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Excluding dwarf planets, such as Pluto which are trans-neptunian objects obviously further away from the sun, so due to inverse square law less area of light and therefore less heat, of course things such as albedo matter here. But considering the principle, a simple question arised in my mind, which is colder Uranus or Neptune? According to this principle, Neptune should be colder, which is true, But other sources state Uranus. According to my analysis, I think that the first one, Neptune holds the record for the average coldest temperature, whereas the later, Uranus has the coldest temperature of a planet (In our solar system, excluding exoplanets such as rogue planets, etc..). Stating that:

Uranus holds the record for the coldest temperature ever measured in the Solar System: a very chilly -224℃. The temperature on Neptune is still very cold, of course – usually around -214℃ – but Uranus beats that.

So why is this? Ideally Neptune should have the lowest mean temperature as well as the lowest temperature recording (according to present data), why is this, Any reasons?

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    $\begingroup$ Don't neglect heat of formation. Even our small Earth still retains a substantial amount of its formation heat, see physics.stackexchange.com/q/152979/123208 & physics.stackexchange.com/a/154514/123208 You may also find this amusing physics.stackexchange.com/a/652569/123208 $\endgroup$
    – PM 2Ring
    Commented Jun 30 at 6:06
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    $\begingroup$ @PM2Ring Thanks, that is interesting for sure. But considering the fact that it is possible that Neptune had some energy retained from its formation from circumstellar ring, so would have Uranus but still some sort of disperency is there. That could be related to initial energy but I am unable to figure it out. Can you plz help? $\endgroup$
    – Arjun
    Commented Jun 30 at 7:23
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    $\begingroup$ I have changed the title because it is already known that Uranus is colder than Neptune. We should ask why. $\endgroup$
    – Nilay Ghosh
    Commented Jun 30 at 7:45
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    $\begingroup$ @NilayGhosh I can think of another, possibly better reason to change the title. $\endgroup$
    – RBarryYoung
    Commented Jul 2 at 15:24
  • $\begingroup$ Here's a link to the original article containing the quote in this question (it's CC-licensed so it can be found on other websites as well). For what it's worth, the article explains the phenomenon by heat loss due to meteoric impacts on Uranus. $\endgroup$
    – Schmuddi
    Commented Jul 2 at 16:01

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The seventh planet from the Sun, Uranus has a mean temperature of -195°C (-320°F). Neptune, the eighth and most distant major planet known orbiting our Sun has a mean temperature of -200°C (-330°F). Neptune is colder than Uranus, as one might have guessed.

But, the coldest spot on any of the eight planets is the winter side of Uranus. Tilted by 97.77° relative to the ecliptic, its poles experience 42 years of sunlight followed by 42 years of darkness, enough time for one side to drop to -224°C (-371°F).

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    $\begingroup$ Similarly, the coldest temperatures among the inner planets are on ... Mercury, which lacks a significant atmosphere for convective heat transfer and has long solar nights. $\endgroup$
    – Oscar Lanzi
    Commented Jul 1 at 14:57
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    $\begingroup$ @OscarLanzi: Indeed! Mercury even has ice on it (due to a low axial tilt and craters)! The only gravitational-rounded body in the solar system with a solid surface but which lacks ice is Venus. Edit: Except maybe Io! $\endgroup$
    – Kevin Kostlan
    Commented Jul 1 at 18:32
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    $\begingroup$ The Jovian moon Io may say hello unless you count some sulfur compounds as ices. I am not sure they found or could find water ice amid all that volcanic actvity. $\endgroup$
    – Oscar Lanzi
    Commented Jul 1 at 18:35
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The intuition that the temperature of the planet is related to the distance between it and the sun is false as Uranus, being nearer to Sun than Neptune is colder. Astronomers came up with two theories:

  • Uranus orientation: It is theorized that billions of years ago, something crashed into Uranus with so much force that it tipped the planet over onto its side. The impact of the crash let some of the heat that was trapped inside (primordial heat) Uranus escape.
  • Chaotic Atmosphere: The atmosphere is constantly stirred by the blows. The stirred atmosphere effuses heat through furious zonal winds. The winds that blow near the equator move at 50-100 m/s in the opposite direction (against the rotation). This causes Uranus to constantly lose heat. One more thing to consider is Uranus atmosphere which is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia which doesn't help retain the heat unlike Neptune which is more methane concentrated.

This post will provide you more insight: Neptune's Internal Heat Source

More video reference:

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https://www.spaceanswers.com/solar-system/why-is-uranus-colder-than-neptune/

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.

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