Su.Ki., I must correct you in that "likely" can be used as either an adjective or an adverb in American English. I'd say it's more common as an adjective. In this case, I'd expect to hear "Extraterrestrials very likely exist" before "Extraterrestrials very probably exist." But please note that the structure "Extraterrestrials are very likely to exist" is also correct and not altogether uncommon in American English.

'Most probably' and 'most likely' are how I would usually phrase it and I am from the UK.

Hi, It would be "very likely" . If you wanted to say "very probably" you would define it as "most probably". As it would not make sense to say very and then probably. So I would use "Michael most probably believed aliens exist" I'm from the UK.

I'm from Britain, and 'Extraterrestrials very likely exist' doesn't quite work in the standard grammar of British English. In US.E, 'likely' is an adverb that can be used in the same way as 'probably'. In GB.E, 'likely' is used as an adjective in phrasings such as 'You are not likely to encounter many extraterrestrials' Or maybe you're aware of this, and are just testing out the theory? Anyway, on a practical point, this post would work better in Discussions. . As we say over here, more people ' are likely to ' see it and respond in that section. Good luck with your survey. You never know, you might even get some responses from Alpha Centauri.