What Could Create the Remaining Two Types of Single-Biome Planets?

Question

There have been numerous questions in this SE about single-biome planets popularized in the Star Wars mythos. Despite the criticisms, these worlds are plausible, and they all involve the right combination of distance to their parent star, the thickness of their atmospheres and how much greenhouse gases are in the atmosphere. The thicker the atmosphere, the more heat and moisture it can hold. So with these factors, worlds predominated by desert, forest, swamp and even tundra are within the realm of possibility. But this leaves us only two biomes left:

• Tropical/temperate grasslands
• Mountains

Within these two different biomes, would the same factors listed above create a world predominated by either one of the remaining biomes, or would other factors be involved?

Answer 1

Recipe for Mountainous Single Biome Planets

1. Start with a desert planet in a young solar system full of small rocky (non-ice) asteroids.
2. Let the asteroids fall into the planet's gravity well to punctuate the planet's crust with craters and crater ridges. Keep the individual impacts down to a size that at most cracks the tectonic plates, not so big that they liquify large swaths of the mantle.
3. With enough asteroid strikes to cause lots of fault lines, but with no engulfing oceans to wear down the resulting mountains, you planet will soon be covered in massive peaks.
4. When your planet reaches the desired level of mountainous-ness, have your solar system run out of nearby asteroids. Stop the ongoing bombardment.

Answer 2

Trees are not a given. You could have a world whose only plants are grasses, or at least grasslike. Evolution on Earth favoured large woody structures in land plants but that doesn't necessarily mean that that is the only viable pathway. It is plausible that land plants could have maintained the frond like structure of the Caulerpa seaweeds which resemble underwater grasses, albeit of potentially very large size. The problem then is simply one of tuning the world temperature and water balance to the point where there are no polar caps etc... to interrupt the spread of these primitive grass-ish plants.

All the land is mountainous and effectively at high altitude is easy but that means having a lot of water and a thin atmosphere. The world is covered in huge oceans divided by narrow mountain chains created by uplift along tectonic collision zones, the thin atmosphere makes sea level feel like 13,000 feet. To have a world with no ocean and all mountains you need gravity and lots of it, the world itself will be relatively small but it will be orbiting close to it's Roche Limit in 2:1 Resonance with another large world that also gets close enough to cause land tides that are a noticeable percentage of the thickness of the crust. This world will undergo continuous and massive crustal stresses that crack apart and mash together its crust creating a surface covered in ridges and valleys that look to an Earthling like mountainous terrain.

Answer 3

Recipe for Tropical Single Biome Planets

1. Have a planet with a dam' good magnetic field and a thick atmosphere, around 20atm may be enough
2. place it in orbit around a hot star, emitting in UV to soft-Xray at the peak of the black-body radiation spectrum - maybe a young white dwarf? You can put the planet way further than in the case of UV-Vis stars
3. watch how the top of the atmosphere gets fully ionized (and kept in place by the magnetic field), ions persisting for longer (so the night is mostly an aurora, no visible star). This "fluorescence bulb" type of receiving the energy, by shifting the wave length of the incoming radiation to something that pumps up a "normal energy" chemistry, will result in distributing the energy more uniformly around the globe.

The last point is to say "no more cosine law for the insolation dependence with latitude, the polar regions included", which allows you to make a uniform climate.

Answer 4

The equatorial ridge planet!

I invented this fast spinning planet for your question about the all-Mediterranean world. But I cannot find that question - you must have deleted it. I wonder why? I thought it was a fine one.

In any case: same principle here but colder. The planet spins fast and so is flattened with a high altitude equatorial bulge. The whole planet is colder than the Mediterranean version I would have liked to link up here. The only biome is the tundra biome, a term which I think is more specific to what you are looking for at high altitude on mountains.

https://www.conserve-energy-future.com/alpine-tundra-biome.php

Tundra is a region found in the coldest places on earth. They are of two types: the arctic tundra and the alpine tundra. The Alpine tundra is located on high mountain regions in the world, for instance, the Swiss Alps. As opposed to the Arctic Tundra, the Alpine Tundra are found away from the pole regions. The word ‘Alpine’ is derived from the Alps which are high mountains that are found in Central Europe.

The characteristic of the Alpine is similar to polar tundra in that it has no trees, has annual temperatures that are recorded to be very low, and most animals migrate to these regions only during the productive summer periods.

Places which would be warm because of latitude are not warm, because of altitude. The tundra at higher latitudes just continues on to the lower latitudes and higher elevations.

Most of the water is locked up in polar glaciers. Precipitation comes from glacial melt in the summer.