Icebergs in a cosmic sea

The habitable ice planet is easiest to explain.

Authors on a time-budget may moot a “Snowball Earth”, on the model as has happened here on Earth. Here is a mostly-water world a few billion years old where has lately arisen a carbon-crashing but oxygenic cyanobacterium [UPDATE 2/15/24 OR: something removed the addition of carbon-dioxide, long after that]. Open water still exists; but it is equatorial, and littered with iceberg. It is not Hoth, since it supports no indigenous complex life; but it can be home. Maybe it will get warmer later; maybe there’s some other mechanism which prevents that. And don't expect much of a spinning magnetic core here.

The offworld habitable planets in Pohl’s first Gateway book are hinted as natural snowballs: Peggy’s World, and that other one. Also that miserable haven in the recent Battlestar Galactica series.

Book critics will complain that natural snowball planets are “boring” but they are also common, so will feature in any story set in an interstellar network of inhabited worlds. Pohl handled the phenomenon about as well as anyone could, by putting them into the background.

Cold planets are more interesting when unnaturally so. Such can happen in the aftermath of some cosmic accident - like Chicxulub. Expect these in systems as haven’t done with the Heavy Bombardment. As far as we know this event demands newly-assembled planets so precedes “blue green algae” by billions of years; but in science-fiction, we can allow that some alien (in which I include, humankind) has terraformed the 9.8g planet already and made it (prematurely) temperate.

Also expect cold snaps in the wake of war, once some attacker deliberately flings comets into someone else’s home.

These worlds might not be snowballs yet but they’re headed that way. They’ll get better later but not during the protagonists’ lifetime.