Why isn't Mercury the hottestplanet?

Mercury isn't the hottest planet in the solar system because, despite being closest to the Sun, it has a very thin atmosphere that cannot retain heat. As a result, temperatures on Mercury can fluctuate dramatically, reaching up to 800°F (427°C) during the day and dropping to -330°F (-201°C) at night. Venus, on the other hand, has a thick atmosphere rich in carbon dioxide, which creates a strong greenhouse effect, trapping heat and making it the hottest planet, with average temperatures around 900°F (475°C).