Notes on Worldbuilding Series

Notes on Worldbuilding Part 10: Weird Worlds

Most of the planets we've been talking about are analogous to the worlds of our own Solar System. But planetary scientists have come up with some planet types which might exist circling other stars. Small variations in element abundances when a system forms can lead to very exotic worlds.

Carbon Planets: Carbon is one of the most abundant elements in the universe — far more abundant than silicon or iron. Yet most of the solid planets in our Solar System are made of rock and metal, not carbon. What if a planet were to form with more carbon and less oxygen than Earth? It would have to be almost waterless, which suggests the most likely place to find a carbon planet would be close to its parent star.

The core would be steel (iron plus carbon), with a mantle of graphite or silicon-carbon compounds, and continents of diamond on the surface. The atmosphere would be pure carbon dioxide or carbon monoxide (and perhaps some cyanide compounds).

Here's the Wikipedia entry, which is about the most informative article I can find online: https://en.wikipedia.org/wiki/Carbon_planet

Chthonian Planets: A gas giant that migrates close to its parent star is going to lose atmosphere as diatomic hydrogen molecules get broken into monatomic hydrogen, and the extreme heat boils off those atoms. As the planet loses atmosphere, it loses mass, and thus its gravity decreases — which means the rate of hydrogen loss will increase.

The end result of this kind of runaway boil-off is the gas giant's dense core left exposed. ("Chthonic" or "Chthonian" has nothing to do with H.P. Lovecraft. It's the ancient Greek term for "Underworld" or "Earth-born" deities, as opposed to the gods of sky and air.) Essentially it's a "Super-Earth" or "Mega-Earth" with very high density, up to 1.5 times Earth's density. Such a world would be very hot, very dry, with massive gravity and an atmosphere of very dense gases (carbon dioxide, sulfur dioxide, and possibly things like mercury and sodium vapor).

As above, the Wikipedia entry is a good introduction: https://en.wikipedia.org/wiki/Chthonian_planet

Coreless Planets: Earth has a crunchy rocky crust and a chewy molten iron center. But if a planet were to form from oxygen-rich planetesimals so that all its iron is bound up in iron oxide compounds, the planet's interior would not differentiate into a metal core and a rock-metal mantle. Instead the interior would be a kind of uniform goo of hot iron-rich rock.

What would this mean in practice? No magnetic field, for starters. And that means a thinner atmosphere as more radiation boils off the lighter gases. It might also mean slower rotation, as the mass would be more uniformly distributed, rather than having a dense core. (I think. If anyone with a better understanding of physics can correct me, please do so.)

It would also mean less tectonic activity (again: I think) as there wouldn't be as much convection in the interior. Less tectonic activity means more elements like carbon and hydrogen get bound up in the crust. So you get a slow-turning, mostly flat world with a thin atmosphere and little or no water on the surface.

Gas Dwarf Planets: A "Gas Dwarf" or "Mini-Neptune" would be a planet with a mass of 5 or more Earths, with a dense rock-metal core, a very deep ocean of water or water mixed with ammonia, and a thick atmosphere of methane, ammonia, nitrogen, and diatomic hydrogen. Such a world would probably form in the outer system, though it might be right at the outer edge of the Goldilocks Zone, if it's big enough to hang on to a massive atmosphere. In the Goldilocks Zone a Gas Dwarf turns into an Ocean World (see below).

A Gas Dwarf would have low density (roughly the equivalent of water, about 1/5 that of Earth) and relatively low gravity at the cloud tops. The combination of a dense atmosphere and low gravity means it would be a perfect environment for flying.

Here's a paper about the idea: https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/

Iron Planets: A planet which forms from metal-rich planetesimals would have a higher iron content than Earth. The planet Mercury is an example of this: it's smaller than Mars but it's got a density more in line with bigger planets like Earth or Venus. Now imagine a Mercury with the mass of Earth, or even more.

An Iron planet would have high density, up to 1.5 times that of Earth, and thus high surface gravity. The abundant iron would soak up any free oxygen, so the surface would look rusty-red like that of Mars.

Ocean Worlds: Earth's oceans cover about three-quarters of the surface, but it's actually kind of remarkable that there's any dry land at all. Double the amount of water on the surface and Earth would have just a few islands. Double it again and there'd be nothing but water.

Now imagine an Earth which could boast thousands of times more water, like a small ice giant but warm enough for liquid oceans. The sea would be hundreds of kilometers deep, with absolutely no land at all. The planet's overall density would be lower (in the 0.5 to 0.75 Earth density range), which means an ocean planet could be substantially bigger than Earth with comparable gravity. Ocean worlds would likely have pretty dense atmospheres, and the enormous amounts of water vapor mean it would be best to stick them near the outer edge of the Goldilocks Zone to avoid a runaway greenhouse effect.

An ocean world would have distinct layers in its immense hydrosphere. The deep layer would start about ten kilometers down and run all the way to the solid seabed. Most of this water would be stagnant and oxygen-poor, with a uniform temperature and little convection. Above that would be the more active ocean layer, just a couple of kilometers deep, where life might thrive.

With no land at all, ocean worlds might boast long-duration storm cells, hurricanes lasting for decades, circling the globe in the warm latitudes. The seas would be fresh or slightly brackish water, as there just isn't enough sodium in the crust to make them salty.

Hycean Worlds: A subset of ocean worlds — and let's all remind ourselves here that all of these planet types except the ones we can see around us in the Solar System are entirely theoretical — is called "Hycean" worlds. Like ocean worlds, the mass of the planet has a large fraction of water. The difference is that the atmosphere is hydrogen, or a mix of hydrogen, hydrogen compounds, and helium. The name is a bit of chemist humor: an "O"cean world has an Oxygen atmosphere, so a HYdrogen atmosphere makes a "Hy"cean world.

The characteristics of a Hycean world would be broadly similar to the description of an ocean world above. One major difference would be any life inhabiting that titanic ocean. Because the atmosphere contains large amounts of diatomic hydrogen, living things would have to rely on hydrogen reactions rather than oxygen respiration. Hydrogen "reducing" reactions are less energetic than oxidation, so life in a Hycean world would presumably be less active, less abundant, and generally slower.

What's interesting from a science fiction perspective is that Hycean worlds look a lot like the pre-Voyager probe model of what gas giants like Jupiter and Saturn might be like. Classic science fiction from the Golden Age often made reference to the oceans of Jupiter. Hycean worlds lets us have those glorious pulp settings again, with the advantage of a more bearable surface gravity.


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