Planetary Profile: Unusual Uranus

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It’s that time again! Time to continue my unofficial weird solar system series (here’s Mercury, Venus, Earth, Mars, the asteroids, Jupiter, and Saturn if you want to catch up). That means it’s Uranus’s turn on the mat.

Not gonna lie, I’ve been putting this one off until the school year was over because the presence of field trip groups in the Planetarium means that I’m dealing with Uranus on a regular basis. Middle schoolers looooove yelling out this planet’s name. 

The thing is, it’s actually a truly fascinating world, and if you can get past giggling at the name even its naming saga is interesting (and particular to this world). Certainly it’s more deserving of respect than it generally is treated. It’s a whole planet after all! So let’s take a trip to the ice giant part of the solar system and see what makes Uranus unique!

Rediscovery

One thing that is unique to Uranus is our relationship with it. It’s technically the first planet to be discovered, and it’s certainly the only one that got rediscovered multiple times. I always tell my Planetarium audiences that in the pre-telescope days humans knew of five planets, because out to Saturn you can see them clearly with your eyes and humans have known about them since they started looking up.

Strictly speaking, I am lying when I say this (sorry to my audiences). Because in a clear, dark sky of the sort they had before light pollution became a real thing, you can see Uranus with the naked eye. Barely, but ancient folks had a serious yen for looking up and cataloging things. There is evidence the legendary ancient Greek astronomer Hipparchus identified Uranus in 128 BCE. Even going into the telescope era there are over a dozen confirmed sightings of Uranus in various records. But if you look up its official discovery date, it’s listed as March 13, 1781.

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That’s because despite all of these people spotting Uranus and even observing it multiple times, sometimes across multiple years, not a single one of them recognized it as a planet. They thought it was a faint If you imagine your only reference for how planets look in the sky is, like, Venus and Jupiter, it’s no wonder. Uranus is dim, and all the other visible planets are bright. 

And it moves so. Very. Slowly. If you look at even Saturn from year to year, you can see it changing its position relative to the background stars easily as it makes its way through its 29.5-year orbit. Uranus, with its 84-year orbit, crawls. I could argue that anyone paying close enough attention should be able to notice that it moved over the course of years—but that only applies if you know for sure you’re looking at the same, faint object and not a different faint object.

Heck, even William Herschel, the guy who spotted Uranus on March 13, 1781, didn’t realize at first that he’d found a planet. He presented it to the Royal Society as a comet—though one that seemed to show off certain planetary characteristics. Still, that is the date and he is the guy credited with making the first definite planet discovery in our solar system’s history.

What’s in a Name?

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So about that discovery. By April of 1781 Herschel’s newly discovered “comet” hadn’t grown a tail, so astronomers who were not Herschel began to consider it a planet. By 1783 Herschel himself had accepted this classification and as the discoverer of record, he proposed a name for the new planet: George. 

Well, George’s Star, technically, in honor of King George III of England. Shocking exactly nobody, no one outside of England liked this name. Everyone eventually settled on Uranus, but it took a while to get there. And it was a strange choice.

Every other planet in the solar system is named for a Roman version of an originally Greek deity. For instance, the Greek deity Aphrodite was Venus to the Romans. The Greek deity Zeus was Jupiter. But the deity for whom Uranus is named, a sky deity, was Ouranos. His Roman counterpart was Caelus. Frankly if 18^th-century astronomers had just gone with Caelus, they would have saved generations of planetarium educators a lot of having to take deep, calming sighs.

But no. For…reasons?...they didn’t use the Roman version of Ouranos’s name, they just Latinized it to Uranus. Which, by the by, has two pronunciations, making this planet the only one with multiple ways to pronounce its name. There is the classic, giggle-inducing, middle school favorite yoor-AY-nus, and the one that more closely resembles the pronunciation of Ouranos (and the one far, far preferred by astronomers and planetarium educators everywhere), YOOR-uh-nus. Technically both can be considered correct, but I know which one I prefer.

