What are Inner Planets?
In our Solar System, which consists of eight planets total, there are four inner and four outer planets, with the asteroid belt between them.
The inner, or Earth-like planets, include Mercury, Venus, Earth and Mars, and are not to be confused with inferior planets, which are those closer to the Sun than Earth (so Mercury and Venus).
This classification of inner and outer planets was made based on their shape, size, structure and number of moons orbiting those planets. In the case of inner planets, they have rocky compositions – their crusts are made up of minerals such as silicates, while their inner layers are made up of metals such as iron or nickel.
The Earth is no exception and has the same characteristics. In addition, inner planets have few or no moons, and not one of them has some sort of a ring system, like those seen with Neptune, for example. Finally, they’re much smaller and less massive than the outer planets, but also warmer.
This temperature range, combined with the fact that all of the inner planets except for Mars allows the other planets to have atmospheres that can create and support weather and climate.
What are Outer Planets?
As opposed to the inner planets, the outer planets are further away from the Sun, thus making them much colder.
Separated by the asteroid belt between Mars and Jupiter, they’re on the other side of it than the inner planets. The outer planets include Jupiter, Saturn, Uranus and Neptune.
They should be distinguished from superior planets, which are those that are further away from the Sun than the Earth, and which have Mars in addition.
They aren’t rocky, but either gaseous or icy, and much more massive than all the inner planets combined.
In fact, the outer planets are 99% of all the mass that orbits around the Sun, and only Jupiter and Neptune are almost 400 times more massive than the Earth.
While the inner planets were mostly made up of minerals and metals, the outer ones are usually made up of hydrogen and helium, the two most common elements found in the universe.
All of them have rings, although the famous Saturn’s rings are most easily observed from Earth. Additionally, all of them also have moons orbiting around them, with the Jupiter’s moons Ganymede, Callisto, Io and Europa being the most common examples.
Similarities between Inner and Outer Planets
It’s obvious from the differences I’ve covered above that there aren’t many similarities between these two types of planets. There are some, however, and they’re listed below:
- They’re all planets orbiting around the Sun
This might seem like a trivial statement, but, with new extrasolar planets discovered each day, it’s becoming an increasingly important distinction.
- They all have their axis of rotation and rotate in the same direction, except for Venus and Uranus
Venus and Uranus are famous for their retrograde rotation, meaning they rotate clockwise, while all the other planets, both inner and outer, rotate in the counter-clockwise direction.
- They were all formed around the same time
There are several theories that try to describe the evolution of the Solar System, but most of them state that all of the planets formed in a relatively short window of time.
- They’re all ellipsoidal or spherical
They all look like balls, but are in fact slightly “squeezed” due to the fact that they also rotate around their axes, and thus are ellipsoidal. They can usually be approximated to be spherically symmetric without many consequences, though
At 2.3-3.3AU from the Sun, there’s the so-called Asteroid Belt, which separates inner from outer planets. While inner planets are rocky, relatively small and relatively warm, mostly with an atmosphere and without moons or planets, the outer planets are completely opposite – they’re very large and massive, either gaseous or icy, mostly composed of hydrogen and helium, and all of them have rings and moons. This distinction is important as it tells something about how the Solar System was formed and how it evolved over time.
Author: Dr. Howard Fields
Dr. Howard is a Clinical Psychologist and a Professional Writer and he has been partnering with patients to create positive change in their lives for over fifteen years. Dr. Howard integrates complementary methodologies and techniques to offer a highly personalized approach tailored to each patient.