Final answer:
There are three broad classes of planets: terrestrial, gas giants, and ice giants, with evidence based on compositions, structures, and formation theories. Terrestrial planets are rocky, gas giants have hydrogen and helium layers, and ice giants have mantles of water, ammonia, and methane ices.
Step-by-step explanation:
We have strong evidence for three broad classes of planets in our solar system: terrestrial planets, gas giants, and ice giants. Terrestrial planets, like Earth, are rocky with metals and have undergone differentiation, where heavier materials sank to the centre during their molten phase. Gas giants, such as Jupiter and Saturn, have dense cores surrounded by thick layers of hydrogen and helium. Ice giants, like Uranus and Neptune, also have a core and are surrounded by a mantle rich in compounds such as water, ammonia, and methane ices.
The evidence for these classes comes from observations of planetary compositions, structures, and formation theories. Terrestrial planets formed closer to the sun, where it was too warm for gases to condense. The gas and ice giants formed farther away, where it was cold enough for gases to condense. Studying exoplanets and moons also provides insight, with thousands of exoplanets revealing a range of sizes and compositions, some similar to Earth and others to the giant planets in our solar system.