Difference Between Inner and Outer Planets

The solar system is a system of eight planets, moons, asteroids and other celestial bodies orbiting the sun. Beyond our solar system, thousands of exoplanets orbiting their host stars have been discovered. Some host stars may have dozens of planets. Most of the properties of planets are different from planet to planet. Anyhow, there are some outstanding differences between inner and outer planets. The main difference between inner and outer planets is that inner planets have a high temperature compared to outer planets. This article mainly focuses on the differences between inner and outer planets of our solar system.

What are the Inner Planets

As the term inner suggests inner planets are the closest planets to the sun. Inner planets are also known as terrestrial planets and are mostly made of rocks and metals. They are small in size compared to the outer planets. They have a solid surface and molten metal core.

As scientists believe, these planets were formed from a nebula. A nebula is a massive cloud of dust and gas. The cloud collapsed as gravitational attraction pulled the material together. Most of the material accumulated at its center to form the sun. The rest of the matter continued to contract while rotating and flattened into a disk called protoplanetary disk. The light gases were swept away from the inner part of the solar system by the sun’s strong radiation. The heavy rocky particles within the inner part of the disk formed planetesimals and eventually, inner planets.

In our solar system, the four innermost planets Mercury, Venus, Earth and Mars are known as inner planets. They lie inside the asteroid belt. They have volcanoes and valleys and are very hot compared to outer planets. The first two planets Mercury and Venus have no moons whereas one moon orbits the third planet earth. Mars, the outermost inner planet has two natural satellites or moons.

Earth is the dominating planet out of four inner planets, and all of the inner planets are nearly similar in size and composition. Therefore, sometimes the four inner planets are also referred to as earth- like planets.

What are the Outer Planets

The light gases and ices within the outer part of the protoplanetary disk attracted and stuck together forming large celestial bodies. Eventually, they collided together and merged into massive gas giants called outer planets. All of these planets lie beyond the asteroid belt.

There are four outer planets in our solar system namely Jupiter, Saturn, Uranus, and Neptune. They are large in size compared to inner planets. Jupiter is the largest outer planet whereas Uranus is the smallest. All the outer planets have a lot of moons and thin ring systems made of dust and rocks. Their thick atmospheres consist mostly of Hydrogen and Helium. They are very cold as they are far away from the sun. Sometimes, the last two planets Uranus and Neptune are referred to as ice giants.

Anyhow, the former ninth planet, Pluto is no longer considered a planet. It is a dwarf planet. (Why is Pluto not a Planet Anymore)

Difference Between Inner and Outer Planets

Synonyms

Inner Planets: Inner Planets are also known as Terrestrial planets, Earth-like planets.

Outer Planets: Outer Planets are known as Gas giants.

Members of the category

Inner Planets: Mercury, Venus, Earth and Mars are the four inner planets of the solar system.

Outer Planets: Jupiter, Saturn, Uranus, and Neptune the four outer planets of the solar system.

Density

Inner Planets: Inner planets are dense compared to outer planets.

Outer Planets: Outer planets are less dense compared to inner planets.

Nature of the Atmosphere

Inner Planets: Inner planet’s atmosphere is thin. (Mercury has no atmosphere).

Outer Planets: Outer planets’ atmosphere is very thick.

Size

Inner Planets: Most of the inner planets are smaller than the outer planets. Earth is the largest inner planet while mercury is the smallest.

Outer Planets: Most of the outer planets are very large. Jupiter is the largest while Neptune is the smallest.

Temperature

Inner Planets: Inner planets have a high temperature.

Outer Planets: Outer planets are cold compared to inner planets.

Spin

Inner Planets: Inner planets spin slowly.

Outer Planets: Outer planets spin faster than the inner planets.

Orbital Speed

Inner Planets: Inner planters orbit the sun quickly.

Outer Planets: Outer planets orbit the sun slowly compared to inner planets.

Orbital Location

Inner Planets: Inner planets lie inside the asteroid belt.

Outer Planets: Outer planets lie beyond the asteroid belt.

Number of Moons

Inner Planets: Only earth and mars have moons. One moon orbits the earth and two moons orbit the mars.

Outer Planets: All of the inner planets have many moons.

Other Properties

Inner Planets: Inner planets have no rings.

Outer Planets: Outer planets have rings.

Image Courtesy:

“Inner Planets” by NASAMercury image: JHUAPLVenus image: JPLMars image: HST – Mercury Globe-MESSENGER mosaic centered at 0degN-0degE.jpgVenus globe.jpgThe Earth seen from Apollo 17.jpgMars 23 aug 2003 hubble.jpg, (Public Domain) via Commons Wikimedia 

“Outer Planets” by NASA – JPL image (Public Domain) via Commons Wikimedia 

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