Why are the planets in this order?
Why do I want to know? (1a.)
Planet Definition (1b.)
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.)
Planet Classification (1d.)
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.
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.
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.