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Our knowledge of exoplanets is changing all the time, but based on observations so far, is it possible to estimate the number of planets in a galaxy the size of our own that would have an Earth-Like gravity? Assume "Earth-Like" to be no more than say 25% higher or lower.

It seems to me the limiting factor for colonization of a galaxy would be more related to gravity than atmosphere, temperature, etc; all of these can be overcome with relatively trivial applications of technology. But no technology can negate a Super-Earth with 2gs of gravity.

I am trying to imagine a galaxy where the primary in-demand resource would be habitable planets, and that one of the the primary factors in determining what is "habitable" is gravity.

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  • $\begingroup$ Well, life on a planet would have been created by God to fit the gravity of that planet, whether it be 0.001g or 1000g. But, for humans, it would be about from 0.5 to 1.5 for us to survive. $\endgroup$ – The Mattbat999 Jul 13 '17 at 2:22
  • $\begingroup$ I understand the science based tag to include science as we understand it currently; and yes, a 2G planet would pose significant health and technical problems to colonize . But, provided adequate energy, strong enough materials, and advanced automation, installing an orbital habitat around that same world is possible, albeit more expensive. $\endgroup$ – Sean Boddy Jul 13 '17 at 3:31
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    $\begingroup$ @SeanBoddy True enough, but assume for this exercise that interstellar travel (and hence exploration) is, for whatever reason, a lot cheaper than hauling millions of tons of soil, atmosphere and strong materials into orbit to construct habitats large enough to sustain a substantial population. $\endgroup$ – Corwin62 Jul 13 '17 at 3:38
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NASA estimates 1 billion ‘Earths’ in our galaxy alone. But their criteria are more rigid - approximately the size of the Earth and are orbiting familiar-looking yellow-sunshine stars in the orbital “habitable zone” where water could be liquid at the surface.

So I would guess something about 10 billion.

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    $\begingroup$ I'm not sure their criteria is more rigid. In the link you provide they constantly refer to Kepler 452b as one of the billion possible habitable planets, yet they state Kepler 452b has 2G gravity - just the kind of planet the OP wants to rule off. $\endgroup$ – Rekesoft Jul 13 '17 at 8:17
  • $\begingroup$ @Rekesoft Though of course, if NASA is looking for size (as in volume or radius) and OP is looking at gravity, one can certainly trade one for the other by adjusting density. For example, a denser planetoid with the same gravity will have a smaller radius. There's only so far you can go in this in either direction while the planet remains Earth-like, but it's certainly one factor to be considered. $\endgroup$ – a CVn Jul 13 '17 at 9:17
  • $\begingroup$ By going from 1 to 10 are you saying that there are 10 galaxies like the Milky Way in the universe? $\endgroup$ – candied_orange Jul 13 '17 at 9:38
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    $\begingroup$ No, he's just saying that if you don't limit to those planets in the so-called Goldilock's zone your number of potential planets grows tenfold. $\endgroup$ – Rekesoft Jul 13 '17 at 9:46
  • $\begingroup$ @MichaelKjörling True, but I think the Kepler's mission has only accomplished to measure both mass and radius for a little fraction of the planets observed. For most of them it only has the size and the orbital period so we'd had to settle for a much smaller sample if we limit ourselves to planets with estimated mass. $\endgroup$ – Rekesoft Jul 13 '17 at 9:49
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The "billions" estimates are probably correct, precisely how many billions is not going to have any significant effect on your plot line. So I will address a different story concern I'd have.

.. would be more related to gravity than atmosphere, temperature, etc; all of these can be overcome with relatively trivial applications of technology.

Sure, but if you are willing to live on a planet without an atmosphere at -200F then you might as well be living in Space!

non-optional Gravity is a detriment, and in space another relatively trivial technology to make it optional is just centrifugal spinning of a habitat; when Earth gravity is necessary for plant growing, or light gravity aids various manufacturing concerns (e.g. 0.2 G keeps everything on tables and floors, and liquids in their vessels, instead of floating about the lab or factory).

Similarly with a crushing atmosphere, or a molten surface.

Which I think kills your story line as implausible; people would not live on planets just for the gravity when they can live in space habitats, with whatever gravity they select, and that is cheaper, safer, and easier than living on a planet. It avoids the gravity well, too: It is easier to relocate, has no weather or natural disasters, planets can be used as shelter from star radiation; solar energy is easier to gather, structures for factories and energy gathering can be flimsy and built with "support" against gravity.

The only plausible reason I can think of, for occupying a planet and requiring close to 1 Earth Gravity, is to farm or ranch on a huge scale, hundreds of times the size of cities; or perhaps mining for the hundreds of minerals that only form over millions of years in the pressures and heats found on a planet.

We can ignore the cost of getting the product out of the gravity well; the important point here is that to be useful as farms or ranches, the planets must be much more like Earth than just having gravity; they must be able to support the kinds of life we find valuable (food plants and food animals, or food producing animals (bees for honey, chickens for eggs, cattle and goats for milk, etc).

Which brings you back to NASA's estimates of maybe a billion. But you can ditch the idea of trivial tech on this scale, so terraforming takes too many years and and your fictional characters discover that upon examination, the number of planets with atmospheres, water, and non-toxic composition suitable for life are rare, closer to 250,000. Then you enter the realm of plausible scarcity.

Alternatively, if you just want lots of habitable planets, presume Nasa's billion is an underestimate, and most planets in the Goldilocks zone capable of holding liquid water end up having it, and oceans, at least plant life and an atmosphere with oxygen. It is entirely plausible that life almost never evolves past that stage, and Earth is unique in having any large multi-cellular animal life. (Not that evolution doesn't work, but it only works by accidental mutation, and the particular accidental mutation that put Earth on the path to developing the first "animal" life may have been a one-in-a-quadrillion six-bank billiard's shot that just hasn't happened anywhere else in this galaxy.)

You can plausibly make terraforming out of reach so livable planets are rare, or make terraforming unnecessary so livable planets are plentiful. The argument that it is easy to live on desolate planets that wouldn't support life makes no sense, when it would be even easier to live in space.

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  • $\begingroup$ In reference to the cost of hauling soil, etc out of the gravity well, you don't have to do that, either. The number of small, low gravity asteroids and moons follows an exponential curve in our solar system; simply take them, or mine them. Take the moon Styx of Pluto, a rock between 6 and 15 miles across. We don't know its composition, but iron is plentiful in the galaxy and iron-rich asteroids are too. A space-going species would not transport anything out of the gravity wells, really, they'd just make what they need in space. $\endgroup$ – Amadeus Jul 13 '17 at 14:19
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In this article they estimate some 30 billion Earth-sized planets in the galaxy. It's not stated clearly, but it looks that their parameters for "Earth-sized" are from 70% to 140% of Earth's radii. Since a large amount of them will have a lot of habitability problems even having the right size, I think I concur with @Vashu's estimation of roughly 10 billion.

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