Is mercury a gas or an icy planet?

Answer 1

Mercury, Venus, Earth and Mars are all rock planets. The more technical term is terrestrial planets and they are composed of almost entirely of rock and metal. Each has a tiny amount of atmospheric gas and Earth has a tiny amount of liquid water.

The other four planets are the gas giant planets, Jupiter, Saturn, Uranus and Neptune. That means they do not have a solid surface that is visible and much of the mass of the planet is the gaseous material. (All must have some small inner region consisting of rocky material, even if it is a tiny fraction of the mass that has been collected from stray asteroids.)

The two gas giants closest to the Sun, Jupiter and Saturn, are the two largest in the solar system with Jupiter the largest of all. Both are composed mainly of hydrogen and helium.

The outermost gas giants are called ice giants. Being colder, Uranus and Neptune have formed with substantial quantities of water, ammonium and methane with smaller amounts of hydrogen and helium in the outer regions. The water, ammonium and methane are solid at these temperatures, so the center of these planets is primarily frozen gasses with hydrogen and helium being predominant as an atmosphere.

Answer 2

mercury is a rocky planet