If you’ve been researching the solar system, you’ll commonly see the planets divided into two groups. Astronomers call these groups the inner and outer planets. You may ask what separates the inner and outer planets?
What exactly distinguishes these two groups from each other?
Well, essentially, the inner planets consist of those which are closest to the sun. The outer planets are much further away and are subsequently colder.
You may be wondering what physically separates these two categories of planets, and if so, you’ve come to the right place. However, you will also discover that the two groups have different characteristics.
We will be taking a closer look at this subject and discussing each of the different planets in the solar system.
To find out more, keep reading as we look closer.
What Separates the Inner and Outer Planets?
So, why not just jump straight in and answer the question? What exactly separates the inner and outer planets from one another?
As mentioned, the inner planets sit closer to the sun, whereas the outer planets sit further away. But what is the physical object which divides them from one another?
Well, the answer is an asteroid belt. This asteroid belt separates the large gaseous Jovian planets and the inner rocky terrestrial planets.
Other than the asteroid belt the inner and outer planets have different characteristics. These characteristics are believed to be due to distance the bodies are from the Sun.
Why are the Inner and Outer Planets Very Different
While we mentioned that the asteroid belt separates the inner and outer planets, that is not the full answer. The character of the two sets of planets is very different. The inner planets are small and rocky, while the outer planets are larger and gaseous.
The reason that the inner and outer planets are different is a result of the distance that they are from the Sun.
The inner planets receive more warmth from the Sun. They also receive stronger solar wind (i.e. a stream of particles ejected from the Sun). The warmer temperatures melt ice and in combination with the solar wind strip gas from them. As a result, the inner planets lost much of the gas that they had originally, and it was ejected into the outer solar system. Only Venus and Earth retained significant atmospheres. Mars also had a significant atmosphere however it was last when the planet’s magnetic field weakened.
The cooler temperatures and weaker solar wind allowed the outer planets to retain much of their original gas, resulting in them becoming the gas giants we see today (Planets and How They Formed).
For more information on why they are different and how the solar system formed, you may like to read Why Do Planets Orbit the Sun?
The Inner Planets
Now that we’ve looked at what separates the inner and outer planets, we can discuss the planets themselves.
First, we will be taking a closer look at the inner planets that orbit closer to the sun.
Below, we’ve compiled a list of each of the inner planets and a description of each so you can identify them. We will start with the closest planet to the Sun and move outwards.
These planets have a solid surface and are made up of materials such as nickel and iron. Some of them don’t have any moons and the ones that do have very few.
Mercury
Mercury is the planet closest to the Sun. It is a small rocky planet that lacks an atmosphere. The planet has many craters on its surface. Many people viewing a photograph of Mercury confuse it with our Moon as it appears very similar.
Due to being close to the Sun Mercury never strays far from the Sun in our sky. As a result, it is only visible in the very early evening or just before the Sun rises in the morning.
With a good level of magnification, you can observe the phases of Mercury. The phases are similar to the Moon but not all phases are visible due to the planet being behind the Sun for extended periods of time.
Venus
Venus is a planet that’s known for having an incredibly thick atmosphere. It’s very similar to Earth in terms of appearance and size.
The atmosphere contains carbon dioxide and nitrogen. Because of the atmosphere’s composition, it has a surface temperature of about 900 degrees Fahrenheit (475 degrees Celsius). It doesn’t have any moons and demonstrates an incredibly slow rotation.
Venus is also called the evening and morning star because it is visible in the early evening and before sunrise. It is very bright and easy to find. Like Mercury it displays phases. After the Moon, Venus is the brightest object in the night sky.
Earth
The third planet from the Sun is our home. Earth is the only planet that we know fosters life and has a combination of nitrogen and oxygen in its atmosphere.
Mars
The next planet that we’re going to be looking at is Mars. Mars has a thin atmosphere of carbon dioxide, argon, and nitrogen.
One of the interesting things about this planet is that it shows signs of having had water running through the surface billions of years ago. It has no rings surrounding it and contains two small moons.
Mars is easily visible in the night sky due to its reddish color. In a telescope, it is possible to see surface markings, polar ice caps and dust storms. With careful observations, Mars demonstrates apparent retrograde motion. For more information on observing Mars you can visit Can You See Mars With A Telescope?
The Asteroid Belt Separates the Inner and Outer Planets
As previously mentioned the asteroid belt lies between the inner and outer planets. The asteroid belt is a region of space between Mars and Jupiter. Within this region are a large number of small irregular-shaped objects. These objects are called asteroids or minor planets.
You may imagine a region with many objects, but this is untrue. On average, the asteroids are separated by approximately 600,000 miles (1 million km).
The Outer Planets
Now that we’ve taken a closer looked at the inner planets and asteroid belt we can move on to discuss the outer planets. These planets sit farther away from the sun.
These are known as the gaseous, or Jovian, planets. They are easily distinguishable by the rings that surround them. They all also have an abundance of moons.
Below are descriptions of each of the outer planets and their distinguishing features.
Jupiter
After Mars the next planet out is Jupiter. Jupiter is the largest planet in the solar system.
It spins at an incredibly rapid rate and is known for having a particularly thick atmosphere that’s composed of hydrogen and helium.
Perhaps one of the most interesting things about this planet is that it has 80 moons surrounding it.
Jupiter’s orbit around the Sun takes 11.8 years to complete. While its year is long, its day is only about 10 hours.
It is suspected to have a solid inner core like the other gaseous planets.
Jupiter is a wonderful planet to observe with a telescope. With sufficient magnification and good seeing conditions banding is visible. Jupiter has thin rings that are not visible in an amateur telescope. However, four of its moons are visible with binoculars.
Saturn
Saturn is one of the most easily recognizable outer planets because it has large rings surrounding it that appear highly complex the closer you look.
There are large gaps between the rings. Astronomers believe that the gaps are due to gravitational forces from Saturn’s moons (Saturn’s Cassini Division).
The planet’s atmosphere is composed mainly of hydrogen and helium. It’s also known for having a particularly quick rotation compared to other outer planets.
Saturn is spectacular in a telescope. It is a sight that you are unlikely to forget. For information on observing Saturn, you can read What Does Saturn Look Like Through A Telescope?
Uranus
The next planet that we’re going to be taking a look at is Uranus. Uranus has a surface temperature of -320 degrees Fahrenheit (-195 degrees Celsius). This planet contains methane, ammonia, water, hydrogen and helium. Due to the very low temperature, these compounds are in ice form. It has the third-largest planetary diameter in the whole of the solar system.
The planet was first discovered in 1781 and was originally perceived as a comet or a star.
From Earth this planet appears very dim. You will need to be at a dark site to have any chance of seeing this planet or Neptune.
Neptune
Neptune is the farthest planet from the Sun. As a result, it’s incredibly cold and is known to be subject to supersonic winds.
It contains hydrogen, helium, ammonia, and water. It’s distinguishable as a result of its stark bluish color.
Like Uranus, it is very dim and difficult to find. If you do find it, it has a distinct blue color.
Final Thoughts
To sum up, two things divide the inner and outer planets. Physically they are separated by the asteroid belt. The small rocky planets with few Moons are between the Sun and asteroid belt. On the far side are the large gaseous planets with rings and numerous moons. The two things that separate the inner and outer planets is the asteroid belt and their physical properties.
All of the inner planets and some of the outer planets are visible without the use of a telescope.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
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