The planet in our solar system with the highest temperature is Venus. Although Mercury is the closest planet to the Sun, it does not have a substantial atmosphere to retain heat, leading to extreme temperature fluctuations. Venus, on the other hand, has a thick atmosphere filled with carbon dioxide and clouds of sulfuric acid, creating a runaway greenhouse effect. This results in surface temperatures averaging about 900 degrees Fahrenheit (475 degrees Celsius), making it hotter than Mercury. The extreme heat makes Venus an inhospitable environment, with conditions that are hostile to life as we know it.