6 Answers
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If you mean planet, that would be Mercury. There are other objects in our solar system that could fit the criteria.
Our moon, Ganymede, and Callisto are about a quarter the size of Earth and have no atmosphere or moon.
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Mercury is about 1/3 the Earth (it does depend on how you measure, though), so I say the Moon is a better answer.
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The Planet Mercury.
It's structure though shows that is but the remaining core of a body much bigger which suffered a massive collision at some time.
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The planet Mercury.
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Mercury.
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In the solar system, that would be Mercury.