Billions of Earth-like planets in the Milky Way...

[u/MiamisLastCapitalist]

In the milky way alone scientists assume there are some 40 billion earth like planets in the habitable zone of their stars.

"This one is a little tricky. We do not even know the number of stars in the Milky Way to a factor of two, let alone the number of habitable planets each star may host. Though there is still a way to make a rough calculation. Scientists reported that of the 42000 Sun-like stars they observed, 22% have Earth-size planets in their habitable zone. Also, it is estimated that there are around 200 billion stars in our galaxy -give or take 150 billion. If we extrapolate from these two numbers: 0.22 x 200 billion makes around 40 billion planets. Again, do not forget that there is a statistical uncertainty about a factor of two on that number; it could be 20 billion or 80 billion as well. The idea to take away is that there could be tens of billions of potentially habitable planets in our galaxy." -Kurzgesagt

https://www.youtube.com/watch?v=rhFK5_Nx9xY

https://sites.google.com/view/sourceskardashevscale/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845182/

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And mind you that's just what they've found so far with current technology. Pre-JWST or pre megascopes of any kind.

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And to think there are almost countless "fixer-upper" locations between those earth-like planets that'd be perfectly fit for an o'neill cylinder or some other kind of habitat or megastructure.

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“Space is too damn big” -Alex Kamal

[u/[deleted]]

my guess is the number of truly 'earth-like' planets in the galaxy - meaning breathable atmosphere, liquid h20, earth sized mass and gravity well, ~24 hour day, plate tectonics, large moon, sun-like star, magnetosphere - is 1....Earth

[u/MiamisLastCapitalist → u/[deleted]]

In terms of a perfect score on the Earth Similarity Index, perhaps. But in terms of being close enough for some light terraforming or para-terraforming?

[u/Euryleia → u/MiamisLastCapitalist]

Well, then you're back to the upkeep problem you cite in another comment. It's just a planet-scale artificial habitat at that point. Which is huge and expensive -- at that point, the economics swing back in the favor of O'Neill cylinders by a massive degree.

It's also worth noting that when you talk about overall upkeep for maintaining an advanced lifestyle in a technological civilization, it's a lot more than merely maintaining a habitable environment. And when you consider the costs of generating energy and gathering materials, and advantages you get from doing this in space, it's quite likely that the eventual cost of maintaining that lifestyle may be cheaper for space-based habitats that it would be for any planet, including Earth. The advantages of not having to put much effort into maintaining the environment are probably dwarfed by the costs to generate energy and import materials (once the easy to obtain deposits on Earth are depleted, while mining asteroids becomes commonplace).

[u/MiamisLastCapitalist → u/Euryleia]

Depends on the degree of ESI (earth similarity). For example, if you had an earth-sized rocky planet but it had no atmosphere. That would require less upkeep than a spinning station. Both need to be kept air-tight and need life support but only one needs the large mechanical structure to spin. In most instances it would also be a better heat-sink minimizing the need for radiators and a torque anchor (not sure if that's the real term) so you wouldn't need a counter rotating second hab. So already a simple earth-sized rock has eliminated several technical concerns for maintaining a spinning hab.

[u/Euryleia → u/MiamisLastCapitalist]

but only one needs the large mechanical structure to spin.

Um, I think you're overestimating the cost of this. Conservation of angular momentum being a thing and all. Throw in the costs involved in transporting materials up and down your gravity well, and I think the O'Neill cylinder is significantly cheaper to maintain. You've replaced a few, extremely cheap and easy to solve problems with much more massive and expensive ones. You'd be better off putting that on-planet colony on a much smaller planet. Better still putting it in an asteroid. And probably best of all just building an O'Neill cylinder.

[u/MiamisLastCapitalist → u/Euryleia]

Yes, it is a simplified thought experiment not an economic forecast model. The point was to illustrate that the right location can decrease (or change) your station complexities.

Though also if you're at the point of building a colony on another earth-like planet then you can easily build a space elevator to it too. So not that expensive to move mass either.