aurelion sol teaser confirms other planets with life

Well I think we already assumed this. I mean, there's the Void after all.

They literally just announced a dragon that comes from outer space. Is that not alien enough for you? If he wasn't born on Runeterra (and no alternate dimension shenanigans) then by my standards he is an alien.

Well they once worked on a champ called Averdrian who definetly looked like an alien http://vignette2.wikia.nocookie.net/leagueoflegends/images/e/ef/Averdrian.png/revision/latest/scale-to-width-down/180?cb=20091030230944

I'd say an alien champ would not be completely out of question,though he would probably be more mystical than alienesque.

_Taric, the Gem Knight, is Runeterra's sole practitioner of this form of magic, having been erratically summoned to Runeterra from a faraway world. _

In a world far away where an ancient war still rages, Kayle was a great heroine - the strongest of an immortal race committed to destroying evil wherever it could be found

There was an Alien champion - Averdian who was reworked into Heimerdinger

https://yurixy.files.wordpress.com/2015/09/a58bf4c8a5b2f4c106ccabb95e32f59bd2f19a3135b207078dpimgpsh_fullsize_distr.jpg

I mean...

Do they not count?

this was confirmed with both bard and illaoi im pretty sure.