system
Given the following post, respond as if you're commenting on Reddit:
human
Is it a coincidence that the first four planets nearest to the sun are all much smaller then the four other planets?
gpt
No, it definitely isn't. The conventional explanation is as follows:

There are three main types of neutral materials in the universe, which (listed in decreasing order of abundance) are:

1. Gases (Hydrogen and Helium)
2. Ices (Water, Ammonia,  Methane, Carbon Dioxide, Carbon Monoxide)
3. Rocks and metals



Materials in category 1 are always gases, while materials in category 3 are always solids. But materials in category 2 are gases close to the Sun and solids farther away (note that we're referring to conditions of low pressure like those in space, where no materials are liquid).

Far from the Sun, solid cores of planets can get much larger because they can contain abundant ices rather than rare rocks and metals. They can even get large enough to capture Category 1 materials (gases) and get really big, like in the case of Jupiter and Saturn. So outer planets should be much larger than inner planets.

That's the conventional story, but things are more complicated due to the possibility of orbital migration. That's why we've seen hot Jupiters close to their stars. But if violent migration of this type doesn't happen, we expect that things will be like our Solar System, with big planets far out and small planets close in.