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The idea is that a species very similar, but not related to humans have evolved on a different planet and are living in a medieval society. Humans from Earth must be able to live on this planet without spacesuits but not necessarily pleasantly.

The planet must have the following traits to support this scenario:

  1. One moon to keep the planet from spinning to fast. (multiple moons would likely collapse into each other or swing the poles too much)
  2. Gravitational pull not stronger than 2~3 times that of Earth.
  3. Terrestrial body with the right elements for the creation of life.
  4. Atmosphere that is breathable and keeps water liquid.
  5. Be within the habitable zone of a star.
  6. Receive enough light for photosynthesis.
  7. Have a magnetic field that protects from radiation.
  8. A climate that supports medieval civilization.

I was thinking about having the planet a little further away from a bigger star or a little closer to a weaker star than Earth is to the sun so the photosynthesis would take on a different wavelength of color and the sun would look different changing the landscape. I also figured gravity and thus planet size could differentiate this place from Earth but I am not sure how this impacts on the other requirements.

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    $\begingroup$ don't worry about photosynthesis, the color of plants has very little to do with the light from the sun, there are many photosynthetic pigments on earth, plants did not become dominate becasue they are green, green became dominant becasue that was the pigment plants happened to have and plants had several advantages. Bright pink,red, orange, violet are all possibilities. There is a whole separate question about it. worldbuilding.stackexchange.com/questions/63259/… $\endgroup$ – John Feb 10 '17 at 21:13
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    $\begingroup$ 3 to 8 are obvious, aren't they? And 1,2 are both disputable and have little to do with visual differences. I'm a bit lost here. $\endgroup$ – Mołot Feb 10 '17 at 21:45
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    $\begingroup$ I really can't see what you are actually asking, consider shortening the question, and/or summarising. Perhaps you are asking: "Are the traits I outlined reasonable for what I want to acheive?" $\endgroup$ – Dubber Rucky Feb 11 '17 at 12:34
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From what you've listed I can only see the strength of gravity as the debatable point as far as your species being human-like.

There are way too many variables for us to say exactly what will happen...but then this isn't a "hard-science" question so I'll be light on the details. For a more in-depth discussion see this resource.

The main points:

  • In the sea likely your creatures don't experience too much of a difference (variations in how strong they have to be to go deeper, perhaps).
  • Once they leave the sea: Stronger gravity increases the weight that bones will need to support.
    • Things probably won't evolve to be as tall: Trees will struggle to transport the water as high, likely they will still fight to be taller than their competitors but a larger spread with a stronger base will have to be a focus too. This will also apply to your creatures, it takes more energy to lift things to your mouth so increases your running costs in terms of energy efficiency. Unless there is a particular benefit things don't evolve.

So your "humans" will probably be shorter and stockier, perhaps walking on two legs wouldn't have been enough of an evolutionary advantage in the first place....but that is debatable enough that we can still assume they evolve. I would recommend implementing some minor differences like that. (Stronger hearts to pump blood around, earth humans would feel light headed after being there for a while).

Birds are another point to consider - they would have to have larger wing-spans or be less massive in general.

The weather would be different too, lower clouds, denser air at the ground, to describe this exactly would require quite extensive research too.

There would be many differences, in this planet just from a higher mass - we can assume a human-like species exist but, like the planet, they wouldn't be quite like their earth analogues.

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    $\begingroup$ Also, not generally appreciated is the need for basic neurochemisty modifications, A "normal" sized human in a 3 g field, even assuming increased muscle and bone strength, will need to react significantly faster than than humans in order to successfully keep his balance. $\endgroup$ – WhatRoughBeast Feb 11 '17 at 13:30
  • $\begingroup$ @WhatRoughBeast Another good point, I know there are probably plenty of other things that would differ, there wasn't a hard-science tag so I thought I would just skim the surface. I wondered about all sorts - lungs, digestion, eye-sight (would the composition of the eye have to be different? I imagine they would get squished out of shape a little) and so on....I think there would be a lot of differences but the scope of this question didn't seem to call for research into all of them. But yes, it is a good point (and probably noticeable within the story) about the reaction times. $\endgroup$ – Lio Elbammalf Feb 11 '17 at 13:36

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