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Most exoplanets making the news are larger than Earth. I wonder about the smaller worlds, how small is too small? Reword to keep the question narrow enough to be answerable.

When the Earth was created in it's orbit around our sun, if Earth had been smaller, how small could it have been and still allowed human life to evolve and survive?

I imagine that the ability to maintain an atmosphere is the key. There are several things impacting atmosphere retention. If we make the Earth smaller and/or less dense, at what point does life as we know it today became impossible to have evolved and survived to present day?

I recognize the Moon is important in the process also, but for simplicity answers may choose to ignore lunar influences.

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    $\begingroup$ The reason we find so many large exoplanets is simply because they are much, much, much easier to find than Earth-sized exoplanets. Odds are, the Earth is fairly typical, and there should be lots of planets like it. They're just harder to find. $\endgroup$ – Phiteros Apr 16 '17 at 18:25
  • $\begingroup$ "I imagine that the ability to maintain an atmosphere is the key." Some exobiologists speculate that Europa might harbor life within the vast oceans beneath the ice. What is 'the key' is difficult to speculate given we have only Earth (a single example) from which to glean data. $\endgroup$ – Andrew Thompson Apr 16 '17 at 20:50
  • $\begingroup$ @AndrewThompson and Mars may have harbored life before it's atmosphere was lost, but neither Europa nor Mars can reasonably been assumed to have supported Human type life. Not a lot of speculation is required here. You have Earth in Orbit, Take away original mass/volume until humans do not come to exist. $\endgroup$ – James Jenkins Apr 17 '17 at 10:04
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As far as we know the life only on the Earth, we could only speculate about human-like life. We don't exactly know what would change depending on one or another condition.

  • On ice moons like Enceladus life could exist - despite the size and distance from the Sun are very different. The key factors are hydrothermal activity and shield of ice. You could imagine about some kind of whales which smart enough to build civilisation
  • Europa has atmosphere so you could imagine moon like this with life on surface. But without light and having small mass most likely forms of life would be differ from human
  • Mars lost its atmosphere for some reasons and it's possible that human-like life could arise on Mars-like planet

So, the atmosphere, energy and chemical elements could be met all together in very different planets and moons. (One could add some factors like magnetic field, ionosphere etc) But it's hard to say about the gravity. We could just imagine and read many fantastic books :)

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