I am designing a planet that humans can live on for a few tens of thousands of years, while they are hiding from some sort of inter-galactic police force (or whatever). Therefore, it's important that they live on a planet that at first glance is not habitable, i.e. outside of the "goldilocks zone", while in reality the planet is habitable.
Here is what I have so far (thanks to Artifexian's great youtube series!):
Mass of star: 1.2 (all numbers relative to Earth, sol, etc)
Lumens of star: 2.07
Distance of planet from star: 5AU (within the frostline, but too far out to be in the goldilocks zone)
Density of planet: 1
Mass of planet: 0.22
Radius of planet: 0.6
Albedo bond: 45
Using the calculator https://www.astro.indiana.edu/ala/PlanetTemp/index.html I get a temperature far too low for human life (138 Kelvin) unless I increase the greenhouse effect to 50 (276 Kelvin).
My question is: Is it possible to get a greenhouse effect of 50 on a planet like this in a way that is still supportive of human life? Alternatively, is there another way (maybe asteroid bombardments?) to raise the temperature of the planet to a level where water is liquid, but the atmosphere, radiation levels, etc. are still OK for human life?