Gas giants are mostly not gas, and they have no surface
The phrase gas giant was coined in 1952 by the science fiction writer James Blish, and it stuck despite being misleading. Deep inside Jupiter or Saturn the pressure is so great that hydrogen stops behaving like a gas at all, and there is no ground anywhere to stand on.
Giant planets, also called Jovian planets after Jupiter's Roman name, are much bigger than Earth and made mainly of substances with low boiling points rather than rock. Many astronomers now reserve gas giant for Jupiter and Saturn and call Uranus and Neptune ice giants. Planetary scientists use rock, gas and ice as labels for kinds of material, whatever state it is in: hydrogen and helium count as gas, while water, methane and ammonia count as ice.
Jupiter and Saturn are mostly hydrogen and helium, with heavier elements making up 3 to 13 percent of their mass. Beneath banks of water and ammonia clouds, crushing pressure turns hydrogen into a liquid metal that conducts electricity and makes up most of each planet, probably around a molten core. Their atmospheres simply thin out with height until they merge into space, so nothing like a surface exists.
Uranus and Neptune differ. Their hydrogen-rich outer layers reach down to about 80 and 85 percent of their radius, below which lie mostly icy materials. Traces of methane in hazy air tint both aquamarine, and both have magnetic fields sharply tilted from their spin axes. Uranus lies almost on its side, giving it extreme seasons, and although lighter, it holds more hydrogen and helium than Neptune, which is denser.
At the heavy end the line blurs. Bodies above roughly 13 Jupiter masses can fuse deuterium and are called brown dwarfs, though that cutoff is a rule of thumb. Planets circling close to their stars, dubbed hot Jupiters, were long the easiest exoplanets to find. Stranger objects have turned up too, such as the pulsar planet PSR J1719-1438 b, heavier than Jupiter yet smaller, and thought to be largely crystallised carbon.
Source: Giant planet
