Did you see my post earlier about how big the observable universe is and approximately how many stars there are in said universe? We keep on discovering new species on Earth every year.
5000 new species discovered this year in one spot:
A wealth of biodiversity has been found in the Clarion-Clipperton Zone, an area earmarked for exploitation by mineral firms
www.theguardian.com
Now, we can't see what's on the surfaces of the planets out of our solar system. There's more stars out there than what there is sand on earth. That's not even to mention how many planets there are for every system.
That's the most basic I can make it for you. If you can't figure it out from there then an alien can come pee in your eye and you still won't believe it.