The Ultimate Guide to Our Solar System

This image shows an artist’s impression of the Solar System.
Credit: NASA/Jet Propulsion Laboratory-Caltech

Our solar system is just one of the billions of solar systems that make up the Milky Way galaxy. The Milky Way is a spiral galaxy and the “arms” of the spiral are active star formation regions. We are located near the Orion Arm, or the Orion Spur. This spur is between two more prominent arms of the spiral, Sagittarius and Perseus.  

The white circle denotes where our Solar System is in the Milky Way galaxy.
Credit: NASA/JPL-Caltech/T

Our solar system is made up of many objects, much more than just planets and moons. The most massive thing in our solar system is the Sun, consisting of over 99% of the Solar System’s mass. The last percent is made up of all of the planets, their satellites and rings, comets, asteroids, and dust combined.

What is a Solar System?

A solar system is one or more stars that have a celestial object(s) orbiting and bound to them by gravity. Solar systems don’t all look like ours. Some systems have multiple stars which orbit around their center of mass. Some star systems only have one planet where others may have many more than ours. The planets from star systems other than ours are referred to as exoplanets. Since other stars are so far away, it is hard to observe the nuances of their systems. But scientists are working hard to discover new solar systems, especially ones with planets that might be habitable for humans. 

The Discovery of Our Solar System

The geocentric model
Credit: NASA/JPL

Theories about our solar system date back to the ancient civilizations. There were many ideas about what the Solar System contained, how it was structured, the cosmos etc. Some thought that the Sun, planets, and other stars revolved around Earth. This idea is called the geocentric model and was widely accepted as correct in the second century and beyond. It wasn’t until the 16th century that the correct heliocentric model, where the planets revolved around the Sun, was accepted. 

The discovery of most of the planets in our solar system was rather arbitrary. Obviously, we have known about Earth for some time now. But we have also known about some of the other planets in our solar system for just as long, we just didn’t know they were planets. To the unaided eye, planets are very bright and look like big stars. But if you track them, as the ancient Greeks did, you will find that they don’t behave like stars. Stars “move” according to where our planet is in its orbit, this is why you can see some stars in the winter that you can’t in the summer. The Greeks found that 5 stars in particular were not changing with the season but rather exhibiting some strange behavior. These were called wanderers, along with the Sun and the Moon. Where stars moved uniformly, the wanderers appeared to periodically slow down, stop, and even change direction. They are Mercury, Venus, Mars, Jupiter, and Saturn.

What about Uranus and Neptune? They are the two planets furthest from the Sun which is why they are very difficult to see with the naked eye. The other objects in our solar system are too small to see unaided which is why more of our solar system was discovered after telescopes were invented in 1608. In 1781, Uranus was discovered by Sir William Herschel. In 1801, the asteroid belt was discovered by Giuseppe Piazzi. Then in 1846, Neptune was discovered by John Couch Adams. 

Origin of Our Solar System

The formation of the members of our solar system started about 4.5 billion years ago. Everything in the Solar System formed from the solar nebula which originally was a spherical cloud of dust and gas. The solar nebula was composed mostly of hydrogen and helium, which are now the two most bountiful elements in our solar system. The nebula collapsed into a spinning disk which then condensed to form the Solar System.

Most of the nebula condensed to form the Sun but the outer ring did not and revolved around the new star. As the nebula cooled, the cores of the outer four planets started to form as well as many other much smaller cores in the inner Solar System. These smaller cores chaotically collided with each other to form the inner four planets. The inner planets were messy, jagged fragments of collided rock until the gravity they developed pulled these imperfections into a spherical shape.

Credit: Space Facts/Laurine Moreau

The remaining pieces of these cosmic rocks had two main fates. One fate was being propelled into the cosmos. Many of these pieces stayed within the orbit of the Sun and formed the spherical Oort Cloud at the edges of our solar system. Some pieces, however, were ejected from our solar system altogether. The other fate was finding a stable orbit around the Sun. One such orbit is located between Mars and Jupiter and is known as the asteroid belt. Another stable orbit is beyond Neptune and is named the Kuiper belt. 

The Planets

There are 8 planets in our solar system: Mercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, and Neptune. We can classify all planets, including our own, into a few different categories but, for now, we’ll just focus on the inner planets and outer planets of our solar system.

The inner planets of our solar system.
Credit: NASA/Lunar and Planetary Institute

The inner 4 planets are also known as the terrestrial planets. These planets have a heavy-metal core, a solid surface, and are rather dense. Their surfaces can be impacted and shaped by collisions and they have varying levels of atmosphere depending on their size. The inner planets have very few moons in comparison to the outer planets.

