Temperatures in Uranus’s cloud tops average \u2013224\u00b0<\/b>C, making it the coldest planet in the Solar System, even though it’s closer to the Sun than Neptune.<\/p>
Neptune has an average temperature of -214\u00b0C, making it the second coldest planet in the Solar System<\/p>
All the planets of the Solar System<\/a> radiate primordial heat left over from their formation, but Uranus appears to be emitting very little heat.<\/p> It is up to astronomers and planetary scientists to work out why.<\/p> Uranus was discovered by William Herschel<\/a> in Bath, UK in 1781, yet to date only one space probe has ever visited.<\/p> That was Voyager<\/a> 2, which flew past fleetingly in 1986.<\/p> In the years since then, we have glimpsed changes in Uranus\u2019s rings and atmosphere.<\/p> Very little weather was detected at Uranus by Voyager. But recent observations from the ground have detected quite active weather systems.<\/p> We need to understand what has changed and why. <\/p> Astronomers tend to group Jupiter<\/a>, Saturn<\/a>, Uranus and Neptune<\/a> together, but the latter two lie much further out in the Solar System.<\/p> Uranus orbits at a distance of 2.86 billion km (1.78 billion miles) from the Sun.<\/p> Take a closer look, and Jupiter and Saturn are dominated by hydrogen and helium with small rocky cores.<\/p> Whereas Uranus and Neptune are made up of a lot more rock and ice.<\/p> There is also more water and methane in their atmospheres and their solid cores are much larger. <\/p> Neptune is the farthest planet from the Sun in our Solar System, so surely it should be the coldest. We know this is not the case: Uranus is the coldest planet. But why?<\/p> One suggestion is that convection in its atmosphere is somehow inhibited so there are no large-scale over-turning motions like in other planetary atmospheres.<\/p> Another is that Uranus lost most of its primordial heat early in its life.<\/p> An odd feature of Uranus is that its axis is highly tilted to its orbit by over 97\u00b0<\/b> so that it rolls around with its pole lying in its orbital plane.<\/p> It is thought that something the size of Mars or Earth may have hit Uranus in the early days of the Solar System and knocked it onto its side.<\/p> That might also have caused a massive loss of primordial heat. <\/p> And with so little heat coming up from inside the planet, Uranus\u2019s atmosphere is completely driven by sunlight.<\/p> The large tilt in its axis also produces very unusual seasons on Uranus in its 84-year orbit around the Sun.<\/p> One pole gets continual sunlight for 42 years while the other is in darkness, and then the situation is reversed for 42 years.<\/p> The atmosphere is very strongly affected by this: a situation we don\u2019t find anywhere else in the Solar System.<\/p> When Voyager 2 flew past Uranus, just one hemisphere was being illuminated by the Sun.<\/p> But in 2007, Uranus reached one of its equinoxes when the equator was getting all the sunlight. This changed the dynamics of the atmosphere.<\/p> Uranus\u2019s ring system is quite different to the ones we find at Jupiter, Saturn and Neptune.<\/p> The rings are very dense, narrow arcs and they have been seen to change since Voyager 2.<\/p> One of the rings has moved in closer to the planet and new rings have been observed.<\/p> A tiny moon called Mab lies at the core of one of these rings and seems to be feeding it material. <\/p> We also need to know more about how Uranus\u2019s own aurora is produced by its magnetic field<\/a>. <\/p>Why is Uranus the coldest planet?<\/b><\/h2>
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Uranus’s rings<\/strong><\/h2>
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