under 10 km in diameter).

–      Saturn – 47 known moons.

–      Uranus – 27 known moons.  Most of the names of these moons come from characters in Shakespeare plays.

–      Neptune – 13 known moons.

–      Pluto – 3 known moons.  Two of these moons were just discovered in 2005.  The largest moon of Pluto is named Charon.

  • Rings

–      All of the gas giants have rings (pieces of rock and ice floating in orbit around the planet).  Jupiter has one very thin ring which is barely visible.

–      Saturn has the most rings (over 1000).

  • Interesting Features

–      Jupiter – One distinguishing feature of Jupiter is the “Giant Red Spot.”  This is actually a violent storm that has been occurring for many years and is larger than the Earth.  Recently, another storm has started on the surface creating another smaller red spot nicknamed “Red Junior.”

– Jupiter is the largest planet in our solar system.

–      Saturn – Saturn is the second largest planet in the solar system and is red/orange/yellow in colour.

–      Uranus – The atmosphere of Uranus is mostly hydrogen gas and it has a ring around it.  It is the third largest planet in our solar system.

–      Neptune – The atmosphere of Neptune is high in methane gas (CH4) and therefore appears blue.  Like Jupiter, Neptune has a “Great Dark Spot” which is also a storm.

–      Pluto – Clyde Tombaugh discovered Pluto in 1930 with data from Percival Lowell.  It has a very weird orbit that has it crossover Neptune’s orbit for about 20 years.  During this time, Pluto is actually closer to the Sun than Neptune.  This last occurred between approximately 1979 and 1999 and will not happen again for over 200 years.


–      According to a decision made by astronomers in the summer of 2006, a new category of planets was created – the “dwarf planets.”

–      Our solar system currently has three dwarf planets – Pluto, the asteroid Ceres (located between Mars and Jupiter), and a newly discovered object located past Pluto that currently does not have a real name.

Some astronomers are not happy with this decision; therefore, it’s hard to say whether this category will exist for long.


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