Possible is not necessarily possible.

[Edwardo Piet]

Anything logically possible is anything that cannot be proven to contradict itself or otherwise be proven impossible through proof by contradiction.

In other words, if you can't prove it to be impossible, then it's logically possible, and that's that. It necessarily is, according to the definition of logical possibility.

As for what is necessarily true, that's anything logically or mathematically proven or anything tautologically true, etc.

As for probability, that's more down to science and statistics and odds and mathmetical theory.