What is the coldest planet in the Solar System? The answer may be a little surprising, but not really too much. Pluto used to be considered the coldest planet with a minimum temperature of -387 degrees Fahrenheit at the farthest point of its orbit, but since its demotion the title of the coldest planet in the Solar System goes to Uranus, with a minimum temperature of -371 degrees Fahrenheit in its outer atmosphere. Neptune is very close behind Uranus with a minimum temperature of about -361 degrees Fahrenheit in its outer atmosphere, although Pluto would have been the coldest with a minimum temperature range of -369 degrees Fahrenheit at its closest distance to the Sun to -387 degrees Fahrenheit at its farthest distance before its demotion to a dwarf planet.
So the answer is that Uranus is the coldest official planet in the Solar System with a minimum temperature of -371 degrees Fahrenheit. To give a more thorough answer to this question, let’s quickly compare the three planets -the two official planets and one dwarf planet – that are in the running for the coldest planet.
How Cold Is Uranus?
Uranus is the seventh planet from the Sun at a distance of about 1.7 billion miles at the closest and about 1.89 billion miles at the farthest point. It takes about 84 years to make one revolution around the Sun and has the 3rd largest volume and 4 greatest mass of all the planets in the Solar System.
Uranus is in fact so cold that it is considered as an icy giant planet as opposed to a gas giant like Jupiter and Saturn. Uranus consists mostly of oxygen, nitrogen, carbon, and sulfur with hydrogen and helium in its upper atmosphere. Because of its much greater distance from the Sun than Jupiter and Saturn and its somewhat heaver element composition, Uranus remains in a mostly icy state with a minimum temperature of -371 degrees Fahrenheit in its upper atmosphere.
How Cold Is Neptune?
Neptune is the eighth planet from the Sun at a distance of 2.77 billion miles at its closest to a distance of 2.82 billion miles at its farthest point, in an almost circular orbit. Neptune takes about 164.8 years to make a complete revolution around the Sun and has the fourth largest volume and third greatest mass of all the planets in the Solar System.
The axial tilt of Neptune gives it complete seasons similar to the Earths seasons, although on Neptune they last for a little over 40 years. Neptune is also considered an icy giant instead of a gas giant since, like Uranus, it is so cold that it remains in a mostly icy state. The atmosphere of Neptune consists of mostly hydrogen and helium although there is some icy methane and water in the form of ice. It is the methane which gives Neptune its electric blue color.
Although Neptune is about 56 percent farther from the Sun than Uranus and only receives about 40 percent of the light(solar electromagnetic radiation), it has a minimum temperature in its upper atmosphere of -361 degrees Fahrenheit as opposed to -371 degrees Fahrenheit for Uranus.
Why Is Uranus Colder Than Neptune?
Uranus is an average distance from the Sun of 1.789 billion miles and Neptune is an average distance from the Sun of 2.796 billion miles – about 56 percent further, so why is Uranus colder than Neptune?
The precise answer to that is still unclear at the present time, although there are two possible answers that seem reasonable. One is that Uranus may have been knocked on its side from some huge impact in its far distant past, which caused heat from its central core to leak out into space.
The second possibility is that since Uranus has an unusually energetic atmosphere at its equinox, where it reaches a maximum, it has caused leakage of heat into space. Which one of these possibilities is correct, or whether it is something else entirely, is unknown at the current time.
How Cold Is Pluto?
Pluto used to be the ninth planet from the Sun but has now been demoted to a dwarf planet. We will still mention here because of the very fact that it used to be classified as a planet, and many still consider it to be one. It revolves around the Sun at the closest distance of 2.757 billion miles and the farthest distance of 4.583 billion miles in a highly elliptical orbit. Pluto has a diameter about 46 percent less than that of the Moon, only one-third the volume, and a mass only one-sixth as great – its demotion to dwarf planet status with new data received from the New Horizons space probe is therefore easy to understand.
The composition of Pluto is mostly icy and rocky material – nitrogen, methane, carbon monoxide, and various rocky substances. As mentioned previously, the minimum temperature on Pluto varies in a range of -369 degrees Fahrenheit to -387 degrees Fahrenheit depending on its proximity to the Sun. If Pluto were still officially classified as a planet, it would be the coldest one in the Solar System.
Dwarf Planets In The Kuiper Belt
Although dwarf planets located in the Kuiper Belt are of course not officially planets, I think for completeness of this discussion they should be mentioned.
Eris & Farout
Eris and Farout(also known officially as 2018 VG18) are two confirmed dwarf planets in the Kuiper Belt and will be discussed briefly below.
Eris is a dwarf planet in the Kuiper Belt which revolves around the Sun roughly every 558 years at a close distance of 5.723 billion miles and the farthest distance of 14.602 billion miles in an extremely elliptical orbit. The diameter of Eris is very close to that of Pluto – 1445 miles versus 1430 – but Eris is about 27 percent more massive.
The temperature on Eris varies from -360 degrees Fahrenheit to -405 degrees Fahrenheit due mainly from the difference of its distance from the Sun during its very long orbit of 558 years.
Farout – 2018 VG18
Farout is another confirmed dwarf planet in the Kuiper Belt even more distant than Eris, and probably with even colder temperatures. In fact, it is the most distant object ever observed in the Solar System! I say ‘probably’ because the minimum temperature of Farout as yet to be determined. It orbits the Sun approximately every 929 years at an average distance of about 11.8 billion miles – around 3 times the distance of Pluto from the Sun. The closest and farthest distances from the Sun of Farout have yet to be determined precisely, although it is much further from the Sun than Eris.
Farout is known to have a diameter in the range of 310 to 372 miles which qualifies it for dwarf planet status. It also has a pinkish red color which indicates that it is an extremely icy object – the color is caused by the frequency change from the bombardment of electromagnetic radiation from the Sun which is reflected off its surface.
Is There Another Unknown Large Planet In The Kuiper Belt?
It is believed by some astronomers that there may be some large unknown planet lurking deep in the Kuiper Belt, awaiting its discovery by us. They base this on the fact that our celestial mechanics do not account for the mass and orbits of the known planets and dwarf planets – there is still a missing amount of mass which is needed. It is thought by some that this planet could be the size of Neptune or even larger. Whether or not such a planet exists is yet to be determined in the future, but if it does exist it would no doubt be the coldest planet of all!
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