The outer planets of our solar system.
Credit: Lunar and Planetary Institute

The outer planets are somewhat separated into two categories, gas giant and ice giant. The two gas giants are Jupiter and Saturn. They have a low density considering their colossal size and are constructed mostly of their atmosphere. Their cores are theorized to be rocky but, because of the immense pressure that such a massive atmosphere creates, the rock is most likely not in a solid form. Gas giants are composed of the same elements as stars and are sometimes called failed stars because of it. Saturn famously has dazzling bright rings, but Jupiter also has rings, albeit they are faint.

The ice giants are Uranus and Neptune. They have an icy, rocky, core which is much larger than that of the gas giants. The atmosphere of these planets is more elementally complex than the gas giants. These two planets also have rings but, like Jupiter, they are faint. All of the outer planets have dozens of moons.

Another planet you may familiar with is Pluto. Pluto is no longer considered a major planet because it is not able to clear its orbit of celestial objects. There are a few other minor planets in our solar system: Ceres, Haumea, Makemake, and Eris. All of these planets are smaller than Earth’s moon yet have up to 5 moons of their own.

Comets and Asteroids

Comets and asteroids are the leftover bits and pieces from the formation of our planets. Nevertheless, they have differences in composition, orbit, and how they interact with our solar system. The compositions of comets and asteroids have been somewhat unchanged chemically or physically since they first formed. This makes them valuable examples of the chemical structure of the building blocks of our solar system.  

Comets are chunks of frozen gas, rock, and dust. Comets mostly come from the Oort Cloud where there is estimated to be around 10 trillion comets. The Kuiper belt is known for ejecting a comet on occasion. Comets begin their journey around the Sun by being thrown from the Oort Cloud or Kuiper Belt. Comets vary in size and can range from a diameter of a few hundred feet to many miles. When comets come close to the Sun in their elliptical orbit, something spectacular happens. 

Comet Hale Bopp
Credit: NASA

The Sun heats up the frozen gas letting it flow to form a vibrant tail that stretches for millions of miles. When a comet comes around the Sun in its orbit, it picks up incredible speed that allows it to slingshot back out into the cosmos. There are around 3,600 known comets flying around in our solar system. Scientists expect many more.

The asteroid Ida observed by the Galileo spacecraft. 
Credit: NASA

Asteroids are rocky lumps that are vast in quantity and live considerably less spectacular lives than comets. Asteroids primarily reside in the main asteroid belt located between Mars and Jupiter. Over a million asteroids can be found in this belt which spans a width of millions of kilometers. The life of an asteroid becomes a little more interesting when it collides with something. When asteroids collide with Earth, we call them meteorites but most of the time they fail to pass through our atmosphere. An asteroid that burned up before making an impact is called a meteor. 

How Big is Our Solar System?

This impression of the Solar System shows some of the minor planets as well as the asteroid and Kuiper belt.
Credit: NASA

It is sometimes difficult to comprehend just how big our solar system is. It is important to note that we consider the end of our solar system to be the Oort Cloud, not the last planet in the Sun’s orbit, Neptune. On Earth, we measure large distances in feet, miles, meters, and kilometers. We can try to use these same measuring devices with space, but since space is so vast, we have to make some adjustments to keep the numbers from being burdens to work with. 

The kilometer (km) is approximately the length of 9 football fields. We use kilometers to talk about the diameters of planets or asteroids and comets. In the context of space, the next largest measurement is the astronomical unit (AU). One AU is the distance from the Earth to the Sun, which is about 147.1 million kilometers. We use AUs to talk about the distance between planets in our solar system. After AU comes light-years (LY) which is 63,241.1 AU. A light-year is how far light can travel in one Earth year. We use light-years to talk about how far away objects are in our galaxy. 

To measure the distance from the Sun to the Oort Cloud, we will use astronomical units. Since the Oort Cloud is still relatively unexplored, scientists can only estimate how far it is from the Sun. Right now, we think the distance between the Sun and the Oort Cloud is between 2,000 and 100,000 AU. Let’s do some simple math to find that distance in the aforementioned example of football fields. 

A reasonable 50,000 AU, or 0.79 LY, will be used as our distance for this example. 50,000 AU is a whopping 7,500,000,000,000 kilometers, that’s 7.5 trillion kilometers. We said that 1 kilometer was 9 football fields so that’s an incredible 67,000,000,000,000, or 67 trillion, football fields between the Sun and the Oort cloud. This number is just the radius of our solar system. With this estimate, we can say that our solar system is 134 trillion football fields, or 15 trillion km, from edge to edge. Our solar system is undoubtably enormous.

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