Neptune is the coldest planet in our solar system. Here's why:

Atmospheric Composition:
Neptune is an ice giant, composed mainly of hydrogen and helium, with trace amounts of methane. The presence of methane in its atmosphere is what gives Neptune its blue color. Methane is a greenhouse gas, which means it absorbs and traps heat from the Sun, contributing to the planet's overall temperature.

Distance from the Sun:
Neptune is located about 2.7 billion miles away from the Sun, making it the farthest planet in our solar system. The distance from the Sun affects the amount of sunlight and heat that reaches the planet. Since Neptune is so far away, it receives much less sunlight and heat compared to the inner planets like Earth, resulting in colder temperatures.

Internal Heat:
Although Neptune is far from the Sun, it still possesses its own internal heat. This heat is generated from the planet's core, which is primarily composed of rock and ice. However, the internal heat of Neptune is not sufficient to counterbalance the extreme coldness of its atmosphere.

Extreme Temperatures:
Neptune experiences incredibly low temperatures due to a combination of factors. The average temperature on Neptune is around -353 degrees Fahrenheit (-214 degrees Celsius). However, temperatures can drop even lower, reaching as low as -391 degrees Fahrenheit (-235 degrees Celsius). These frigid temperatures make Neptune the coldest planet in our solar system.

Other Cold Planets:
While Neptune is the coldest planet, it is worth mentioning that Uranus, Saturn, and Jupiter are also quite cold. Uranus, the seventh planet from the Sun, has an average temperature of -371 degrees Fahrenheit (-224 degrees Celsius). Saturn, the sixth planet, has an average temperature of -288 degrees Fahrenheit (-178 degrees Celsius). Jupiter, the fifth planet, has an average temperature of -234 degrees Fahrenheit (-145 degrees Celsius). Although these planets are cold, Neptune still holds the title of the coldest planet in our solar system.

In conclusion, Neptune is the coldest planet in our solar system due to its atmospheric composition, distance from the Sun, internal heat, and extreme temperatures. While other planets like Uranus, Saturn, and Jupiter are also cold, Neptune's average temperature of -353 degrees Fahrenheit (-214 degrees Celsius) makes it the coldest of them all.