Ice Planet is a subtrope of Single Biome Planet. The defining terrain feature is snow and ice. There may or may not be some exposed rock. If you're really lucky you'll get ice floes and icebergs in a really cold ocean - but it's not unheard of for a planet to be so frigid there's no liquid water. Frostbite and Hypothermia are constant dangers.
Hypothetically, there's a couple ways such a planet might come to exist:
- A very lush and wet terrestrial planet might undergo a really bad Ice Age and end up looking more or less like this, though probably never to quite the extent it's typically presented in sci-fi. Even in the worst Ice Ages, our Earth has always had at least a temperate zone along the equator. See Ice Age for more ideas.
- Some planets form out beyond the snow line, so far from their star that all the naturally occurring water is frozen. These come in a two varieties, Ice Giants and Ice Dwarfs, with differing histories and physical properties. See those corresponding pages for details.
The defining factor that seperates the sci-fi ice planet from it's real-life analogs is oxygen. Realistically, a big ball of ice shouldn't have much oxygen on it. Since it has no forests, and no warm oceans full of algae, there's no way to replenish oxygen you use. So, the longer a colony exists on the planet, the thinner the air will get, until eventually everyone suffocates. The same problem faces the Desert Planet, and that page has a few suggestions how to get around it. Technically, you've got an extra option on an ice planet, because there might be liquid oxygen deep beneath the ice that can be mined and pumped to the surface.
On a similar note, it's worth mentioning that oxygen is more soluble at lower temperatures - cold water can carry more oxygen than hot. This means there may be some sort of aquatic life on an ice planet. In our own oceans, there solubility of oxygen directly translates to a greater density of life in the arctic ocean than in the tropics.
One theoretical way to supply oxygen is electrolysis. Theoretically, massively heavy lightning could electrolyze the water ice into oxygen and hydrogen. If the planet is Earth-sized or somewhat smaller, the hydrogen could then escape to space, leaving the oxygen. However, the amount of lightning needed would be quite ludicrous, probably not really possible without some handwaving, and would pose a great threat in itself. Precursor technology might use the same principle to *make* an ice planet habitable.
Game and Story Use
- Ice Planets are good places for Precursors to hide their lost technology or megastructure. Extreme cold and thick layers of snow and ice will make it hard to find.
- Once such tech is turned on, though, the heat of it's operation might convert the planet to a Water World.
- For more ideas, see Grim Up North.