A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.
Some astronomers think that the two gas-giants do not sport solid surfaces secreted beneath their immense and heavy gaseous atmospheres, although others suggest that the jumbo-size duo do, indeed, harbor relatively small cores of rocky-icy stuff. The two other large inhabitants of the outer limits of our Sun's family are Uranus and Neptune, which are both classified as ice-giants, because they harbor large icy cores secreted deep down beneath their heavy, dense gaseous atmospheres which, though very massive, are not nearly as heavy as the gaseous envelopes possessed by Jupiter and Saturn. As time passed, the region would have cooled down considerably and contracted--thus pulling away from its surroundings and forming fractures akin to the cracks that form in mud as it becomes dry--but on a considerably larger scale. But there is an important difference. On our own planet, lakes and seas are flowing with water, while Titan's lakes and seas are filled primarily with methane and ethane, that slosh around within these liquid reservoirs. In this never-before-seen cycle, the hydrocarbon molecules evaporate and condense into clouds that send an exotic "rain of terror" back down to this strange moon-world's carbon-slashed surface.