They exist in case we ever make contact
with an ALIEN civilization.
The Big Bang Theory Season 9, Episode 10
I'm even on some
ALIEN conspiracy blog.
The Big Bang Theory Season 12, Episode 20
No ALIEN lady ever told Captain Kirk,
"Hey, get your thing out of my nose."
The Big Bang Theory Season 1, Episode 13
...an ALIEN named Ford Prefect
pretended to be human in order to blend in...
The Big Bang Theory Season 8, Episode 8
...who's been replaced with an ALIEN pod.
The Big Bang Theory Season 2, Episode 6