By Jonny Lupsha, Wondrium Staff Writer
Uranus is a unique planet in our solar system for a number of reasons. It takes 84 Earth years to complete a single orbit; it smells like rotten eggs; and it rotates on its side. Why is Uranus so different from other planets?
Moving outward from the Sun, beyond the gas giants, we reach the ice giants: Uranus and Neptune. Uranus has two sets of rings, has completed less than three full orbits since its discovery in 1781, and has only had two seasons pass since Voyager 2 passed it in 1986.
Speaking of the changing seasons of Uranus, recently released photographs taken by the Hubble Space Telescope show that the distant ice giant is getting paler, possibly owing to seasonal change. This discovery leads to more questions than answers, especially about Uranus’s cloud cover.
So, what’s going on with this odd planet? In her video series A Field Guide to the Planets, Dr. Sabine Stanley, a Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Sciences at Johns Hopkins University, answers some questions about Uranus.
Why Does Uranus Rotate on Its Side?
Uranus rotates at an angle of 98° from its orbital axis. This gives it one of its most compelling features: It appears to be turning on its side.
“Planets like Earth, Mars, and Saturn have tilts around 25°; other planets have little to no tilts, like Jupiter and Mercury,” Dr. Stanley said. “Then, of course, there was Venus, whose upside down tilt is a whopping 177°, but Uranus is the only planet rotating on its side. The most common explanation for the tilt of Uranus is that while the planet was forming, it suffered a glancing blow from an Earth-sized protoplanet.”
If such a collision occurred, it would certainly change Uranus’s rotation vector. According to Dr. Stanley, this explanation has precedent: It’s the same one used to explain other planet’s rotational tilts. In fact, angled impacts also explain the creation of Earth’s Moon.
“What’s a bit unsatisfying is that this means a major property of yet another planet was the result of yet another chance encounter early in the solar system,” she said.
How Do Seasons Change on Uranus?
On Earth, we experienced the vernal equinox last week. On Uranus, the vernal equinox last occurred in 2007—and summer won’t come until 2028. And since the planet rotates at a 98° angle, the changing of seasons differs greatly compared to other planets.
“Let’s say you lived in the northern hemisphere on Uranus, at latitudes similar to the U.S. or Europe,” Dr. Stanley said. “In northern summer, you would constantly face the Sun, experiencing no night. It would be a single summer day that would last 21 Earth years! If you don’t head south for winter, you’ll have a single night lasting another 21 years.”
Spring and fall are an entirely different story. Different latitudes of the planet face the Sun as Uranus moves along its orbit. Around the times of the spring and fall equinoxes, Uranus will feature far more typical day-night cycles.
“On planets like Earth that have a smaller axis tilt, the equatorial regions experience the most solar heating when averaged over the year,” Dr. Stanley said. “But on Uranus, if you average over the Uranian year, it’s the poles that get the most sunlight. But there are huge chunks of time—decades—where each pole receives no sunlight at all.
“We have to completely rethink how weather patterns and climate are related to seasons for a planet like Uranus.”
A Field Guide to the Planets is now available to stream on Wondrium.