A Black Hole is an area in space where the gravitational pull is so high that not even light can escape from it. Black holes are thought to be the result of stars dying and collapsing into themselves due to high gravity. As no light can escape from the gravitational field of a black hole, they are invisible and can be of any size ranging from as small as an atom to “super-massive” black holes which have the mass of around a million of our Sun. Albert Einstein was the first person to predict the presence of black holes.
The existence of black holes is accepted by the majority of scientists today, although we still haven’t found any tangible proof of that. What we know about black holes is mostly circumstantial or based on the reaction of other bodies in space. Evidence suggests the presence of black holes in almost every galaxy that we have come across. In certain areas of the universe, scientists have observed groups of stars revolving around an invisible body.
Black holes have also been known to emit Hawking radiation, which has been observed by scientists in the centre of almost all galaxies that they could observe. All these suggest that black holes really exist although if you ask for proofs, nobody will be able to provide them to you.
The simple effect that a Black Hole will have on a planetary system is that it will suck the whole system inside itself eventually. The same will happen with the star of that planetary system. If we talk about the solar system, it is under no such threat at the moment. However, it is the black holes at the centre of each galaxy that holds the galaxy together. The Black hole that holds the Milky way galaxy together is called Sagittarius and its mass is around four million times the mass of the sun.