Mercury is the hottest planet in the solar system.
Why it is true
Mercury, despite being closer to the Sun, has no atmosphere to retain heat. Venus, with its dense carbon dioxide atmosphere, has an average surface temperature of 465 degrees Celsius, much higher than Mercury s.
Source: NASA
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.