The planet closest to the sun is Mercury. It is the smallest planet in our solar system and completes one orbit around the sun every 88 Earth days. Due to its proximity to the sun, Mercury has a high temperature range from 800 degrees Fahrenheit during the day to -290 degrees Fahrenheit at night. It has a cratered surface with long, steep cliffs known as scarps. Despite being closest to the sun, Mercury is not the hottest planet in our solar system; that title belongs to Venus because of its thick atmosphere that traps heat.