I define a "bioplanet" as any single organism that completely covers the surface of a planet. In my scenario, humans pass Kardashev Type I status and build a Matrioshka brain around the core of a large asteroid, but it is biological in nature and not a digital computer. They informally call this kind of bioplanet a "brainstar".

Why not just build a digital, chip-based supercomputer? There are a couple reasons for this:

  1. Issues with AI. Humans can't figure out how to solve the alignment problem, so instead of building bigger AIs they build bigger human brains. Brainstars have the same moral architecture as humans, so they are thought to be inherently more aligned with human values. A brainstar can't edit its own weights or self-replicate like an AI can, so there is no fear of uncontrollable replication either.
  2. Easy job of scaling the human brain. In real life, Craniopagus twins can develop functional thalamic bridges spontaneously. In my story, humans are able to get similar results through bioprinting, tissue engineering and other technologies, and they find cross-brain connections relatively trivial to develop. They know how to scale up any region of the human brain they desire, or wire groups of them together.
  3. Power. A human brain consumes 12 to 20 watts of power. A supercomputer consumes 1 to 10 megawatts. Even if supercomputers get more efficient, they are still orders of magnitude away from the energy efficiency of a human brain. The same is true for volume: human brains take up orders of magnitude less space than supercomputers.

But there is a big problem: what is the motivation for making the brainstar a Matrioshka brain proper and building it around a moon or asteroid? Why not just build it on Earth? There are plenty of sensible reasons to do this:

  1. Space is too complicated. Sending hundreds of ships to an asteroid is expensive and dangerous, the vacuum of space is lethal and you'd have to spend a huge amount of resources terraforming the surface.
  2. High maintenance. Biological organisms like brainstars can't just be powered by a bunch of solar panels. They need food and oxygen, they perspire, they excrete, they have medical issues and sometimes even need surgery. All of the necessary resources need to be shipped through space (although this could be solved by growing food and other resources on the brainstar itself).
  3. Latency. There will be communication delays if the brainstar is far from Earth.
  4. Real estate. There's plenty of room on Earth. Why not just build it underground, or in an ocean or desert?
  5. Secrecy need not be sacrificed. If there's a need for it, a brainstar could be built deep underground or in some remote locale on Earth for a tenth of the budget.
  6. No need to cover the whole celestial body. It might not make sense to build a brainstar that covers an entire moon or asteroid. You could use the rest of the surface for other things.

There has to be a reason to build it in space and make it cover a celestial body. Otherwise, the construct ends up as a "brain building" or "brain city", not a brainstar.

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    $\begingroup$ (a) If you read the tag wikis, the science-fiction and hard-science tags are mutually exclusive. (b) The hard-science tag is ruthless (answers that don't meet the tag's expectations are under threat of deletion)... and I'm having an incredibly hard time believing any hard science can be found to support an answer. (c) We do not host questions about motivation. Per the help center, " If ... you aren’t sure what a character (be it an individual or organization) should do, that is out of scope for the site." Motivations = choices = decisions = storybuilding = off-topic. $\endgroup$
    – JBH
    Commented Sep 19, 2023 at 2:00
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    $\begingroup$ Brainstorming character motivations is outside the scope of this site. $\endgroup$
    – sphennings
    Commented Sep 19, 2023 at 2:13
  • $\begingroup$ Matrioshka brain is a structure build around a star. Not around a moon or an asteroid. It's purpose is to use the whole output of a star for computation. Also, computers are way more energy efficient than human brain.... The supposed energy efficiency of brain is a myth. $\endgroup$
    – Negdo
    Commented Sep 20, 2023 at 15:21
  • $\begingroup$ Section 1 of why not digital: . "can't edit weights" is that in reference to activation weights of a neural net? If so those weights must be editable for the system to learn. This applies to both digital and biological neural nets.. How can you get it to a usable state if it can't learn? $\endgroup$ Commented Sep 21, 2023 at 22:17