Just heard that Marvel's Asgardian gods are aliens, can someone please explain this?

That was an explanation given in the Earth X universe, and is not canon in the main Marvel universe. In the main universe they are normal divine beings.

They were created by The One Above All most likely, and there are gods that don't originate on Earth. Such as the Cosmic Gods (Galactus). As for the Asgardians being aliens, I think they are based off of the Aesir in Norse Mythology and are then considered gods, just not the same as the Greek Pantheon which were much more powerful.

In the Marvel Universe, Human physiology is much more malleable than real humans are. A gamma explosion the size of which created the Hulk, would have killed any man caught inside it not turn them into a Hulking creature that crushes gods for fun. So to compare a fictional god with a fictional mortal really isn't fair, considering in these universes the "mortals" have the potential of becoming godlike in strength themselves.

In Marvel these humans who naturally progress that way are called Mutants, in DC Universe they are called Metahumans, although the phrase there is more widespread for anyone who has superpowers it still applies to the "DC Mutant".

@jacksper: There is no such thing as the One Above All. The name has only been used twice: 1) As the name of a Celestial. 2) the mutant named Protege', who attempted to replicate the power of this being, but instead was only able to look like the Living Tribunal. The Living Tribunal killed him for his presumption.

That's it.

THEY ARE GODS