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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?

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  • $\begingroup$ If the planet is habitable then won't the intergalastic police be aware of the fact? If they are coming by space craft then presumably they can build a habitat under the surface in a wide variety of environments regardless of the temperature above? $\endgroup$
    – Slarty
    Feb 13, 2020 at 22:17
  • $\begingroup$ You can add greenhouse gases to the atmosphere (CO2, water vapour, methane, even nitrous oxide) and lower the planet's albedo. Building underground habitats as suggested by @Slarty does make a lot of sense. $\endgroup$
    – a4android
    Feb 13, 2020 at 22:43
  • $\begingroup$ Living underground is fine, but let's assume my humans want to live on the surface. My question is: is it possible to add enough greenhouses gases to the atmosphere to raise the temperature while still keeping the environment supportive of human life? In response to @a4android, if I add the gases you listed at high enough concentrations to increase the temperature, would it be toxic to humans? $\endgroup$
    – Doh
    Feb 13, 2020 at 23:21

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Frame challenge: The "habitable zone" is the only region where habitable atmospheres are stable.

While a simple definition of the habitable zone is "the region around a star with the right temperatures to support liquid water" this is misleading. Distance from a star is not the only major factor: calculations of the habitable zone factor in whether or not Earth-like atmospheres are stable at a given temperature.

A planet with an atmosphere conducive to water would be in the HZ by definition because atmosphere defines the HZ. If you place your world "outside the habitable zone" because you want it to seem too cold for water, it is going to be too cold for an atmosphere conducive to water on large timescales, regardless of how much you crank up the greenhouse effect. "Outside the HZ" literally means "outside the range of a stable greenhouse."

Past the outer edge of the HZ, carbon dioxide becomes less efficient as a greenhouse gas. As distance increases, it scatters sunlight better than it absorbs it - and instead of linear cooling with distance, planets surprisingly begin to cool rapidly. No matter how much insulation you add, it won't be efficient enough to keep a planet above freezing past the outer boundary of the HZ. A positive feedback loop will kick in, a runaway refrigerator effect will dissolve existing CO2 in the ocean, and the atmosphere will leave within a billion years.

The converse of this argument is that if such a planet could exist, the galactic police would broaden their definition of the HZ to include this type of world.

You could try using non-CO2 greenhouse gases, but that's a dangerous game to play. Developing an Earth-sized planet beyond the HZ risks accumulating lighter gases in abundance. Whether those gases are good insulators outside the HZ is beyond my knowledge - but I do know that you will reach dangerous-to-humans, mini-Neptune-level pressures quickly.

The galaxy is vast enough that your human colonists could probably live on any uninhabited world in small colonies without being detected. Depending on how easily life develops, there may be plenty of "habitable" worlds out there that never developed microbes. And if life is everywhere habitable, you could always live on an inhospitable world underground.

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    $\begingroup$ Ah, I understand. Thank you! I felt like I was missing something about habitable zones. $\endgroup$
    – Doh
    Feb 14, 2020 at 0:58
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One of the most powerful greenhouse gases is the one you can see with your eyes in the sky. Water vapour. You might have a planet with a lot of geothermal activity generating a lot of water vapour.

Downside is that a planet with a dense cloud cover would attract the attention.

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Depends the tech level

If humans can travel the stars, they can build underground on an uninhabitable planet and grow food hydroponically as long as they have a energy source.

Mars and the Moon have lava tubes they could be moved into and live underground with virtually no trace that anyone lives there.

Humans don't have to live on the surface

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