Geek Anders Exoself (yes, we think it is a pseud too) dismisses the hope of finding a huge Earth naturally (“We can do better if we abandon the last pretence of the world being able to form naturally (natural metal microlattices, seriously?)”) and considers the issues around just building a giant habitable planet from scratch:
Why aim for a large world in the first place? There are three apparent reasons. The first is simply survival, or perhaps Lebensraum: large worlds have more space for more beings, and this may be a good thing in itself. The second is to have more space for stuff of value, whether that is toys, gardens or wilderness. The third is to desire for diversity: a large world can have more places that are different from each other. There is more space for exploration, for divergent evolution. Even if the world is deliberately made parts can become different and unique.
Planets are neat, self-assembling systems. They also use a lot of mass to provide gravity and are not very good at producing living space. Artificial constructs can become far larger and are far more efficient at living space per kilogram. But in the end they tend to be limited by gravity.
Our search for the largest possible world demonstrates that demanding a singular world may be a foolish constraint: a swarm of O’Neill cylinders, or a Dyson swarm surrounding a star, has enormously much more area than any singular structure and few of the mechanical problems. Even a carefully arranged solar system could have far more habitable worlds within (relatively) easy reach.
One world is not enough, no matter how large.
See also: Why fine tuning of the universe for life is such a problem, unless we did it ourselves.
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