This is an interesting question, because although it looks as if the two alternative sentences are very similar, they are, in actual fact, completely different constructions. First let's consider:
The discovery has excited scientists.
This has the clause structure:
- Subject, Predicator, Object (where predicator is the function carried out by the verb)
The subject is the discovery, the predicator is has excited, and the object is scientists. The verb form has excited is the present perfect form of EXCITE. Has is the perfect auxiliary and excited is the past participle. Excited is definitely a verb in this sentence.
The verb form in this sentence is present perfect, so we do not know when the act of exciting the scientists took place: it might have been yesterday, last week, or over the last century (of course, given that the example is from Time we might guess that it was probably recently - but this isn't encoded in the sentence meaning).
It is worth taking a second to consider the function of scientists here. We have said that is the direct object of the verb. Direct object has a very particular meaning. We use it to mean that the word is a complement of the verb. However, direct object means, more specifically, that the word indicates the patient, or recipient of the action described. So in the sentence:
- The elephant ate the donut.
... the donut indicates the recipient of the eating action. Compare this with:
- Our guide seems a nice guy.
Here a nice guy is the complement of the verb seems, but it is not a direct object. There is no nice guy who is being 'seemed' by the guide. Rather, a nice guy is indicating a perceived quality of the guide. We call this type of complement a predicative complement because it 'predicates' (describes) something about one of the entities in the sentence, in this case the subject. Predicative complements can be adjectives as well as nouns:
Here the adjective ecstatic is denoting a quality of the cat.
Now let us consider the second sentence:
The discovery has scientists excited.
Although it looks similar to the first sentence, it is not. In this sentence, the subject is still the discovery, but the predicator is has - not has excited. In this sentence scientists and excited are both complements of the verb.
Scientists is a direct object of the verb and excited is the predicative complement of the verb. However, this time it is describing the object of the sentence, the scientists, not the subject. It is the scientists who are excited.
The parts of speech in the sentence are: The dicovery, a noun phrase; has, present simple of the lexical verb HAVE; the scientists, a noun phrase; and lastly excited, which is an adjective, not a verb.
The meaning of the sentence is that the discovery is causing the scientists to be in a current state of excitedness. This use of the verb HAVE is often called 'causative have', because it shows that the subject caused the object to be in a particular state. The structure of the sentence is the same as we often see with the verb MAKE:
The flowers made her happy.
Here again we see the form:
- Subject (the flowers), predicator (made), object (her), predicative complement (happy)
The answer to the question, then, is that both sentences are correct, although they have very different structures. Importantly, excited is a verb in the first sentence, and an adjective in the second.
Hope this is helpful!