Sciency Words: Ice Giant

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Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related vocabulary. Today, we’re looking at the term:


Depending on whom you ask, our Solar System has either four gas giants or only two. Uranus and Neptune are sometimes classified as ice giants instead.

Oc04 Vanilla Ice Giant

Why do we have to make this distinction? Because in the 1990’s (around the time that annoying Vanilla Ice song came out), astronomers began to realize that Uranus and Neptune are fundamentally different from Jupiter and Saturn.

While Jupiter and Saturn are composed of over 90% hydrogen, Uranus and Neptune have a more interesting mix of chemicals: methane, ammonia, water… They have hydrogen too, but the ratio of hydrogen to other stuff is much lower.

It’s believed that during the formation of the Solar System, Uranus and Neptune accumulated vast quantities of ice (hence the name ice giant). By ice, I mean any volatile substance in a solid state, not just water ice.

In describing modern Uranus and Neptune, astronomers continue to call substances like methane, ammonia, and water “ice” even though these substances aren’t necessarily in a solid form anymore. Also, don’t let any of this terminology mislead you into thinking these planets are cold. Their interiors are extremely hot, regardless of their so-called “icy” composition.

Perhaps the biggest difference between gas and ice giants relates to us humans. We don’t honestly know much about the gas giants, but we know even less about their icy cousins. Jupiter and Saturn have been visited by a handful of space probes. Uranus and Neptune have only been visited once each, and that was back in the 80’s.

Oc04 Uranus Spins Right Round

NASA is currently considering a flyby mission to Uranus or Neptune (or both) similar to the recent flyby of Pluto by New Horizons. Approval for that may come in the next year or so.

Of course, if we really want to understand what ice giants are like and why they’re so different, we should send an orbiter, not just a flyby mission. Unfortunately, entering orbit around Uranus or Neptune is much easier said than done. More on that next week.


NASA’s Next Big Spacecraft Could Visit an Ice Giant from Astronomy Magazine.

The Atmospheres of the Ice Giants, Uranus and Neptune from NASA’s Jet Propulsion Laboratory.

Are There Oceans on Neptune? from Universe Today.

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