Tilt-a-Whirl

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Obviously all of that is not about Uranus itself, but about the way we humans have dealt with it over time. But the truth is that it can be hard to find physical characteristics of the planet that are truly unique. It’s not the densest giant planet, nor the least dense. Not the largest, nor the smallest. Its orbit is neither particularly round nor particularly elliptical, neither especially flat nor notably tilted. And the planet itself has so much in common with Neptune it’s nauseating.

But of course there’s that one thing. That one thing that is absolutely unique purely to Uranus. And that is its tilt. 

If you want to be super technical about it, Venus has the most extreme planetary tilt in the solar system at 177 degrees. That means it spins backwards, which is certainly weird, but in terms of total tilt away from being upright, the kind of axial tilt that determines how rough your seasons are, it functions like a tilt of 3 degrees. Piddling.

For a tilt that really throws its whole planet out of whack, you’ve got to go to Uranus, which is rolling around the solar system on its side at an angle of nearly 98 degrees. Or 82 degrees. It depends which way you say is north, which is hard to define on a sideways planet.

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The International Astronomical Union states that north is the pole which lies on the same side of the plane of the solar system as Earth’s north pole. That would give Uranus a tilt of 82 degrees and a retrograde spin. But nobody really likes that definition (that would also mean Venus’s official tilt would only be 3 degrees) and for once everyone seems to quietly agree to ignore the IAU definition. Instead a common convention is to use the right hand rule. If you wrap the fingers of your right hand around the equator in the direction a planet is spinning, your thumb will point north. By this definition, by far the more popular, Uranus has a 98-degree tilt. 

How exactly do you get something the size of Uranus (which can fit 63 Earths inside of it) over onto its side? The go-to explanation in a case like this is that something hit it. It must have been a heck of a something, on par with Earth for size and mass. And it must have been a very, very long time ago because Uranus’s gossamer rings and its innermost moons line up with its current equator, which they wouldn’t do if the knock over hadn’t been primordial.

To Everything There is a Season

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That means that Uranus has, by far, the most insane, whacked out, intense, extreme, insert-your-favorite-adjective-here seasons in the solar system. It points its poles directly at the Sun. Each of its poles spends 42 years in constant sunlight in summer before experiencing 42 years of total darkness in winter.

This, as you might guess, plays absolute merry hell with Uranus’s weather. If you’ve seen a picture of Uranus, it was probably bland, blank, pale cyan ball seen when Voyager 2 flew past it in 1986 (the only time a spacecraft has ever been near it). For a long time we assumed this planet just didn’t have much interesting to look at. 

But that’s the danger of not having a lot of data. Turns out Uranus was having an off day the day Voyager 2 flew past and wasn’t showing much activity. Longer term observations from Earth have revealed a roiling atmosphere with incredible, enormous storm events! Heck, we have a picture of it from 2018 taken by Hubble that shows most of its northern hemisphere (northern using the right hand rule, anyway) topped by a single, giant cloud cap during summer. Uranus is crazy.

Only From a Distance

The more we observe Uranus from Earth, the more we realize how much we missed out on seeing during that brief 1986 Voyager 2 flyby. Uranus and Neptune remain the only planets never to be orbited by a spacecraft. And that is one thing astronomers have been clamoring to change.

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An ice giants mission to either Uranus or Neptune (but let’s face it, Uranus is over a billion miles closer than Neptune, so we know which one we’d be targeting) has been on astronomers wish list for decades. Just think of the incredible depths of knowledge we gained by having Cassini in orbit around Saturn for half of its year! 

A mission around Uranus for an equal length of time would still only be 15% of a Uranian year. But that’s still a whole heckuva lot more than the few hours Voyager 2 got! My personal wish list of missions I would love to see developed switches around frequently, but an ice giants mission is definitely top 5 at all times.

And then we could really dive into just how unique Uranus is. You just know this sly critter has more secrets to reveal!