Asked by Chris Jones

With Uranus at an average distance of 2.88 billion kilometres from the Sun and Neptune at an average distance of 4.5 billion kilometres it would be very easy to point out which of the gas giants is the coldest, but if you were you were to say that Neptune was the coldest, you’d be wrong.

Given that we expect planets further from the Sun to be colder than those closer, this does make Neptune and Uranus quite a mysterious pair. Uranus and Neptune are brimming with volatiles such as water, methane and ammonia and due to their composition in comparison to Jupiter and Saturn, which are comprised mainly of hydrogen and helium, are labelled the ice giants. Scientists have measured how hot Uranus and Neptune should be and have found that Uranus is very cold and very dim, but why?

There seems to be two possible answers to this question. One is that Uranus seems to have been knocked on its side, possibly by a giant impact long ago, which has caused the heat from within its core to spill out into space. Alternatively, some astronomers point the finger at the ice giant’s incredibly energetic atmosphere during the planet’s equinox where it is most lively. It is here that scientists believe that the stirred atmosphere could be oozing heat. Even with these two suggestions, however, we still have very little details to hit on a definite answer and will have to wait for many more missions to unravel the answers hidden by the ice giants.

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