Difference Between Inner Planets and Outer Planets (With Table)

Over the years, it has always been man’s persistent theme to understand and explore our cosmos. A tour of our solar system shows us the stunning beauty of all worlds.  

Our solar system is shaped by all sorts of unseen forces that we do not understand. From the dark matter to undiscovered planets, there is so much to delve into this strange world.

The Planets of our solar system did not appear from anywhere. Neither was our Sun. There was a big cloud of gas and dust in the center of which gravity collected a lot of materials thus creating the Sun. The waste left out circled round the forming Sun. The small grainy particles, gas, and dust collided and collected together to stick to each other thus forming planets.

A lot of disputes arose among scientists over what a planet is. Eventually, they came up with a definition in 2006 according to which a planet must do three things. The first obvious one is that it has to orbit around the Sun. 

The second is that it must be that big enough to have gravity enough to force it to form a spherical shape. The last one is that it should be big enough to clear away any object of smaller size near its orbit by its gravity.

Planets are divided into inner planets and outer planets depending on their distance from their Sun.

Inner Planets vs Outer Planets

The difference between the Inner Planets and Outer Planets is that the Inner Planets are close to the Sun, smaller and rockier whereas the Outer Planets are further away from the Sun, large and mostly made up of gases.

Comparison Table Between Inner Planets and Outer Planets

Parameters of ComparisonInner PlanetsOuter Planets
DefinitionInner planets are the planets whose orbits lie between the Sun and the asteroid beltOuter Planets are the planets whose orbits lie beyond the asteroid belt
Names of planetsMercury, Venus, Earth, and MarsJupiter, Saturn, Uranus, and Neptune
Composed ofInner planets are small planets mostly composed of rockOuter planets are mostly big and composed of gas
Position from SunInner planets are closer to the SunOuter planets are further from the Sun
MoonsInner planets have very few moonsOuter planets have lots of moons
RingsInner planets have no rings around themOuter planets have rings around them
SurfaceInner planets have a solid surface and are terrestrial planetsOuter planets do not have solid surface and are gas giants
RevolutionInner planets have short periods of revolution around the SunOuter planets have long periods of revolution around the Sun
DensityInner planets have great densityOuter planets have less density
OrbitsInner planets have close orbitsOuter planets have separated orbits

What are the Inner Planets?

Inner planets of the solar system are the planets whose orbits lie between the Sun and the asteroid belt. They are also called terrestrial planets. It is believed by astronomers that these planets have an iron core.

The inner planets have a solid surface and each of the four planets have a vastly different environment. These planets have short orbits around the Sun and they all spin slowly. 

The terrestrial planets have a rocky surface that has mountains, valleys, plains, etc. The temperature inside these planets is very low which makes the rocks exist as solid surfaces. They are made up mostly of silicate rock and heavy metals like iron and nickel.

Earth is the only planet in all the terrestrial planets that has life on it. Some believe that Mars does have the possibility to support life at one point, but no proof has been found yet for this. Venus and Mercury are just not suitable for life and it is said that life never existed on these planets.

The inner planets are Mercury, Venus, Earth, and Mars.

  1. Mercury: This is the closest planet to the Sun and the smallest of the inner planets. It looks very much like the Earth’s Moon and is also a little greyish. This planet has deep craters and particle silicates make a thin layered covering. Its thin atmosphere makes the temperature go to extremes, as hot during the day as 430 degrees and freezing at night as low as -187 degrees. Mercury has no moons of its own and is one of the densest planets of the solar system. It is comprised mostly of iron and nickel.
  2. Venus: This planet is about the same size as the Earth. It has a thick topic atmosphere composed of about 96% carbon dioxide, nitrogen, and other gases. This atmosphere traps heat making it the hottest planet in our solar system. The crater impacts on Venus makes it difficult for man-made objects to penetrate without disintegrating. The surface of Venus is mostly marked with volcanoes and deep canyons. Venus is also called the morning star as it is a bright object. It has no moon on its own.
  3. Earth: This is the largest of the inner planets and the only one having life on it. Earth is the only planet to have extensive regions of liquid water and water is the most necessary for life. The water vapor in the atmosphere of the earth helps to moderate the temperature. The surface of Earth is rocky with canyons, mountains, and heavy metalcore. The seasons on earth are regular except for regions closer to the equator and the poles.
  4. Mars: The final inner planet and also known as the Red Planet because of the rust of the iron-rich elements formed on the surface of Mars. An interesting feature of this planet is that it has the largest mountain of the solar system – Olympus Mons- which has a height of about 69,649 ft. Most of the surface of Mars is filled with craters. The polar ice caps at the Martian poles shrink during the planet’s spring and summer. The atmosphere of Mars is thin which makes astronomers believe that water must have existed there in liquid form once. This planet has two moons Phobos and Deimos.

