memes

It's an ice planet!

Carter, after exiting the second gate on Earth

Or a really cheap single set piece that vaguely fits the theme of an ancient earth culture that has managed to not change at all in millenia, and then there is a single high tech alien device in the middle of it.

BTW, I say that with love. Stargate is the best.

Also Star Wars... Star Wars even have a city covering an entire planet.

From Irregular Webcomic!, #87 via https://tvtropes.org/pmwiki/pmwiki.php/Main/SingleBiomePlanet

Imperial Officer: Lord Vader, the rebels have fled the ice planet of Hoth. After going to the swamp planet of Dagobah, Skywalker has rejoined his friends on the desert world of Tatooine. And now the rebel fleet is massing for an attack on the forest moon of Endor.
Darth Vader: I sense a great disturbance in the Force.
Imperial Officer: My lord?
Darth Vader: How else can so many worlds be totally covered with only one terrain type without regard to latitudinal variations?

Mars is rocky desert or rocky desert with canyons.

Mars has river deltas. It has flat plains. It has shifting rolling dunes. It has mountains and valley. It has a twisting series of canyons so constricted they're called the Labyrinth of Night. It has vast ice sheets and polar caps of frozen carbon dioxide and water. It has caves and frozen mud flats and a thousand other varied forms.

Mars is a world. It is a place. It has biomes as varied and unique as those of Earth.

Pluto is ice ball or rocky ice ball.

There are more things in Heaven and Earth than are dreamt of in your philosophy.

Some Sci-Fi planet types are reasonable.

The Kepler program found a lot of exoplanets and has categorized them generally as Hot Jupiters, Cold Gas Giants, Ocean Worlds & Ice Giants, Rocky Planets and Lava Worlds.

If you ignore the gas giants because there's no surface to land on, rocky planets (and maybe desert planets) would be extremely common. Water or ice planets would also be incredibly common. And, if you're really unlucky, you might end up on a lava planet -- one that's small and very close to its sun.

What wouldn't be common are things like an entire planet that's a swamp, or an entire planet that's a forest of Earth-style trees. I'm sure it's entirely possible that on some planet there's a life-form that becomes the dominant form and that looks vaguely like Earth-style trees, but not the kind you see on a typical SciFi show filmed near Vancouver.

Hmm. Still no resolution on the egg sac question, though.

Moss is to be sorted by color not taste!

Lower Decks vibes!

And only a few specific exports of a single variety. Apparently Romulans found one fermentation technique and never experimented again.

California, such diverse film locales

They're making a new game with that exact premise actually. Only one planet but still

No man’s sky also did it because of lazy. People may have forgotten, but that game released as pure hot garbage and only got better after tons of updates.

And how there's usually a single culture on each planet.

Also there have been eras in earths history where it was basically like one of two environments. Like before the continents emerged from the oceans properly, or the several snowball earths, or the multiple times a super continent formed and created swamp land and desert land because of the fucken Appalachian mountains.

Water doesn't have to be the thing that brings variation. Titan has a methane "hydrology" with clouds, rivers, valleys, and beaches whose sand is made of ice. On Triton, ammonia cryovulcanism powered by tidal forces from Neptune create plains with ammonnia snowfall, ice mountain ranges, and underground lakes. On Miranda, the planet is ice, but there are massive terrain differences from 10 km cliffs to flatlands. Io has a massive variety of volcanic planes with color differences visible from space because of their entirely different chemical compositions. The turbulent atmosphere of Jupiter is streaks of water vapor clouds, upwellings from deep beneath the surface, cyclones and massive pressure drops that dent the atmosphere inward by kilometers, with ionosphere above and gas as dense as water below. Even an atmosphere-less grey rock like Mercury has basalt plains, craters, ridges, highlands and dust plains.

In No Man's Sky, many planets have life, which requires complex chemistry being possible at the temperatures the planet has using the chemicals that are available on that planet. This then naturally creates temperatures that are "too cold" for that life and "too warm" for that life, and complex adaptations made by that life to take resources from places that get "too cold" or "too warm" with less risk of predation or competition. Similar adaptation is possible to other extremes/variations, such as "submerged", "on land", "flying", "too dry", "too few nutrients", "too acidic", "too basic", "too steep", "cave", etc. And thus we get complex biospheres that vary across the planet.

I mean, not really? There's lots of reasons to use aliens in a story and I'm struggling to think of one that only works if you assume low-diversity planets

"This is all one castle."