A Jupiter brain (J-Brain) is a computational substrate comparable in mass and size to a gas giant planet, typically in the 1X1027 kilogram range.
The computronium basis of a modern Jupiter Brain usually consists of a colloidal structure of plasma, monopolium/magmatter, and/or diamondoid/feroid. Heat dissipation is often a major concern with Jupiter Brains; the denser they are, the faster they can run, due to the distances involved in transmitting internal data. But increasing the density of the computronium also increases the problems associated with the heat of operation. The number of (non-reversible) computations that a Jupiter brain performs in a given time is directly related to the amount of waste heat the brain must emit, and since this heat must be emitted at the surface of the structure, very often a J-brain has a very large external radiating surface which helps to cool the object.
A Jupiter Brain can be considered to be an AI, posthuman or Local Archailect Node of extremely high computational power and size. This is the typical megascale concentrated intelligence. Such complexity is often considered to be the bare minimum housing for an archailect at the full S4 rating.
Some Jupiter Brains are sufficiently luminous to act as the centre of a system of habitable moons or other megastructures, which gather the waste heat given off by such an object and use it to provide energy for their environment.
As part of a general discussion on terraforming projects in the Sol System the gas giant Jupiter was examined by Birch. Birch knew that Jupiter's surface ( if you could call it a 'surface') gravity was far too high for habitation by humans. So he proposed creating a new surface above Jupiter at 100,000 km from Jupiter's center of mass, where gravity is felt at Earth standard. This surface would be suspended using so-called Mass Stream Technology.
Mass Stream Technology uses mass particle beams which encircle the planet or star to support structures above it; by exerting thrust magnetically against these beams (known as dynamic orbital rings), suborbital structures can be suspended at any height. Dynamic compression members and dynamic orbital rings using mass stream technology would be configured into a framework around the planet which would support platforms, which could in turn support a large biosphere. Individual platforms could then be extended into bands which could later be widened into a complete shell.
In most ways the details of the biospherics are similar to those of other space habitats. Airwalls at the edges of the platforms keep the biosphere in place until the shell is completed and supporting the shell with orbital rings avoids the need for 'unobtainium'. Jupiter, with a mass of 317 Earths, would have a shell (at 1 standard gravity) with 317 Earths surface area. [a simple calculation shows this ratio holds true for any supra-planetary shell - an 'underbody' with a mass of 100 Earths would have a shell (at 1 standard gravity) with 100 Earths surface area, etc.]
Although Supra-Jupiter would remain only a proposal, other supramundane planets would be built in the millennia to come. As Birch pointed out a supramundane planet could be built around any heavenly body, however if a standard gravity was required on the shell then the underbody needed to have greater than standard gravity at its surface, also if the underbody in question is hotter than a small red dwarf star then active cooling systems will be needed.
Shellworld (Type 1 + 2)
Type 1 (medium tech): Given an inactive world, such as a Selenian (dead world) or Hermian (Mercurian) terrestrial 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. 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.
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. Waste heat from the various levels is often a problem, often requiring elaborate cooling systems to maintain a comfortable climate on each level.
Type 2 (ultra-tech): 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.
A Banks' Orbital is an extremely large hoop-shaped artificial habitat that rotates once per day to create artificial gravity along its inner surface. These large constructs can provide natural-seeming and self-maintaining environments over surface areas that are hundreds to thousands of times that of a typical rocky planet like Old Earth. The tremendous size of a Banks' Orbital and the stresses involved mean that exotic matter is required for its construction. A Banks' Orbital is not to be confused with circumstellar 'ringworld' megastructures. A typical Banks' Orbital consists of a ribbon of material arranged in a ring that has a radius measured in millions of kilometres. The spin and radius of the orbital are set so that the sunlight and surface acceleration along the inner surface simulate the day length and surface gravity of a planet.
Its inner surface has one bar of pressure, experiences a 24 hour day, and provides 1g (9.8 metres per second squared) of acceleration. The sidereal rotation period of such a ring (assuming an orbit of 1 AU around a star with the same mass as the Sun) is 23 hours, 56 minutes and 4 seconds, (86,124 seconds); this is the time it takes to revolve once with respect to the fixed stars, and is the relevant figure when calculating spin gravity.
Such a ring would have a radius of 1,842,509 kilometres and a circumference of 11,576,800 kilometres. The hoop spins on its axis once per 86,124 seconds; the velocity of the rim is therefore 483,913 kilometres per hour, or 134 kilometres per second. Along the rims of the inner surface are high walls that prevent water and air from escaping into space; these are foamed diamondoid to a height of 100km, in most cases topped by a lightweight, transparent inflated wall 400km high. Many Orbitals are somewhat smaller than this and produce significantly less gravity; the rate of rotation also depends on the year length (which itself depends on the mass and luminosity of the local star).
Solids, liquids, and gases comparable to those of the desired planetary type are held against the inner surface by centrifugal effects and prevented from slipping off the edge into space by walls hundreds of kilometres high. The entire structure orbits a star within whatever constitutes the life zone for the inhabitants. From the point of view of an inhabitant, a completed Banks Orbital functions as a cylindrical planet, and in fact an unsophisticated inhabitant might not see any difference. Typically it has the equivalents of a lithosphere, hydrosphere, and atmosphere, and a functioning biosphere and/or mechosphere. In the case of a Banks' orbital these elements are sometimes referred to as the lithotorus, hydrotorus, and so on, since they conform to the shape of the Orbital.
A fully grown tree is a spherical structure up to a hundred kilometers across. It generally consists of 4-6 trunk structures growing out from a comet nucleus. Branches grow from the top of each trunk and intertwine and merge with each other to form a single structure. The trunks and primary branches are hollow and contain a breathable atmosphere and symbiotic ecology as well as a space adapted ecology on their exteriors. Various sub-species have been engineered to survive at a variety of distances from a star and under various wavelengths. Generally found at distances ranging from .5-4 AU around stars in the G, K and M class. Often a popular habitat for space adapted bionts.
Growth of a new tree begins when a suitable comet is diverted into a close solar orbit and a seed is planted on it. Over several years the seed extends a root system into and around the comet and then begins the growth of the primary trunk systems. Depending on the material supply in the comet and the distance from the star, full growth may take up to a century. However, most trees are ready for initial habitation within 10 years of planting. Average tree lifetimes run to a millennium even without life-extension bio-nano, and a mature tree may support a population in the millions.
In systems where the trees have been long established entire 'orbital forests', consisting of hundreds of trees spread across millions of cubic kilometers, may be found in the most desirable orbits.