Question: what are planets made of?

Andrew McKinley answered on 21 Nov 2014:

Planets, ultimately, are made of “stardust”! All the elements in the planets came from the remnants of a star which exploded billions and billions of years ago! That dust started to clump together and rotate, and this eventually formed the solar system.
Much of the gas was drawn to the centre – and eventually formed our local star, the Sun. As the other clumps came together (a process called ‘accretion’), they formed bigger and bigger clumps – eventually these became the planets as we know them.

Because much of the gas was drawn to the centre of the solar system, the planets nearest the centre are mainly ‘rocky’ planets, mostly consisting of the heavier elements of the Periodic Table, while the outer planets still have much of the gas surrounding them.

Most planets will have an iron core – iron is the heaviest element that can be normally made inside a star, so it sinks to the centre.