If the largest planets are mostly liquid, why are they sometimes called “gas giants”?

This is an interesting question, which leads many scientists to avoid using the term “gas giants.” But here’s what’s going on: Jupiter, Saturn, Uranus, and Neptune are “giants” in the sense that they are much larger and more massive than the rocky inner planets (Mercury, Venus, Earth, and Mars). The reason they are sometimes called “gas giants” is because they are made mostly of hydrogen, helium, and a variety of molecules (including methane, ammonia, and water) that are commonly found as gases on Earth. In that sense, the term “gas giants” might seem appropriate.

However, the combination of pressure and temperature in the interiors of these planets actually compresses most of their “gas” into liquid form (and sometimes even to “weird phases” in which matter behaves differently than we’d usually expect for either a liquid or a solid). For that reason, many scientists prefer to refer to planets that formed mostly from gases as “jovian” – which means “Jupiter-like” — because after they form, the gases tend to be compressed into liquid phase. In this book, we will refer to them as “hydrogen-rich,” since that accurately describes the fact that a large proportion of their masses is made from hydrogen atoms or from molecules that contain hydrogen.

For those of you who have read more about the planets, you might also be wondering about an even more confusing vocabulary problem that arises with Uranus and Neptune. While Jupiter and Saturn are made mostly of hydrogen and helium, which are always gaseous on Earth (except when we compress them with machinery), Uranus and Neptune are made mostly of molecules like methane, ammonia, and water. Because these molecules freeze to make solid ices at cold temperatures, you will sometimes hear people refer to Uranus and Neptune as “ice giants.” But this is a terrible term, at least in the opinion of the authors of this textbook, because these planets are far too hot inside for any of their material to be in the form of ice. In fact, the only place where there is any ice at all in Uranus and Neptune is in their clouds, and even there it just miniscule amounts in the form of tiny “snow flakes.” So rather than referring to these ice-free worlds as “ice giants,” your textbook authors suggest categorizing them along with Jupiter and Saturn as “hydrogen-rich” or “jovian” planets.