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Table of Contents

  1. Introduction/The Sun
  2. Inner Planets
  3. Asteroid Belt
  4. Outer Planets
  5. Kuiper Belt
  6. Oort Cloud
  7. For Educators
Embed from Getty Images

Now that we know of the moon and Earth, we will be diving into the planets. This post will be an overall description of what we call our solar system, which is everything that orbits the Sun. This orbit is caused by the gravitational pull of the Sun. I won’t describe how that works currently but if I do, I will replace this line with a link to the blog post explaining what is known as Newton’s Law of Gravity and Kepler’s Laws(these help us describe planetary motion).

For now, lets talk about the solar system. It starts with our Sun, which is a very large(2*10^30 kg and radius of 700,000 km) “ball” of mainly hydrogen plasma(superheated gas for our purposes). The sun is 1 Astronomical Unit, or AU, from us(Earth), which is equal to about 93 million miles. The important thing is that the Sun releases a lot of energy in the forms of light and charged particles, which account for us having heat and many other interesting phenomena. Also, the Sun is what holds everything together, because the Sun is 99.8 percent of the total mass of the Solar System(Jupiter is literally insignificant in mass to the Sun).

The Terrestrial(Inner) Planets

Next we have the four terrestrial planets, which are, in order of increasing distance from the Sun, Mercury, Venus, Earth, and Mars. These are called the terrestrial planets for two reasons, one, they are primarily rock and do not have much of a gaseous component(less atmosphere) and they are separated from the rest of the planets by the Asteroid Belt. The term terrestrial is used to say that each of these planets are like Earth. They all have rocky cores made of iron and nickel.

Asteroid Belt

The Asteroid Belt is composed of small rocks that could not aggregate together like moons and planets do. These small rocks are asteroids(I say small but the smallest asteroids are more than a half mile across). They separate Mars and Jupiter. This belt is huge, but most of the mass is aggregated into 4 large asteroids, the largest being called Ceres. This asteroid is considered a dwarf planet and is 500 kilometers in radius(The Earth is 6400 kilometers for reference). The asteroids are very sparse, meaning most of the belt is empty space. Finally, these asteroids are also called planetesimals because they are considered the pre-requisites to the formation of a planet.

Jovian Planets

Pass the asteroid belt to find Jupiter, the first of four Jovian Planets. Jovian means “Jupiter-like”, so all of these planets are like Jupiter. The planets after Jupiter are Saturn, Uranus, and Neptune. These planets are gas giants, because they are much larger than the terrestrial planets and are mostly gaseous. Note that these planets are much farther away from the Sun than you might think, the distance between planets is not proportional. From Earth to the Sun, as we said, is 93 million miles, while Jupiter to the Sun is 500 million and Uranus is 1.8 billion miles away from the Sun. Jupiter is the largest, Saturn has the rings that you probably know about, and Uranus and Neptune are very similar, the biggest difference being how far from the Sun they are(Neptune’s year is double that of Uranus’s approximately).

Kuiper Belt

Beyond the Jovian Planets lies the Kuiper Belt. This begins around 50 AU(astronomical units/ 1 AU= distance from Earth to Sun) beyond the Sun. The belt is very similar to the asteroid belt, except you will find more comets and ice rocks because of the extremely cold temperatures. This belt is also very sparse in rock density(mostly empty space), but is also much larger than the Asteroid belt.

Oort Cloud

Finally, let’s discuss what is known as the Oort Cloud. This is just barely part of the Solar System, it is 2,000 AU away from the Sun(remember the Kuiper Belt is only 50 AU). This ‘cloud’ is believed to be a large sphere of sparsely populated ice rocks. However, sparsely populated does not mean underpopulated. It is believed to have trillions of different objects. Because of how far away it is, this cloud is just speculative and we do not know if anything really exists there except for very long period(time per revolution around the Sun) comets.

For Educators

After doing the five minute presentation on the subject(make it however you feel like, just stay away from talking too much, visuals and audio are more useful). Afterwards, you have a couple of main goals.

  1. Explain how the composition of the gas giants and terrestrial planets differ
  2. Get relative distances between the planets

For goal 1, the easiest way is to explain gas giants as being like cotton candy(you could fill a bag with cotton candy to make it round like a planet). Since they are gaseous, they do not have much solid like behavior and will be fluid(this is much less dense than the actual densities of Jovian Planets, but will convey the point). Note, however, since they are so massive, they will make you feel heavier on these planets(gravity is more than here on Earth.) For terrestrial planets, you could use a rock because these terrestrial planets are very dense(mostly rock). Make the rock round to reflect a planet. Even better is have the students make these planets from some materials and paint them(you could use something like Styrofoam for the Jovian Planets and actual rocks for the terrestrial planets). Please remember that for Jovian Planets are much denser than Styrofoam or cotton candy, I am just using those to convey the “texture” of the planets in a easy to understand way. Most of the density would come from the mass of the Planets packing the gases in the core to make it solid.

For goal two, I am using a very easy to understand description from NASA. ” One way to help visualize the relative distances in the solar system is to imagine a model in which the solar system is reduced in size by a factor of a billion (109). The Earth is then about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40,000 km away! “(See the link below for the source). ” What they are saying here is that if you scaled everything down to a very small size, you could use these numbers to explain the relative sizes and distances of the planets. Students do not need to remember any numbers, rather they should get the concept that things are not proportional. I would recommend taking students on a walk for things like the distance to Jovian Planets since they are quite large.


Thank you

Thank you for reading! Any questions or comments, let me know at

One thought on “Astronomy: Our Solar System

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