happen upon vs discover

happen upon and discover both are verbs.

Word	Noun	Adjective	Verb	Adverb
happen upon	No	No	Yes	No
discover	No	No	Yes	No

As verbs, happen upon and discover are synonyms defined as:

Other synonyms of happen upon include attain, chance on, chance upon, come across, come upon, fall upon, light upon, strike.

happen upon (verb)	discover (verb)
find unexpectedly	get to know or become aware of, usually accidentally
	identify as in botany or biology, for example
	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
	make a discovery, make a new finding
	see for the first time; make a discovery
	discover or determine the existence, presence, or fact of by perception with the eyes
	find unexpectedly