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My story revolves around a group of human astronauts exploring various planets via a new revolutionary type of drive. One of the planets I'm envisioning is a Cold Super-Venus, i.e. larger in size, mass, and gravity. While I do want a similar Greenhouse Effect with Atmospheric pressure in the 200 atm - 400 atm range, since I'm creating biological aliens to populate this planet I need it to be much cooler than Venus is. The obvious answer is to push it out way past the normal Goldilocks Zone, so the temperature range is manageable.

Any ideas and suggestions would be most welcome. Assume a Temperature range of 50F - 180F, so that biological life is plausible. The plot point is that the aliens can't leave the planet because the lower pressure would kill them, just like deep sea creatures on Earth can't survive on the surface. Any help would be greatly appreciated.

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You already pointed out one of the possible solution to get a cooler temperature:

  • moving the planet farther away from the central star

The trick is lowering the amount of radiation reaching the surface.

Another possible solution is to have a layer of highly reflective material in the high atmosphere, such as clouds or dusts. In this way a good amount of the light reaching the planet would be reflected back before warming it up. It is the same effect behind volcanic or nuclear winter.

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Okay, let’s work with a planet with a gravity of 1.8 g, or 17.7 N/kg. And let’s put the atmospheric pressure at the surface at a median of 300 atm, and the albedo at 70%. For the sake of convenience, we will assume that it’s in a single-sun system with a sun of approximately the same mass as Earth’s sun. Assuming an atmospheric composition just like Earth’s, but EXTRA THICC, at an orbital radius of 1 AU, we’d have a surface temperature of 833 F, or 718 K.

That’s too hot. And the information is coming from https://www.astro.indiana.edu/ala/PlanetTemp/index.html#fourthpage, a source I always use to predict the surface temperature of the planets in my story. If we were to change the orbital radius while keeping the solar luminosity the same, the planet will be in the required temperature range, if it orbits between 4.1 AU and 6.45 AU. But with a smaller sun, a lower greenhouse effect, or a brighter albedo, it needs to come in closer, and it would need to be further away if the sun was larger, the greenhouse effect was stronger, or the albedo was darker.

There are a good amount of assumptions, but they are backed with real science, and should work.

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