What is the coldest planets in order?

Answer 1

Pluto held the distinction of being the coldest planet until it was reclassified as a dwarf in 2006.

Which is "coldest", depends on whether you want extreme (the coldest recorded/estimated) or mean (average) temperatures. It also depends on whether you're recording upper atmosphere or surface temperatures. For example, Jupiter's surface temperature is about 20C, which is about "room temperature". Its upper-atmosphere temperature can dip to -145C. Also, a planet such as Mercury can vary from -184C to 465C, due to its slow rotation (e.g. the "dark side" gets no sunlight for long periods of time).

Scientists generally measure the cold extreme by the upper atmosphere temperatures.

Going by the lowest-recorded or lowest-estimated upper-atmosphere temperature, the ranks are: Uranus (-224C), Neptune (-218C), Mercury (-184C on the side not facing the sun), Saturn (-175C), Jupiter (-145C), Mars (-140C), Earth (-89C), and Venus. Some scientists rank Neptune first.

Going by mean/average temperature is trickier, since some planets have relatively warm surface temperatures, and since variance on some planets (e.g. Mercury) is extreme. A rough guess by mean temperature would be to simply rank the planets by their distance from the sun, inverting Mercury and Venus.

Note that Neptune's coldest upper-atmosphere temperature is nearly the same as that of Uranus, which is over 1.5 billion miles closer to the sun, while Jupiter's temperature approximates Mars, which is over 300 million miles closer. This is due to Neptune and Jupiter being "gas giants", which gives them an internal heat source.

Answer 2

The coldest planets in our solar system are Uranus and Neptune, with average temperatures around -224°C (-371°F) and -218°C (-361°F) respectively. Uranus is colder on average due to its greater distance from the Sun.