They exist in case we ever make contact
with an ALIEN civilization.
The Big Bang Theory Season 9, Episode 10
There was no ALIEN.
[chuckles]
The Big Bang Theory Season 12, Episode 18
No ALIEN lady ever told Captain Kirk,
"Hey, get your thing out of my nose."
The Big Bang Theory Season 1, Episode 13
A semi-truck might be an ALIEN robot...
The Big Bang Theory Season 6, Episode 2
finds proof of ALIEN life.'
The Big Bang Theory Season 12, Episode 20