Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Scientists compared Pluto with other objects in the solar system and found that Pluto is smaller than Earth's moon. This finding caused scientists to question whether Pluto is a planet because
the other planets are much larger than Pluto
its size is less than Earth's moon.