How many possible haiku combinations are there in English?

The haiku poem is a description of something told in 17 syllables and three lines. The lines have 5 (top) 7 (middle) and 5 (last) syllables, which makes 17. So you can only use real words in English, you cannot split words over two lines. How many unique haikus are possible? (Or take a guess because it may take years to figure this out)