Why are the planets in this order?

Olivia Hobson

Why do I want to know? (1a.)

I want to understand if certain characteristics make some planets go to the outside and some to the inside. I think that researching this will help me understand our solar system better.

Planet Definition (1b.)

Technically, there was never a formal definition for the word planet before 2006. The International Astronomical Union voted on a definition for the word planet in August 2006.


A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Planet Order (1c.)

The solar system has 8 planets. The order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are 9 planets if you count Pluto, a dwarf planet, at the end.
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Planet Classification (1d.)

Planets are classified two main ways.


Terrestrial planets

The inner four planets - Mercury, Venus, Earth, and Mars - are known as terrestrial planets. These planets that are closest to the Sun all have rocky surfaces.



Jovian planets

The outer planets - Jupiter, Saturn, Uranus, and Neptune - are known as Jovian planets. Jovian planets are huge compared to the terrestrial planets and are gaseous in nature instead of having rocky surfaces. Jupiter and Saturn are known as gas giants while Uranus and Neptune are called ice giants. All four are made up of mainly hydrogen and helium.

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History (2)

Solar systems begin as a collection of dust spinning through space. After time, the gas collapses in the center, and as gravity builds a star forms. Some form into planets or moons.


Planets form when pebbles moving around the sun have enough gravity to pull other pebbles, gas, and dust into them. This makes them bigger. The bigger pebbles attract smaller ones. They gain mass and gravity, forming discs of matter. They become protoplanets, then planetesimals, and finally planets.


Because the earliest planets had more mass and gravity, they grew faster and faster. For hundreds of millions of years, planets continued to form. Eventually, the planets in our solar system developed rocky cores. Some had more mass and gravity than others. They were able to hold onto their heavy, gaseous atmospheres. The rocky bodies of the inner solar system were not big enough to hold onto the large atmospheres.


It also has to do with the heat. Hotter and lighter gases move faster and escape smaller planets easier than they could from the gas giants. Astronomers think that the inner solar system was about 40 times hotter than the outer solar system. For the outer planets, it was easier to hold onto their atmospheres.

Conclusions (3)

The question of planetary formation remains an ongoing debate because we have never seen a planet form. The conclusion that I found is that the planets are in the order they are in because of how the gaseous cloud, that was the proto-solar system, was spinning and coalescing. Each planet either absorbed or ejected the stuff in the orbit around the sun. When the solar system formed, it was chaotic. There was not a plan for order or success, it just happened the way it did because of all the information we have learned about planet formation.