I was actually asked if it is possible for humans to live on this planet, if it exists, but it's been merged into this question and doesn't seem to be any way to unmerge. So anyway just to answer that point.

First, it's not been proven to exist. Nowhere near. Just the best candidate for planet X for a long time. The 1 in 15,000 seems impressive until you realize there are thousands of astronomers looking through reams of data every year, and 1 in 15,000 chances would pop up anyway from time to time just through pure chance. If it is just an observational accident like that, then you'd expect it to be disproved as they find more dwarf planets as it would be just an accident that the first few found suggest this new planet.

But then - it might exist also, it's intriguing enough to search up further. So what if they do find it?

No there's not any chance, not on the surface. Because it would be far from the sun and get less sunlight than Pluto. Our atmosphere would freeze out completely into ice, it's so cold. So no matter what it is made of, then its not habitable, at least not by living on its surface and heated by sunlight.

But what if it has internal heat? It surely has, like Neptune, heat from formation still trapped inside the planet.

So, it's a bit different if you are able to live beneath its surface. Depending on its size, if it is similar to Neptune, the surface gravity could be similar to Earth.

Neptune as photographed by Voyager 2 - Gravity on Neptune - Universe Today

So the gravity wouldn't be a problem.

It would depend what it is made of. Could it have an ocean? If like Neptune it could have a fair bit of heat from formation that it hasn't lost yet. So could be warm enough to keep an ocean liquid. Neptune has a subsurface ocean of very hot dense fluid.

This subsurface ocean on Neptune is definitely not habitable as temperatures range from 3,140 degrees Fahrenheit (1,727 degrees Celsius) to 8,540 F (4,727 F). - See more at: What is Neptune Made Of?

But what if this new ice giant (as Uranus and Neptune are sometimes described) has a higher percentage of ice than Neptune? Could the outer levels of its deep subsurface ocean be at a low enough temperatures to be habitable?

I've no idea if that's possible. But if so, perhaps deep below the surface humans could swim around in a planet girdling ocean :).

Or if not, wait for it to cool down and eventually Neptune's ocean will cool down and be habitable. And so same for this new ice giant if it exists. If you have the patience and longevity to wait a few billion years, or longer, you might eventually find it becomes habitable.

But we'd still need to make our own air to breathe. And the water might be under high pressure at the level where it is warm enough to be habitable. And if it was habitable, as usual the first priority would be to make sure that we don't introduce Earth life to the planet in case it has its own native lifeforms. We wouldn't want to make them extinct before we know about them.

So, who knows, it could in principle have a civilization too :). Probably of fish, and very hard for them to develop technology without fire. And fire very hard in an ocean.

But civilizations don't have to be technological. Philosophy, ethics, music, art, that's all civilization too. They could be millions of years ahead of us in, say, ethics, philosophy and maths, and yet have no technology.

But I don't know how likely it is that it could have such a cool ocean.

If the planet itself is not habitable, it could easily have moons with subsurface oceans that are habitable, heated by resonances with other moons, like Saturn's moon Enceladus.

https://en.wikipedia.org/wiki/En...

If anyone knows more about possible temperatures of the liquid water layers of ice giants in the Kuiper belt - or more distant ice giants in the Oort cloud, do say!