what is the smallest planet in the universe?
Â· just now. 1  The smallest planet we know about is Mercury, the closest planet to the sun. 2  Well no one knows what the smallest planet in the Universe is because it's so vast. 3  Yes, Mercury is the smallest planet in our solar system. 4  No idea what the smallest is in the universe. 5  No way to answer. 6  Simple - planet B612. 7  Nobody knows.