Uranus, the ice giant , is one of the most peculiar planets in our solar system . It is nearly identical in size and composition to Neptune , yet considerably less massive. In addition, being closer to the Sun than Neptune, it has a lower temperature and does not show the banding patterns that Jupiter or Saturn do, and hardly any spots or clouds like those we see on Neptune. But the most peculiar thing about Uranus is not that, but that it seems to orbit on its side . Its axis of rotation is so tilted that the poles point directly at the Sun at certain times of the Uranian year.
Earth’s axis of rotation is also tilted , of course, but its slightly over 23 degrees is nothing compared to Uranus’s 98 . This tilt of the earth’s axis of rotation is responsible for the different seasons of the year . The hot summer and cold winter have nothing to do with the distance from the Earth to the Sun, but with which part of the Earth points directly to the Sun and which part does not. If this were not the case, summer and winter would occur at the same time in both hemispheres, instead of at opposite times of the year. In addition, the closest moment of the Earth’s orbit to our star, the perihelion, is around January 4 , in the middle of winter in the northern hemisphere.
Of course Earth is not the only planet with seasons. Jupiter and Venus have their axes very close to the vertical, so their seasons are very smooth , while Mars , Saturn or Neptune have an inclination similar to that of Earth. On each planet the seasons will last different times , depending on the time it takes that planet to complete an orbit around the Sun. On Earth the seasons last about 3 months or 90 days. On Mars they stretch out to last about 7 months each. In the giants of the solar system it is even more exaggerated and we can go from 3 years per season for Jupiter to more than 40 years for Neptune . In the case of Uranus, the seasons last around 21 years, since its orbital period is 84. This means that Uranus has completed less than 3 times around the Sun since it was discovered in 1781 by the British astronomer William Herschel .
As you can imagine, the unusual tilt of Uranus’ spin axis will also cause some pretty atypical seasons . During the summer the corresponding hemisphere (currently the southern hemisphere) will be completely illuminated permanently . As the south pole is currently oriented towards the Sun, it constantly receives light. In fact, if we could place ourselves over the south pole of Uranus, we would see how the Sun circles the sky without setting . At the beginning of summer it would rise day after day from the horizon until it was above our heads, and later on it would continue to describe these circles in the opposite direction. In the northern hemisphere , the opposite situation will be experienced. Right now it will be plunged into a period of darkness that will last until the spring equinox (which will take place in 2049).
But although these seasons are much longer lasting than the terrestrial ones, in essence they are quite similar . In summer it is hotter because it receives more sunlight, while in winter it is colder because it receives less sunlight. Not so for Uranus’s equator. On Earth, the inhabitants of the regions near the equator do not experience a great difference in the equations , since they always receive the same amount of sunlight. In short, whether it is summer, autumn, winter or spring, it is always hot near the equator. In the case of the icy giant, both summer and winter are similar and are also the coldest seasons . At this time of year the Sun barely rises above the horizon, never reaching more than 8 degrees above or below the horizon. Therefore, it would not be done completely at night either.
In spring and autumn, on the contrary, the more familiar situation of alternating day and night would occur. As we approach the equinox , the Sun would rise above the horizon during the day and submerge at night , but as it passed this date it would go down again, until it reached the solstice again. Thus, the spring and fall of Uranus’s equator would be warmer seasons than summer .
All this on the assumption, of course, that on Uranus, no matter the time of year, it’s never really hot . The average temperature of the upper layers of its atmosphere is around 58 Kelvin, or approximately -220 degrees Celsius. Since the Voyager 2 probe visited Uranus in 1986, we have not been able to visit the icy giant. There are currently no planned missions to visit this distant world again , although that could change soon.
Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032, National Academies Press, 2022, doi:10.17226/26522
Munsell, Kirk, 2007, NASA: Solar System Exploration: Planets: Uranus: Facts & Figures, NASA