Astronomers Reimagine the Making of the Planets

When I was much younger, I was introduced to the "failed star" theory for gas giant planets. That theory is a lot more than just failed stars. Other relevant parts of the theory include that the gravity created by both stars and these failed star gas giants create the dense but not always solid cores that sometimes eventually become planets if something catastrophic occurs - which is not uncommon in the stellar grand scheme of things.

They tend to change a lot once iron starts to form, unless they are stars (and related bright things) that are big enough to create enough pressure for the creation of other elements like gold and platinum that need more nuclear pressure than most others.

We know it rains diamonds within the layers of near-twin planets Neptune and Uranus, so it's not really all that far fetched.

There are a number of theories, and once in a while someone will go beyond the one they teach without debate in grade school.