# Is it safe to have an earth-like temperature for a 48-hour day-night cycle on a big planet?

I want this planet to be very earth-like on all aspects except for its size and the day-night cycle. The circumference is three times greater than the earth's circumference. The day-night cycle lasts 48 hours.

With that I want to have the same temperature variation for the corresponding hour here on earth. e.g. Noon at that planet should feel the same as noon here on earth. As well as midnight, and so on.

I don't know about tidal locking and all that but I don't mind any celestial bodies and don't really mind how big the moon should be. Of course, since this is a fantasy world, I know I can have anything I want and just blame it all on magic or whatever. But I at least want it to have enough consistency or be realistic enough as not to be absurd.

• What do you mean by "safe"? Do you mean if it is possible that such a planet exists? (without magic involved) Mar 25, 2019 at 12:10
• Yes something like that. Basically just trying to reduce the need for supernatural stuff. I am still ready to add some cheats here and there. Mar 25, 2019 at 12:14

Okay, so let me sum up the question in order to answer it right:

Is it possible for such a planet to have an atmosphere that has an earthlike maximum and minimum temperature?

Long answer: Why wouldn't it? Because the planet takes longer for a full rotation and so the atmosphere heats up / cools down way more than on earth.

But that does not have to be the case. Maybe you have already realized that some nights are way colder than others with similar day temperatures. A rule is, that a cloudy night gets less cold than a clear one. Also a cloudy day gets less hot.

So basically clouds can dampen the temperature volatility. During the day, they keep heat out, during the night, they keep it in.

Your planet does not need to have a lot of clouds though. It suffices to have a denser (and or bigger) atmosphere that takes a longer time to heat up and cool down.

The biggest problem is gravity. With a circumference of 3 times that of the Earth the radius would be 3 times that of the Earth, but the volume would be 3 cubed or 27 times that of the Earth. Making Earth 27 time more massive would have a massive effect upon the gravitational pull on the surface. Some relief might be gained by decreasing the amount of iron and nickel in the core and replacing them with lighter element such as oxygen, silicon and magnesium. But this would not be sufficient to counteract the massive increase in volume and the gravitational pull on the surface would be several times that we experience on the Earth.

One way round this would be to have an “engineered” planet that some advanced civilization built from scratch using the very lightest of elements for 99% of the planets interior just leaving a standard Earth like crust. The problem being that crust would sink so some means would have to be found to keep it “afloat”. Or just building a thick shell made of crust material, although astronomical forces would have to be applied to prevent it collapsing in on itself.

By comparison arranging the temperature would be fairly easy to do, although if such a planet did somehow exist there might be more Hadley cells on it depending on a range of variables. So instead of polar, temperate and tropical cells in each hemisphere there could be 4 or 5 zones.