As the planets in our Solar System get further away from the Sun, they generally get colder. So what is the coldest planet of our Solar System. That would be Neptune, the 8th and final planet in the Solar System. If Pluto was still a planet, it would be colder, but now it’s Neptune.
Neptune is located 4.5 billion km from the Sun, on average, and it receives a fraction of the sunlight that the other planets receive. In fact, even though it’s only a little further from the Sun than Uranus, it sees only 40% of the solar radiation received by Uranus.
The surface temperature of Neptune dips down to 72 kelvin, or -201° C. That’s cold. You might be surprised to know that the temperature on the surface of Uranus isn’t much warmer. It only gets as cold as 76 kelvin, or -197° C. You would expect Neptune to be much colder than Uranus, but astronomers think that it has a higher internal temperature, and so more of its internal heat reaches the surface and prevents it from being truly frigid.
Even though Pluto isn’t a planet any more, its average surface temperature is only 44 kelvin, or -229° C. Pluto doesn’t have the same internal temperature as the ice giant planets, so it’s able to cool down much further.
We’ve also recorded an entire episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.