The coolest planet in the solar system is Neptune. Its average temperature is about -353 degrees Fahrenheit (-214 degrees Celsius), making it the coldest planet in the solar system. This frigid temperature is due to its distance from the sun, as Neptune is the eighth planet from the sun and receives only a small amount of solar energy. Additionally, Neptune has a thick atmosphere composed primarily of hydrogen, helium, and methane, which helps to trap heat and keep the planet cool.