What are Outer Planets?

The four large planets residing in the outer part of the solar system beyond the asteroid belt are called the outer planets. These planets are also called Jovian planets after Jupiter. They are substantially large and each planet has a somewhat different composition.

All the outer planets have thick atmospheres. Their gravities and low temperatures retain the gas particles and do not allow them to escape into space. These planets generate hurricane-like storms because of the Coriolis effect. Such long term storms have been tracked by astronomers such as the Great Red Spot on Jupiter and Great Dark Spot on Neptune

The densities of the outer planets are close to that of water and in fact, Saturn has a density less than water. All these planets have rings around them with Saturn dwarfing the others in this. The outer planets possess numerous moons when compared to the inner planets.

Deep within the outer planets, there are strong magnetic fields that are powered by electrical currents generated by the fluid movements that of liquid hydrogen. The magnetic fields of these planets are many times greater than any of the inner planets, including Earth.

A large planet that is composed mostly of gases like hydrogen and helium and a relatively rocky core is called a gas giant.

Such gas giants exist in our solar system – Jupiter, Saturn, Uranus, and Neptune.

  1. Jupiter: This is the largest planet in the solar system and has a mass of more than 300 times that of the Earth. Jupiter also has the most number of moons with 63 identified so far. The brightest planet in the sky, it has a very stormy atmosphere. A major storm, Great Red Spot is as big as the Earth.
  2. Saturn: The most beautiful planet in the solar system, it is really hard to miss this planet with its distinctive large rings. Even though all the outer planets have rings, the most visible ones are that of Saturn. It was only after the discovery of the rings of Saturn that astronomers discovered that all outer planets have rings. The exploration by the Voyager missions shows that the three rings of Saturn have hundreds of smaller rings which are made up of unknown particles and frozen water.
  3. Uranus: This planet is the smallest of the outer planets. Neptune is the only planet to rotate on its side. Even though there are a lot of theories, scientists are still not sure why this planet rotates this way. Some say it could be because of a major collision and some others say it could be because of the small shifts during the planet’s formation which caused this unusual rotation. The atmosphere of this planet contains ices. These ices consist of water, methane, and ammonia.
  4. Neptune: This is the final of the outer planets. The winds of this planet are the fastest of any of the planets and can reach more than 1,200 miles per hour. The atmosphere of this planet contains significant amounts of ices(as called by astronomers). The contents of these ices include water, methane, and ammonia. It is believed that the methane from Neptune is what gives Uranus its blue color.

Main Differences Between Inner Planets and Outer Planets

  1. The basic difference between the Inner Planets and Outer Planets is that the Inner Planets are between the Sun and the asteroid belt whereas the Outer Planets are beyond the asteroid belt.
  2. The Inner Planets are closer to the Sun whereas the Outer Planets are further away from the Sun.
  3. The Inner Planets are smaller and made of rocks whereas the Outer Planets are large and gaseous.
  4. The Inner Planets include Mercury, Venus, Earth, and Mars whereas the Outer Planets are Jupiter, Saturn, Uranus, and Neptune.
  5. The Inner Planets have slow orbits and hence have a slow revolution whereas the Outer Planets have faster orbits and hence faster revolution.
  6. The Inner Planets have no rings whereas Outer Planets have beautiful rings around them.
  7. Geologically, the Inner Planets consist of cooled igneous rocks and iron cores whereas the Outer Planets are made up of gases like hydrogen and helium.
  8. The density of the Inner Planets is greater than the Outer Planets.
  9. The atmosphere of the Inner Planets is all different from each other whereas the atmosphere of the Outer Planets is all the same.

Conclusion

A Planet is an object in space that spins around a star. All the eight planets of our solar system spin in their orbit.  

The study of our solar systems has always been a topic of interest since ancient times. From what makes Earth so special to the question of why there is no life on other planets, all these have been encouraging factors to learn more about our planets.

An interesting thing discovered by some planetary systems is that the outer planets are quite close to the Sun. This makes forecasting how our solar system was formed an interesting exercise for our astronomers.

References

  1. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/GL006i002p00121
  2. https://www.sciencedirect.com/science/article/pii/0019103571900728