The closer you are to a fire, the hottest you get. The rule doesn't actually work when it comes to planets in the solar system. There are eight planets and the closest is Mercury so conventional wisdom probably says that's your answer, well, no.
According to Caltech, the temperature of Mercury is between -173 Celsius (at night) and 427 Celsius (during the day). The next closest planet is Venus. Venus should be cooler than Mercury as its further away. Venus is actually hotter than Mercury.
Venus is actually about 464 Celsius, therefore it is the hotter of the two closest planets. To understand why the planet is hotter, you need to look at the planet. The planet is surrounded by a thick cloud and atmosphere of noxious gasses. These clouds trap heat from escaping, it has gone through a global warming event of unprecedented scale. It is not a place you'd want to be going for your summer holidays.
Venus comes closer to the Earth than what Mars does but because of its high temperatures and toxicity, interest is not that high on the planet. Mars is a cooler planet and therefore scientists can study it in much greater detail than Venus. The further out you get from Earth, the colder the planets become.
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