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_____ is the smallest inner planet.

Explanation
At an average distance of 36 million miles, Mercury is the closest planet to the Sun. Because of this it receives about seven times as much heat and light as the Earth does. In fact, it is hard to see the planet from Earth without a telescope because it is so close to the Sun. Mercury is the smallest inner planet; except for Pluto, it is the smallest planet. It is also a very dense planet. It would take 16 times the mass of Mercury to equal the mass of Earth. The planet's diameter is about 3,030 miles - that's about two-fifths that of the Earth. Mercury's force of gravity is about one-third as strong as Earth's.