Analysis / Single-Biome Planet

Single Biome Planets in real life, while not as common or diverse as the trope makes them out to be, are still quite possible - one would say that for the universe, a Single-Biome Planet is the norm, because it's very easy to get either really hot or really cold - so here is a list of Single Biome Planets which could possibly occur naturally with our current understanding of physics, and their likelihood:

The Common Ones

Ocean Planet

This one is relatively easy: If a planet attracts a lot of icy comets throughout its formation and has the means through which to melt the water (either through external or internal heating), and can hold onto the subsequent water-vapour atmosphere, then you end up with a very humid world covered entirely in water.

However, this only works if you - like the trope - consider 'Ocean' to be a single biome. The trope makes no distinction between the different kind of undersea biomes, which can vary wildly (just look at what goes on beneath the Earth's ocean surface).

Ice Planet

Take an Ocean Planet and remove whatever is heating it, and you will end up with a very cold world covered in ice. Because little energy is provided, don't expect this one to be anything but ice, glaciers, and frost. If the planet has tectonic activity that nonetheless can't heat the ice, the ice might shift, but this world is still unlikely to be harboring much in the way of life.

That said, since cold is an excellent preservative, these kinds of worlds tend to keep remains - whether biological or not - around for a long time

Desert Planet

Park your planet close to its orbiting star, but not so close as to make it a Tidally Locked Planet, and the star will eventually boil away the oceans, leaving barren rock and sand behind on the surface.

Dark Planet

Take any planet - Literally any planet - and move it to the outer limits of a star's gravitational pull - or completely out of it, and you end up with a planet where all you can see is the night sky. One of those stars might be bright enough to stand out, but it is not enough for it to have any measurable impact.

Volcano Planet

You just need to place a planet in a situation where it is being squeezed by tidal forces of the nearby star: This will cause the planet to be close enough to be scorched by the star, and the tidal forces will ensure there is plenty of plate tectonics to form the volcanoes needed. Alternatively, a planet that had suffered a sufficiently massive impact event would exhibit the requisite volcanism from the induced internal heating for millions of years, as would many protoplanets.

Cloud Planets

Technically, any Gas Giant counts as this. Don't expect floating cities to come naturally, nor for their construction to be easy.

The Probable Ones

Mountain Planet

Humans have evolved on Earth, and are adapted to its atmosphere. It is very important for us that there's not only enough oxygen around, but also that there isn't too much of it - oxygen is incredibly reactive and throughout your life, your body is constantly repairing the damage caused by breathing it. If there's too much oxygen around, or what oxygen there is is under too much pressure, you end up dying from oxygen poisoning.

On a world with too much oxygen, the lower pressure of the mountains might be the only place that humans can survive, and so the only question that remains is whether or not it is possible for a world to be entirely covered in mountains.

The answer is yes: All that is needed is for the planet to have an active core causing a lot of plate tectonics. Not only will you have mountains everywhere, but volcanoes are practically a guarantee as well.

Twilight Planet

As noted on the page, this really is three biomes in one, but all you need a Tidally Locked Planet inside a habitable zone, and you will end up with these.

The Improbable Ones

Jungle Planet

Jungle worlds are basically one step removed from Ocean Worlds - to make a jungle, you need to ensure a lot of rain over the course of many years - this means that the world is likely to be hot and humid, constrated with the Desert World that is hot and dry. What puts this in the Possible category rather than a common one actually has to do with topography: If you have a mountain range, you can be almost certain that one side of that mountain will be hogging most of the rain, leaving the other side unable to form jungle.

Thus, to make a jungle world you need to have a world that:

1. Is very flat
2. Has enough water to be humid
3. Has enough heat to be hot,
4. Doesn't have enough water to form oceans while still fulfilling the other three.

It's telling that a subcategory of "Tropical Planet" is often used, allowing for oceans and thus rendering point number 4 unnecessary. This moves it to "Probable" on this scale.

Swamp Planet

See: Jungle Planet. But make the temperature less hot.

Forest Planet

See: Swamp planet. But make it less humid while still having enough water to support forests.