What is the scientific reasoning for the order of the planets?
The order of the planets is partly due to where they were formed and partly from the evolution of the Solar System.
The Sun and planets formed from a nebular of gas and dust which collapsed under gravity. Once the Sun was formed the planets started forming.
In the region within about 4 AU of the Sun it was too hot for volatile compounds to condense. The remains of the nebula formed an accretion disc around the Sun. Grains of dust orbiting the Sun collided and started to clump together. The clumps further collided to form the four inner rocky planets Mercury, Venus, Earth and Mars.
The rocky planets formed closer to the Sun than they are to day. Friction with other material in the disc they were forming in slowed them down which made them move to larger orbits.
There would have been more small planets in the inner Solar System soon after formation. One of which, the size of Mars, is thought to have collided with Earth to form the Moon.
The giant planets Jupiter, Saturn, Uranus and Neptune formed further out. Beyond the orbit of Mars it is cool enough for compounds such as water and methane to be solid enough to start the clumping together process.
The giant planets, particularly Jupiter and Saturn, interact with each other gravitationally. It is thought that Jupiter formed further out and migrated inwards to its current position. It is also though that Uranus and Neptune formed further inwards and migrated out to their current positions.