The hottest planet in the solar system is Venus. Despite Mercury being closer to the Sun, Venus has a much thicker atmosphere composed primarily of carbon dioxide, along with clouds of sulfuric acid, which creates a powerful greenhouse effect. This leads to surface temperatures averaging around 900 degrees Fahrenheit (475 degrees Celsius), making it the hottest planet. The high pressure and temperature conditions on Venus are inhospitable to life as we know it, and the extreme greenhouse effect highlights the importance of atmospheric composition in regulating planetary environments.