Recently I added support for ocean planets - the planets consist almost entirely of water, and can have oceans several hundred kilometers deep. Technically, these would be Terra planets (Earth-like planets with water) but without continents. I've also made the icy polar caps more bumpy. Theoretically, if ocean planets become cold enough, their surfaces may freeze completely, and these planets will become ice worlds, similar to Europa - an ice-covered moon of Jupiter which has a liquid ocean under its icy crust. From this point of view, all icy satellites of giant planets (and even icy planets) could conceivably be called a "frozen ocean planet," or vice versa a "hot ice planet." The boundary between these two classes of planet can be considered as the presence of open (unfrozen) water on a planet's surface.
Below, the first two screenshots show an ocean planet from space, with and without clouds. The third picture shows the surface of the polar cap boundary (Note that all images on this page have a reddish hue given off from the lighting by Red Dwarf stars):
A more interesting planet is a tidal-locked one to its sun; its always turned toward the sun on one side, as the Moon is to Earth (this is known scientifically as a Spin-Orbital Resonance 1:1). This is possible if the planet is close enough to the sun so solar tides are able to slow down a planet's rotation. Most often this occurs in systems of Red Dwarfs, because these systems are compact, and Red Dwarfs have a low luminosity, so planets can be located close to them and still be pretty cool.
On a planet tidal-locked to its sun, one side has eternal day, the other side has eternal night. If the planet has no atmosphere, then the volatiles from the day-side may evaporate and settle on the night-side as a snow layer. If the planet's atmosphere is tenuous, it can completely freeze-up on the night-side.
Much more interestingly are the climates of planets with dense atmospheres. According to scientific numerical models, there is a particular atmospheric circulation: Where hot air at the center of the day-side hemisphere (called a "hot pole") rises and moves towards the night-side hemisphere; where it cools and descends in order to return to the day-side hemisphere again. As a result, powerful winds blow constantly near the planet surface in the direction of the hot pole of the planet, and a never-ending cyclone forms around the hot pole. If the planet has an ocean, it may freeze on the night side, but not completely at the ocean floor - this will be prevented by the warm underwater flows.
The climate of planets with very dense atmospheres will be different. A dense atmosphere transports heat efficiently, so that the planet's climate will be smoother, with almost no difference between the day and night sides. The circulation of the atmosphere will resemble a Venusian super-rotation; where the whole atmosphere is likley to revolve around the entire planet, and will be much faster than the rotation rate of the planet itself.
A tidal-locked Earth-like planet as seen from space, with and without its clouds. And another picture showing the planet surface:
This is an interesting development. I feel thrilled at the prospect of being on a planet that has eternal day on one side and eternal night on other-side. I can imagine SE having interesting weather effects like snowstorms or hailstorms on these planets' eerie dark-sides. This'll bring even more awesomeness to the game. Great stuff as usual.