[Rudder]

Because of the very cold temperature, its surface would be covered with ice, icy ammonia and methane

Hmmm. I don't see how lack of heat produces water, methane and ammonia.

[aruanan]

Hmmm. I don't see how lack of heat produces water, methane and ammonia.

That wasn't claimed. But based on the fact that all the outer planets (except Pluto) are gas giants and many of them have moons that are covered with ice and methane slushes, it's not unreasonable to posit a planet that is even farther out than Pluto would be cold enough to be covered with ice, ammonia, and methane.

[SunkenCiv]

aruanan’s right, the low temperatures make liquid methane possible, and keep (for example) the ammonia from doin’ stuff like leaving. That’s unlike the situation here, where temperatures are too high for that. Also, methane would easily combine with oxygen here to form CO2, H2O, whatever else. :’)