{"id":32579,"date":"2022-05-27T09:37:45","date_gmt":"2022-05-27T09:37:45","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=109010"},"modified":"2022-05-27T10:03:10","modified_gmt":"2022-05-27T10:03:10","slug":"why-is-uranus-the-coldest-planet-in-the-solar-system","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/why-is-uranus-the-coldest-planet-in-the-solar-system\/","title":{"rendered":"Why is Uranus the coldest planet in the Solar System?"},"content":{"rendered":"

<\/p>

By Chris Arridge\n \t\t<\/p>

Published: Friday, 27 May 2022 at 12:00 am<\/p>

\n\n

U<\/span>ranus, seventh planet from the Sun, is one of the giants of the Solar System: i<\/span>ts diameter is more than 51,000km at the equator.<\/span><\/p>\n

One of the big mysteries about Uranus<\/a> is that it doesn\u2019t give out much heat. Uranus is the coldest planet in the Solar System, with temperatures in its cloud tops averaging \u2013224<\/span>\u00b0<\/b><\/span>C.<\/span><\/p>\n

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. It is up to astronomers and planetary scientists to work out why.<\/span><\/p>\n

Uranus was discovered by William Herschel<\/a> in Bath in 1781, yet to date o<\/span>nly one space probe has ever visited.<\/span><\/p>\n

That was Voyager<\/a> 2, which flew past fleetingly in 1986.<\/span><\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A shot of crescent Uranus captured by Voyager 2 on 25 January 1986 from a range of 600,000 miles. Credit: NASA<\/span><\/figcaption><\/div>\n

In the years since then, we have glimpsed changes in Uranus\u2019s rings and atmosphere.<\/span><\/p>\n

For example, very little weather was detected from Voyager. But more recent observations from the ground have detected quite active weather systems.<\/span><\/p>\n

We need to understand what has changed and why. <\/span><\/p>\n

Astronomers tend to group Jupiter<\/a>, Saturn<\/a>, Uranus and Neptune<\/a> together among the planets, but the latter two lie much further out in the Solar System. Uranus orbits at a distance of 2.86 billion km (1.78 billion miles) from the Sun.<\/span><\/p>\n

Take a closer look, and Jupiter and Saturn are dominated by hydrogen and helium with small rocky cores, whereas Uranus and Neptune are made up of a lot more rock and ice.<\/span><\/p>\n

There is also more water and methane in their atmospheres and their solid cores are much larger. <\/span><\/p>\n

Why is Uranus so cold?<\/b><\/h2>\n
\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Uranus (left) and Neptune (right), as seen by the Voyager 2 spacecraft. Credit: NASA\/JPL-Caltech; NASA<\/span><\/figcaption><\/div>\n

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. But why?<\/p>\n

One suggestion is that convection in its atmosphere is somehow inhibited so that there are no large-scale over-turning motions such as you get in other planetary atmospheres.<\/span><\/p>\n

Another is that Uranus lost most of its primordial heat early in its life.<\/span><\/p>\n

An odd feature of Uranus is that its axis is highly tilted to its orbit by over 97<\/span>\u00b0<\/b><\/span> so that essentially it rolls around with its pole lying in its orbital plane.<\/span><\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
An image of Uranus captured by the Hubble Space Telescope in 2003. Credit: NASA, ESA, and M. Showalter (SETI Institute)<\/span><\/figcaption><\/div>\n

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.<\/span><\/p>\n

That might also have caused a massive loss of primordial heat. <\/span><\/p>\n

A consequence is that with so little heat coming up from inside the planet, Uranus\u2019s atmosphere is completely driven by sunlight.<\/p>\n

The large tilt in its axis also produces very unusual seasons on Uranus in its 84-year orbit around the Sun.<\/p>\n

One pole gets continual sunlight for 42 years while the other is in darkness, and then the situation is reversed for 42 years.<\/p>\n

The atmosphere is very strongly affected by this: a situation we don\u2019t find anywhere else in the Solar System.<\/p>\n

When Voyager 2 flew past Uranus, just one hemisphere was being illuminated by the Sun.<\/p>\n

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>\n

Uranus\u2019s rings<\/strong><\/h2>\n
\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
An image of Uranus and its rings captured by the Keck II telescope. Source: W. M. Keck Observatory (Marcos van Dam)<\/span><\/figcaption> Lawrence Sromovsky, University of Wisconsin-Madison\/W.W. Keck Observatory<\/i><\/span><\/div>\n

Uranus\u2019s ring system is quite different to the ones we find at Jupiter, Saturn and Neptune.<\/span><\/p>\n

The rings are very dense, narrow arcs and they have been seen to change since Voyager 2.<\/span><\/p>\n

One of the rings has moved in closer to the planet and new rings have been observed.<\/span><\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Uranus\u2019s rings, as seen by the Voyager 2 spacecraft. Credit: NASA\/JPL<\/span><\/figcaption><\/div>\n

A tiny moon called Mab lies at the core of one of these rings and seems to be feeding it material. <\/span><\/p>\n

We also need to know more about how Uranus\u2019s own aurora is produced by its magnetic field<\/a>. <\/span><\/p>\n

It will take many years for a probe to reach this distant ice world, but returning to Uranus and Neptune<\/a> would enable us to solve some of their many mysteries.<\/span><\/p>\n

This article originally appeared in the May 2011 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p><\/body><\/html>\n

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By Chris Arridge Published: Friday, 27 May 2022 at 12:00 am Uranus, seventh planet from the Sun, is one of the giants of the Solar System: its diameter is more than 51,000km at the equator. One of the big mysteries about Uranus is that it doesn\u2019t give out much heat. Uranus is the coldest planet […]<\/p>\n","protected":false},"author":24,"featured_media":32580,"template":"","categories":[1],"acf":{"readingTimeMinutes":"4"},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system.jpg",721,406,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system-300x169.jpg",300,169,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system.jpg",721,406,false],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system.jpg",721,406,false],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system.jpg",721,406,false],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/05\/why-is-uranus-the-coldest-planet-in-the-solar-system.jpg",721,406,false]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"By Chris Arridge Published: Friday, 27 May 2022 at 12:00 am Uranus, seventh planet from the Sun, is one of the giants of the Solar System: its diameter is more than 51,000km at the equator. One of the big mysteries about Uranus is that it doesn\u2019t give out much heat. Uranus is the coldest planet…","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed\/32579"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/rss_feed"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/32580"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=32579"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=32579"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}