Astronomers categorize the planets in our Solar System into two groups: the inner planets and the outer planets. The inner planets are smaller and rockier since they are closer to the Sun. The outer planets are larger, farther away, and primarily formed of gas.

Inner and Outer PlanetsMercury, 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 amazing thing is that the gas giants in some other found planetary systems are really quite close to the sun.

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, and each of the Outer Planets has several things in common with its fellow 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. Some extrasolar systems, on the other hand, contain “hot Jupiters” that orbit close to their Sun.

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. Because these planets are regarded to be heavy, they move slowly. They are tiny planets with an average diameter of roughly 13000 kilometres.

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.

Difference between Inner and Outer Planets

The differences between the two groups of planets are detailed in the table below.

INNER PLANETS

Mercury, Venus, Earth and Mars

OUTER PLANETS

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 of 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 the 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.