Liquid metallic hydrogen. Uranus and Neptune may have molten cores, because they are smaller, and have less pressure at their centers. They are not massive enough to compress H into a liquid . The interiors of Uranus and Neptune are probably hot, dense liquid water, methane, and ammonia . The planets have internal heat sources.
More heat energy flows out of Jupiter and Saturn than they receive from the sun.
Neptune has a small amount of heat. Temperatures in the planets' interiors may be tens of thousands of Kelvins. Pressures in the interiors are tens of millions of times Earth's atmospheric pressure.