What type of planet is not composed of silicate rock?

Answer 1

We use Earth as a reference. 'A planet not composed mostly of silicate rocks' would likely be a gas planet, although one could imagine a planet consisting of mostly water ... a so-called "water world". Within our sloar system, we call these gas planets "Gas Giants", because in reference to earth they are all very large. The 4 Gas Giants in our solar system are Jupiter, Saturn, Uranus, and Neptune. Sometimes Uranus and Neptune are referred to as "Ice Giants" based on the chemistry of their outer atmospheres and their interior cores. See this link for some info on orbital periods, masses, densities, etc: http://www.solstation.com/stars/jovians.htm

Answer 2

The majority of planets referred to as gas giants, such as Jupiter and Saturn, are composed of lighter elements, mostly hydrogen and helium. Uranus and Neptune have a higher fraction of compounds like methane in their atmospheres; but all four evidence an outer envelope with a preponderance of non-silicate elements. Note however that all the outer planets are believed to have a relatively small rocky/silicate core.

Answer 3

A gas giant planet, such as Jupiter or Saturn, is not composed of silicate rock. Gas giants primarily consist of hydrogen and helium, with some traces of other gases like methane and ammonia.