Jupiter Brain
A Jupiter Brain is a theoretical computing megastructure the size of a planet. Unlike a Matrioshka Brain, a Jupiter Brain is optimized for minimum signal propagation delay, and so has a compact structure. Power generation and heat dissipation are formidable concerns for a Jupiter Brain implementation.
While a rigid solid object the size and mass of a rocky planet or gas giant could not be built using any known material, such a structure could be built as a low-density lattice with a mass comparable to a large moon or a small rocky planet but a far larger volume, or as a solid but non-rigid structure with the mass and density of a planet (as long as the internal heat gradient is carefully controlled to prevent convection).
Fiction
- Earth is a form of Jupiter Brain in the Hitchhiker's Guide to the Galaxy by Douglas Adams. Built by the Magratheans and commissioned by mice, it is frequently mistaken for a planet by its inhabitants (who are part of the program to divine the question of "life, the universe and everything" to the answer 42 found by Deep Thought).
- One of the main "characters" in Ken MacLeod's Cassini Division is the planet Jupiter, which is the home of transhumanists who uploaded their personalities.