Do you believe in extraterrestrial beings?

Yes, I do.

You may not realise it but, nowhere does it say in your scripture that Earth is the only livable planet in this cosmos (regardless of whatever religion you adhere to).

But you may have other arguments if you are an atheist in the first place, because by then you have positioned yourself in a certain denial of God (and hence, His non-earthling creatures too).

In the case of agnostics, I do think that you have the fullest privilege to doubt the existence of aliens. After all, you have chosen to be a sceptic towards God in the first place... and I suppose it does not hurt to doubt the existence of ET beings too.