How do I add an atmosphere to a planet?

[PyroSaphuron]

Topic. I loaded up the simulation for Planet Nine and am experimenting with adding an atmosphere to it, but there are no options for it anywhere. I read elsewhere it should be in the Temperature tab, but that only has Surface and Effective Temperature, Albedo, Radiative Power, and Tidal Power. Where are the rest of the options like you see on Earth?

[OuijaMawl]

You are in the right menu, in the temperature tab there is a number of options to look through. What you are looking for is the drop down menu Atompshere in this tab. Those are Atompshere Mass, Surface Pressure, Scale Height, Greenhouse Effect and Infrared Emissiivity. This is what essentially defines the atomsphere of the body you are selecting.

Adding mass will also increase surface pressure and greenhouse effect. IR emissivity is internal heat from the core. There are more options for Mars and Earth as far as climate goes however all bodies that are not a star or black hole will use the 4 generic options

[PyroSaphuron]

There is no drop down menu at all. The only things are Surface and Effective Temperature, Albedo, Radiative Power, and Tidal Power. Literally nothing else. Is there something in the options I need to check to unlock all the options or something? There's nothing in this tab other than those five things. No drop downs, nothing. And it's completely different than what I see people doing on YouTube, which have all the options on every planet. I only have those options on Mercury, Venus, Earth, and Mars. Nothing beyond that.

[Jar]

This is because Planet Nine is a gas giant in Universe Sandbox ². Gas giants do not have the atmosphere properties that rocky planets do, as it doesn't quite make sense for them (it could be argued that some of the properties would make sense, but they would be different from how they're defined for rocky planets).

Earth and Mars have more advanced climate models, and all rocky bodies (whether randomly generated or known like Mercury, Venus, Pluto, etc) have the generic atmosphere properties Callipso mentioned.

So if you want to experiment with a Planet Nine with an atmosphere, you'll have to replace it with a rocky planet.

I hope that helps. You bring up a good point that this can be a bit confusing because it's not clear why it doesn't have those properties that are seen on other planets. We'll discuss how we can improve this. :)

[PyroSaphuron]

Thanks for the answer! Is there a chance in the future to be able to modify an already-placed planet from gas giant to rocky planet and/or vice versa? Perhaps through the planet composition tab?

EDIT: Actually, how would I replace the gas giant with a rocky planet, and have the exact same orbit and mass?

[Jar]

Agreed that the ability to replace one object with another would be a good feature. It is somewhere on our to-do list. :)

For now, you will have to manually do it. Add a rocky planet orbiting the Sun, then pause the sim and set its mass and orbital elements to match Planet Nine's.

Also, just yesterday we were talking about how we can improve object categorization. In the future, this should be more flexible, so it wouldn't be "stuck" as a gas giant, for example.

[PyroSaphuron]

Awesome! Thanks and keep up the great work!