[dogscoff]

If realy, how can you know we wont find some unique element even deep in mars, in the asteroid belt or on Pluton?

Because there are no more elements to be discovered (none that would be useful for this thread, anyway), and we know there can't be.

Here's how:
An element is a "type" of atom. All atoms are made up of neutrons, protons and electrons.

The thing that makes one element (say, hydrogen) different to another (say, helium) is the number of protons they contain. For example, a hydrogen atom has one proton. To put it another way, any atom with only one proton is Hydrogen. Any atom with 2 protons is helium, any atom with 3 protons is {insert element name here}, and so on up to >200 protons. There might be a few more up the top there yet to be discovered, but they can't exist in nature- they could only be made in labs and would self-destruct almost immediately after creation. This makes them pretty much useless when you're considering things like natural biology.

The point is, you can't have 1.5 protons, it's either 1 or 2. You can't have 2.4 protons, it's either 2 or it's 3, and so on. This means that just about every atom in the entire universe is of a type we have already discovered.

By the time you eliminate the ones that can only exist in a proton accelerator/ the centre of a star/ supernova/ black hole etc, you find that there are only about 100 or so elements actually available to Mother Nature for making life. We know the properties of all of those elements, and so we can make some pretty good assumptions about what aliens will be made of.

===============

Disclaimer: I'm no scientist, aIdpooTV. Doubtless there are loads of glaring errors in the above, but I'm pretty sure it's getting the right general message across.