The short answer is that Neptune has the coldest overall average temperature and Uranus has the coldest temperature recorded.
The long answer is that once upon a time the answer to this question was simple. Pluto was the planet furthest away from the Sun and also the coldest. However, Pluto was declassified as a planet in 2006 and is now known as a dwarf planet. So, what is the coldest planet in our Solar System now?
It sounds like a simple question, but actually, there are two planets in the running for this title. It all depends on whether we are talking about the average temperature of the planet or the lowest temperature the planet reaches.
Most would say the coldest planet in our solar system is the frosty Neptune. This is because it is the eighth planet in our solar system and therefore the furthest away from the Sun. The Sun is our primary heat source so it would make sense that the planet with the greatest distance from it would be the coldest. Neptune is known as an Ice Giant and for a good reason.
It has an average temperature of around -214 degrees Celsius. That is much chillier than Earth’s average of 15 degrees Celsius. Neptune lacks a solid surface as such and instead has an icy water layer that serves as the planet’s mantle. This makes finding a surface temperature difficult, but research conducted from Earth and fly by missions have managed to take this average temperature from the upper atmosphere.
Curiously though, Neptune only holds the title for the coldest average temperature, and it is the seventh planet from the Sun, Uranus, that has the record for the lowest temperature. This has been recorded at a very low -224 degrees Celsius.
This is unexpected as Uranus is more than a billion miles closer to the Sun than Neptune. That’s a long way! So how can it be colder? Well despite the large distance between these two ice giants, they are both so far out into the solar system that the Sun has little effect on the temperature of the planets. Any heat is mostly caused by the motion of the planets and their interiors which is where the differences between them lie.
One theory is related to the curious position of Uranus. Uranus sits on an axis of 98 degrees meaning the planet is rolling around the Sun on its side. This is unique, although some are slightly tilted, none of the other seven planets in our Solar System do this.
Astronomers are not entirely sure of the cause of this, but it is believed that long ago, when the Solar System was first forming, Uranus was struck by a large object. This object would have had to be very big, maybe even as large as planet Earth, to be able to knock an entire planet off its axis. This would have caused huge disturbances to the planet’s core, and a lot of heat would have been lost in the process with heat continuing to spill from the planet’s atmosphere due to its strange orientation.
Both Neptune and Uranus have very similar atmospheres that contain high levels of methane. Methane is a gas that traps heat very well. It will try and retain any heat that reaches the planet. Neptune has a slightly higher level of methane in its atmosphere, and so it is more efficient at keeping in the heat generated at its core. So Neptune has more heat to begin with and an atmosphere that is better at keeping it.