If an event is unlikely to happen its probability is?

If the even is never going to take place, and it is for sure, then it's probability will be 0.

For example, if the event is : A child being born with powers like Superman. For this event, the probability would be 0!

But in practicality, if the event is unlikely to happen, it's probability is likely to be nearer to 0. Keeping in mind the difference between "unlikely to happen" and "never going to happen for sure".

Because there is a possibility that the event that is unlikely to happen may just happen once in a thousand years. So in this case, the probability will be very very close to 0, but it will not be 0 exactly.