Problem: What are the most important words?
in outer space a green planet with gaseous rings surrounding its surface slowly rotates
Answer: ring, rotate, space, surface, surround

Problem: What are the most important words?
Man aiming a rifle with his dog at his side.
Answer: aim, dog, rifle

Problem: What are the most important words?
Three workers are performing their jobs while on scaffolding.
Answer:
job, scaffolding, worker