Astronomers have categorized the planets in our Solar System into two groups: the inner and outer planets.

The inner planets are smaller and rockier since they are closer to the Sun. The outer planets, on the other hand, are larger, farther away, and primarily formed of gas. 

The Inner and Outer Planets

Inner and Outer Planets

 Mercury, Venus, Earth, and Mars are the inner planets (in order of distance from the sun, nearest to farthest).

The outer planets, Jupiter, Saturn, Uranus, and Neptune, appear after an asteroid belt.

The Asteroid Belt, an area of thousands of asteroids orbiting the Sun between Mars and Jupiter, separates the Inner Planets from the Outer Planets.

Although no two planets are alike, each of the Inner Planets has several things in common with its fellow Inner Planets. So is the case with the outer planets.

The Inner Planets, on the other hand, bear few resemblances to the Outer Planets.

As a result, astronomers find forecasting how our Solar System developed to be a fun exercise.

According to popular belief, the young Sun pushed the gases out to the Solar System’s outskirts, which explains why there are so many massive gas giants there.

Inner Planets

Because their surfaces are solid (and, as the name implies, somewhat comparable to Earth), the four inner planets are referred to as terrestrial planets.

However, the phrase can be deceptive because each of the four has very distinct habitats. 

The inner planets are solid because they are formed of rock and metal and because these planets are regarded to be solid and heavy, they move slowly.

They are tiny planets with an average diameter of roughly 13000 kilometers.


Slightly larger than the Moon, Mercury is the smallest in the family.

Despite being so small, Mercury is actually visible to the naked eye. It was discovered in the seventeenth century by Galileo Galilei. 

The Sun rays that strike the surface of Mercury are seven times stronger than the rays received by the Earth. 


For us, Venus is the third brightest object in the Sky after the Sun and Moon.

Venus is often referred to as our sister planet because of its similarity in size and mass. Venus is also the nearest planet to Earth.

Although Mercury is the closest planet to the Sun, Venus is the hottest. 


The only livable planet in the solar system is the 3rd closest to the Sun and is also the 5th largest. 

Although the Earth is the only planet that supports life, the astronomers are on what looks like a never-ending hunt for one more planet that we could migrate to. 

Earth has the ozone layer in its stratosphere which is one of the major reasons it supports life.

Most of the surface of the Earth is full of water and hence it is also referred to as the “Blue Planet”.


Of all the planets in the Solar System Mars is most likely to support life after Earth. The evidence of water found on the surface of Mars means that life is possible however with a lot of restrictions. 

Mars has two Moons and is named Deimos and Photos. These are very small and can easily be mistaken for asteroids.

It is also claimed that they are just asteroids trapped by the massive gravity of Mars

Outer Planets

The outer planets (also known as Jovian planets or gas giants) are massive worlds engulfed in gas.

They all have rings and a large number of moons. Despite their vastness, only Jupiter and Saturn are visible without telescopes.

Uranus and Neptune were the first planets discovered since antiquity, demonstrating to astronomers that the solar system was far larger than previously believed.


Also known as the ‘Gas Giant’, Jupiter is the largest planet in the Solar System. It is massive and has a powerful magnetic field.

It has more moons in the Solar System than any other planet Jupiter is 318 times larger than the  Earth and 2.5 times larger than all other planets combined. Due to its rapid rotation, Jupiter has the shortest days. It rotates around its axis every 9 hours and 55 minutes.


Saturn is a fascinating planet in the family, particularly because of the extensive rings around it that are made of small rocks, ice, and dust. 

Saturn is the second largest planet in the Solar System and is visible to our naked eyes. It is also the least dense planet.

Its density is less than that of water meaning it can actually float on water. 


It is often referred to as an “ice giant” and its upper atmosphere is dominated by methane.

It is the coldest planet with an atmospheric temperature of -224 degrees centigrade. 

Uranus rotates on its axis every 17 hours 14 minutes and it completes one revolution around the Sun every 84 years. Some parts of the planet get direct sunlight for 42 years. 


Neptune has become the farthest recognized planet from the Sun after Pluto was dethroned from its “planet” status.

Neptune is similar to the Earth, but it has a very rocky core. Also, it has a thick atmosphere that prohibits the existence of life.

Physically, Neptune is dark, cold, and covered by supersonic winds and giant ice.

Difference between Inner and Outer Planets


Mercury, Venus, Earth, and Mars


Jupiter, Saturn, Uranus, and Neptune

1.Size Small in size

With a diameter of 12,756 kilometers, Earth is the largest of the Inner Planets (7,926 miles). With a diameter of 4,878 kilometers, Mercury is the smallest of the planets (3,031 miles).

Massive in size 

Jupiter is the largest planet in the solar system, with a diameter of 142,984 kilometers (88,846 miles). With a diameter of 49,532 kilometers, Neptune is the smallest of the outer planets (30,779 miles).

2.Shape They have solid surfaces and thin/no atmospheres. Theoretically, you could stand on each of the Inner Planets, but you’d only be able to survive on Earth.They are gaseous balls with no surface. The Outer Planets are mostly comprised of gas. They are more likely to have a considerably smaller solid or liquid center. Standing on any of the Outer Planets would be impossible.
3.Density The density of the components that make up the planets determines the size and composition of the planets. The Inner Planets’ elements are more closely packed together, making them smaller and more solid with greater density. The elements that make up the Outer Planets are less densely packed together than the components that make up the Inner Planets, resulting in them being very light for their size. Hence, they have a lower density. 
4.Atmosphere The atmospheres of the Inner Planets differ in composition from planet to planet. Although Sodium and Helium can be detected above the surface of Mercury, it does not have an atmosphere. The atmosphere of Venus is primarily carbon dioxide with a little nitrogen. The majority of the gases in the Earth’s atmosphere are nitrogen, with a minor quantity of oxygen and even smaller amounts of other gases. Mars has a comparable carbon dioxide and nitrogen makeup to Venus, but its atmosphere is significantly thinner.The Outer Planets’ atmospheres are largely made up of hydrogen and helium, with some methane in the atmospheres of Uranus and Neptune. Other gases can be found but in much lesser amounts. Moreover, they have similar atmospheres. 
5.Discovery For thousands of years, people have known of the existence of the Inner Planets. The four objects (including Earth) were discovered by early astronomers, who were unaware that they were planets.Only Jupiter and Saturn were observed by ancient astronomers among the outer planets. It wasn’t until later that Uranus and Neptune were discovered. Uranus and Neptune were discovered in 1781 and 1846, respectively.
6.Moons Earth and Mars are the only planets with moons around them. Earth has one moon, and Mars has two small moons.There are several moons around each of the Outer Planets. There are 63 moons that orbit Jupiter, 60 moons that orbit Saturn, 27 moons that circle Uranus, and 13 moons that orbit Neptune.
7.Rings There are no rings orbiting any of the Inner Planets.There are rings orbiting all of the Outer Planets. The rings are thin discs of dust and rocks that may have formed as a result of moons breaking up or failing to fully form while orbiting the planet. Saturn’s ring system is the most visible of all the planets.
8.Orbiting Speed  The Inner Planets complete their orbits swiftly because they are so close to the Sun. Mercury orbits the Sun in only 88 days. It takes 687 days for Mars.The Outer Planets orbit the Sun at a distance of millions of miles and must cover a significantly greater distance to complete an orbit, hence they take much longer. Jupiter takes over 12 years to complete an orbit, whereas Neptune’s takes over 164 years.
The differences between the two groups of planets.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.