Now your probably thinking, how can one argue that to a non-believer from a logical point of view? Clearly fully believing that theory requires some pre-disposition to viewing The Bible as the final authority on all matters.
But I do feel I can demonstrate that the "Aliens" people claim to be seeing, (if they do really exist which is a big if) don't fit in well with an Atheistic Evolutionary-Uniformitarian-Big Bang Theory view of the Universe.
I don't unfortunately remember where to source it from, but I remember Carl Sagon once said "A human being could reproduce with a Petunia before he could an Alien, because they at least evolved on the same planet".
There is a massive variety of what kind of Life exists just on this Planet, Land, Sea, Deep Sea ect. And not all even fit the basics of biological body design we bi-pedal mammals are used to. So indeed, life on a completely different Planet with almost certainly completely different even basic atmospheric conditions (The Conditions on Earth are dependent on a lot of "convenient coincidences") would produce very different kinds of Life.
And indeed, the Creationist's favorite argument against Evolution and Atheism is that the odds were highly against Life or even the conditions to support life occurring by chance even once here on Earth. If it did happen again, it certainly couldn't be the same basic kind of life.
Yet the Aliens people keep seeing, all fit into three types.
1. Pledians or Nordics, they look like humans (white Aryan looking humans) except kind of taller and vaguely "godlike". Sort of like Tolkien Elves (who Tolkien never actually described as having pointy ears).
2. Reptilians or Draconians, various types of Pi-Pedal or Humanoid lizards or amphibians, many resembling the Vleiciratpors as their depicted in Jurassic Park.
Both of those are clearly merely slightly different from Terrestrial life we see every day.
3. The Greys, they are the most truly Alien looking ones. But when you get right down to it even they're clearly based on the basics of vertebrate terrestrial biology. They have 2 legs, 2 arms, a torso and head with eyes ect. Those features aren't even universal on Earth.
The typical cliche Sci-Fi movie explanation is that their taking a from we're used to to make us more comfortable. But that only works for option 1, the Reptilian and Grey forms don't comfort people, they terrify them.
The following observation is mostly made I suspect by people discrediting the sightings all together. "their just imagining what their used to seeing in horror and sci-fi films" ect. And I'm not saying any of it is provable. But if you believe in Intelligent Design, then yes God could certainly design life like we see here elsewhere if he so desired to.
The reason why Biblical Creationists are not fond of believing in natural life (or at least not sentient natural life) independent of Earth isn't really a matter of refusing to believe God left something out of Genesis 1 and 2. It has to do with the New Testament message.
The Sacrifice Jesus made can only be made once, The Godhead became Human not just temporarily but permanently, it can't be repeated.
So if Alien life exists, they are Beings that either never fell and are thus sinless, or beings that are fallen with no hope of redemption, and would essentially become like Tolkien's Balroqs, Orcs and Trolls and other perverted life forms created by Melkor The Morgoth.
Yet Adam's fall effected the entire Universe, not just Earth (unlike what Lewis's Space Trilogy Cosmology implies), that's why things like Black Holes and Supernovas happen. So it't be unfair for benevolent Sinless sentient life to be out there suffering because of what Adam did.
In a sense though, that is exactly what Angels are, the only other sentient life God created, and they're all either perfectly loyal servants of God, or completely lost fallen gods of evil.
Ephesians 6:12 "For we wrestle not against flesh and blood, but against principalities, against powers, against the cosmic-despots of the darkness thereabout the universe, and against spiritual wickedness in celestial places." Because the word translated "Heavenly places" is defined by the Strongs thusly
Epouranios (ep-oo-ran'-ee-os);Now, as to the 3 varieties of Aliens.
Word Origin: Greek, Adjective, Strong #: 2032
existing in heaven
things that take place in heaven
the heavenly regions
heaven itself, the abode of God and angels
the lower heavens, of the stars
the heavens, of the clouds
the heavenly temple or sanctuary
of heavenly origin or nature
KJV Word Usage and Count
in heaven 1
1. Besides the Cheirbuim/Seraphim, Angels when they appear to humans look like Humans (though often of above average Badassness). It's suggested that you could have entertained angels without even knowing it, in Hebrews 13:2.
2. Genesis 3:15 foretells a hatred between the "Seed of the Woman" and the "Seed of the Serpent". This is the first Messianic Prophecy (and I can document that Jews have viewed it as such) but is also felt to be possibly of the "Antichrist" as well. The idea of the Serpent's seed is alluded to elsewhere too, like Isaiah 14:29 (my own translation but based on the JKV). "for out of the serpent's root shall come forth a Basilisk, and his fruit shall be a Flying Fire-Dragon."
So perhaps they're the direct offspring of the Fallen Cherub (Ezekiel 28:11-19, defines Satan as a Cherub, the Issiah verse above used Saraph, a synonym for Cherub). Or perhaps Fallen Angels who've taken on the form of their new master, perhaps by Drinking Lizard Blood (the connection between Fallen Angels and Blood drinking could be a whole new discussion), and then the offspring with Human women with those Angels.
3. The greys are the most well known, probably because they're the lowest of them, doing most of the dirty work. In-spite of people's assumptions, many who have experienced and studied them view the Greys as not very intelligent. And their physical form while based on basic Human mechanics as I said, is very flimsy and incomplete looking. I've seen people like David Ike suggest they're really just some sort of robotic suits for much smaller reptilians.
My own hypothesis is that they're artificially created bodies being wielded by demons. Demons in my view are distinct from Fallen Angels, their spirit beings that need to posses a host in order to interact physically with our four dimensional realm. I believe their origin is that their the spirits of Angel-Human Hybrids who perished in the Flood. Based on Isaiah 26 (The word rendered "descended" is really Rephaim, introduced in Genesis 14) their not going to be Resurrected (which all Humans will be, saved or unsaved) which likely means they don't go to the same Afterlife.
That took longer then I expected. I hope I've at least intrigued some people.