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