Yes, Yağmur and Mikkel are correct. 'Likely' is an adjective. Don't be fooled by the 'ly' suffix - not all words which end 'ly' are necessarily adverbs. Think of 'He took a leisurely walk' or 'She had a friendly smile'. These are both adjectives ending in 'ly'. As for your sentence, think of it as a shortened form of 'The life form [which is] most likely to develop..'. When you look at the full form, you can see that 'likely' is an adjective. In British English, 'likely' is always an adjective. US English sometimes uses it adverbially, as in 'He'll likely need surgery'. In the case of your sentence, though, it is clearly an adjective.

I agree with Yağmur that in this instance “likely” is an adjective. A simple way to show it is to replace “likely” with a synonym and see what happens. As an adjective “likely”=probable and as an adverb “likely”=probably. You can’t say “The life form most probably to develop on a planet would be bacteria.” But you can say “The life form most probable to develop on a planet would be bacteria.” The following sentence is an example of “likely” being used as an adverb. Notice that it can be replaced with “probably”: The bacteria most likely developed because of low usage of the camp's drinking water. A couple of examples of "adjective vs adverb": The person most willing to admit his mistakes would be John. John willingly admitted his mistakes. The person most happy to comply was Ben. Ben happily complied.

It can be used as an adverb or an adjective, depending on the context but in the example you give it's an adjective in my opinion. Because it doesn't define the verb, it's just a rephrased version of "most likely bacteria form to develop on a planet" so it is again, related to a noun. We don't say that "happy" is an adverb when we say "She's happy to be here." or say it defines the verb "to be". I hope it helps, have a nice day.

Hi David, In your sentence, "likely" would be an adverb. "Most likely" qualifies or characterizes the verb "to develop." This makes it an adverb that answers the question, "How likely is it to develop?" It is not an adjective because "most likely" here is not used to qualify "life form." I hope this helps.