How likely is it that a black hole could enter the Solar System? Well, you’d have to define likely; it is more likely that the Earth will get swallowed by a black hole than, say, winning the lottery ten times in a row, but less likely than being struck by lightning. In fact the odds of a black hole devouring our planet are estimated at one in a trillion.
There are two predominant types of black hole in the universe. The first are supermassive black holes found churning at the centre of galaxies. These don’t really pose any threat to us, until our galaxy collides with another like the Andromeda galaxy in a few billion years.