Sciency Words: Ice Giant

Today’s post is part of a special series that first appeared on Planet Pailly. Every week, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s sciency word is:

Ice Giant

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.


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.


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.

Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • Leonardo Faria

    “Unfortunately, entering orbit around Uranus or Neptune is much easier said than done.”

    Who wants an easy space exploration? It’s boring.

    • “Who wants an easy space exploration? It’s boring.”

      Star Trek, Star Wars, Babylon 5, Total Recall, Red Planet, Galaxy Quest, The Expanse, Farscape, Starship Troopers, Solaris, Avatar, Pandorum, Prometheus, the Aliens franchise, Mass Effect, Halo, Killzone, Dead Space- actually, you know what? Forget that last one, it’s better for us in the long run.

      Basically any setting where one would expect interplanetary, let alone intersolar, colonization within a reasonable time frame.

  • John H Reiher Jr

    I wonder if we could do it using an ion drive like we did for the Dawn probe. Have it accelerate until it gets to the midpoint and then turn around and decelerate into orbit about Uranus. You could do an aerobrake as well to save on propellant.

    • Not a bad idea, but I can’t help but think that the whole aerobrake maneuver would actually cut into the payload even more. Not to mention that aerobraking is an extremely tricky maneuver: too shallow and you become a blazing meteor, too much and you bounce off to an eons-long voyage to some distant part of the galaxy.