Worlds with several concentric habitable layers

Image from Steve Bowers
This shellworld (seen in cross-section) consists of several Earth-like levels

The concept of shellworlds was first proposed in the early Information age, but did not become possible until the development of mature nanotech. Medium-tech (Type 1) shellworlds consist of a series of habitable shells constructed using the crust of a tectonically quiescent world, extending above and below the surface.

A more advanced design (Type 2 shellworlds) consist of several dynamically supported shells, essentially a series of concentric artificial planets.

Shell world under construction
Image from Darren Ryding
A Type 2 shellworld (an artificial planet with several habitable layers) under construction on the edge of a Dyson cloud. For a scale reference, there are 16 thousand ships in the foreground, each the size of a sea-going freighter.

Medium-tech (Type 1) Shellworlds

Given an inactive world, such as a Selenian or Hermian terrestial planet, the construction of a shellworld is one way of creating a large habitable area. A layer of earth below the planet's surface is excavated with self-replicating technology to produce a series of habitable caverns beneath the planet's surface. Eventually the caverns are connected together into a nearly continuous shell. This shell is filled with with air and water, often imported from elsewhere in the system; artificial light sources on the ceiling provide sunlight. The power for these light sources may come from fusion, or solar power collectors on the surface or in orbit.

However, the true stroke of genius is what is done with all the excavated stone. It is formed into new shells that rise above the planet's original surface, which themselves must be made habitable. With assembler technology the rock can be assembled into materials that will support the weight of all above them with huge pillars (usually diamondoid or corundumoid), and when excavation is complete the planet has many concentric habitable layers and has many times more surface area than before.

Naturally, finding a largish but non-tectonically active body (lacking conventional or large large amounts of radioactivity in the deep crust, mantle and core and small enough that the friction heat wouldn't be enough to melt rock and form tectonic plates) is not easy. Suitable candidates that have been used are a few unusually stable very old Mars-like planets, some rocky outer worlds in old white dwarf systems, and even "lost" or "rogue" planets. The minimal seismic activity that occurs even on these bodies is manageable using large scale allocation of basic nanotechnology.

Medium-tech shellworlds were originally powered by intercepted sunlight or fusion power plants, or a mixture of the two. Many shellworlds now use conversion technology as a power source instead.

Waste heat from the various levels is often a problem, often requiring elaborate cooling systems to maintain a comfortable climate on each level.

Galrecon is an example of a type 1 shellworld, unfortunately damaged in the Version War.

Shellworld Landscape
Image from Steve Bowers
The diamondoid towers separating the layers of Yoredale shellworld were originally coloured luminous blue, the same as the sky/ceiling. When gliding became popular on this world there were several unfortunate accidents, so the towers are now clearly visible by day and night

Ultra-tech (Type 2) Shellworlds

By constructing a network of dynamic orbital rings above a planet or even an artificial black hole, an artificial planetary surface of almost any size can be constructed. This technology allows an arbitrary number of concentric layers to be suspended one above another, with the only practical limits being the energy requirements for each shell and the removal of waste heat. The different layers can each have very different environmental conditions, but the connecting lift-shafts must be evacuated to prevent contamination between levels.

Ultra-tech shellworlds may be powered entirely by the central black hole, or by conversion technology. Light from nearby stars may also be used where available. Drifting Stevensonian planets in interstellar space make excellent candidates for conversion. More than a few clades, both mainstream and hiders, have constructed shellworlds in this manner.

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Development Notes
Text by Michael Beck, amended by Steve Bowers
Initially published on 31 December 2001.

Additional Information
Shell Worlds -2009 paper by Roy, Kennedy, Fields

Isaac Arthur's YouTube video about Shellworlds here