Why is Uranus on its Side?

Article written: 5 Aug , 2016
Updated: 22 Feb , 2017
by

It’s impossible to do an article about Uranus without opening up the back door to a spit storm of potty humour. I get it, there’s something just hilarious about talking about your, mine and everyone’s anus. And even if you use the more sanitized and sterile term urine-us, it’s still pretty dirty, in an unwashed New York stairwell kind of way. You’re in us? No.

This is a no-win solution. It’s a Kobayashi Maru scenario here. We’re all doomed.

Can we call a truce? I dare you commentators, to keep the YouTube comments as pure and clean as driven snow, so we can focus on the super interesting science. Think of the children.

Let’s set the stage, I’m going to let planetary astronomer Kevin Grazier give you the proper pronunciation to clear our minds and let us move forward with grace and civility.


Kevin Grazier:
Strictly speaking, it’s pronounced Youranous, is the  pronunciation.


As you probably know, Uranus… I mean Ouranus. No, I can’t do it, my brainwashing is too far along. Save yourselves!. Anyway, Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Jupiter and Saturn get all the spacecraft and Hubble space telescopes, but Uranus is an incredibly worthwhile target to visit.

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

It’s almost exactly 4 times larger than Earth and has its own set of strange dusty rings – perhaps left over from a shattered moon. It has at least 27 moons, that we know of, and many more interesting features that would fascinate astronomers, if we had a spacecraft there, which we don’t. Which is ridiculous. We’ve only made one close flyby of Uranus by Voyager II back in 1986.

We’ve seen Pluto up close, but there are no plans to visit Uranus? Madness.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Anyway, perhaps one of the strangest aspects of Uranus is its tilt. The planet is flipped over on its side, like a Weeble, that wouldn’t unwobble.

Actually, all the planets in the Solar System have some level of axial tilt. The Earth is tilted 23.5 degrees away from the Sun’s equator. Mars is 25 degrees, and even Mercury is 2.1 degrees tilted. These tilts are everywhere.

But Uranus is 97.8 degrees. That’s just 0.2 degrees shy of a 90s boy band.

You might be wondering, why have it be more than 90 degrees. High school geometry tells me that 97.8 degrees is the same as 82.2 degrees. And that’s true. But astronomers define the angle as greater than 90 degrees when you take its direction of rotation into account. When you describe it as turning in the same direction as the rest of the planets in the Solar System, then you have to measure it this way.

What could have done that to Uranus, how could it have happened?

The fact that Uranus is flipped over on its side tells us that the calm clockwork motion of the Solar System hasn’t always been this way. Shortly after the formation of the Sun and planets, our neighborhood was a violent place.

The early planets smashed into each other, pushed one another into new orbits. Some planets could have been spun out of the Solar System entirely, while others might have been driven into the Sun. Our own Moon was likely formed when a Mars-sized object crashed into the Earth. Other moons might have been captured from three body interactions between worlds. It was mayhem.

The Solar System that you see today contains the survivors. Everything that wasn’t delivered a death blow.

And something really tried to deliver a death blow to Uranus, very early after it formed. We know this because the moons of Uranus orbit at the same tilt as the planet’s axis. This means that something smashed into Uranus while it was still surrounded by the disk of gas and dust that its moons formed from.

When the massive collision happened, the planet flipped over, wrenching this disk with it. The moons formed within this new configuration.

Astronomers think it was more complicated than that, however. If it was a single, massive collision, models suggest the planet would just flip over entirely, and end up rotating backwards from the other planets in the Solar System.

It’s more likely that another collision or even a series of collisions put the brakes on Uranus’ end over end roll, putting it into its current configuration. It boggles the mind to think about what must have happened.

Uranus' tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)

Uranus’ tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)

Having such a huge axial tilt makes a big different to Uranus. As it travels around the Sun in its 84-year orbit, the planet still has its poles pointed at fixed locations in space. This means that it spends 42 years with its northern hemisphere roughly pointed towards the Sun, and 42 years with its southern hemisphere in sunlight.

If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later. Then you wouldn’t see it for a few decades until it finally reappeared again. It would be very very strange.

Of course, it’s a gas planet, so you can’t stand on it. If you could stand on it, we’d all be marveling at your ability to stand on planets.

Here we are in our calm, ordered Solar System, everything’s business as usual. But if you look around, you realize it’s pretty amazing that our planet is even here. Poor sideways Uranus is a testament to our good luck.

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14 Responses

  1. weeasle says

    I was trying to keep my head out of the gutter until this sentence:
    This means that something smashed into Uranus while it was still surrounded by the disk of gas

    Gas around Uranus?
    Back to science, this is fascinating.. I also wonder how Venus got its upside-down orientation (reversed rotation compared to all other planets excepting Uranus and Pluto)..

    Thanks again Fraser for another nice article and my Fav science website 🙂

  2. Pvt.Pantzov says

    my favorite planet, and yes, partially because I laughed at the name as a kid.

  3. r1xlx says

    Uranus is on its side and Venus rotates the other way because God thought we humans would enjoy them that way.
    No way could your expanding BigBangery produce either the tilt of Uranus or the backspin of Venus – just do some thinking about dynamics and gravitational attractions.
    And as for the moon being bashed out of Earth – did the bash destroy all Earth chemistry to leave the Moon so unlike Earth?
    I’m sure God put the Moon exactly where it is and build of the best materials to light the nights for us – which it does perfectly.

    Take all the ‘mights’ ‘perhapses’ ‘coulds’ and ‘likely’ out of your piece and you have nothing left except the fact that God made us a nice solar system.

    • BCstargazer says

      no, no, no, no, no, no, no …..

    • weeasle says

      Believing in a creator does not exclude science having a valuable role asking and answering our questions. I also wanted to point out that the Apollo sample returns confirmed the Earth and Moon’s crust appear to be the same composition – Primarily basalt based.. And afaik this discovery made the theory of Moon being composed of bashed Earth bits the leading theory. If a person believes in a higher conciousness, surely he/she/it gave us brains for a reason and would enjoy its children asking healthy questions..

    • Forty says

      Rotating counterwise planets could only mean there was more than one “god”. One was a left-hander, the other one a right-hander. Think about it.
      Oh, forgot you can’t.

    • chfosmith says

      Dear r1xlx,
      I must repeat with the same type of answer.
      You need to read * Wolde, Ellen J. van. 2009. Why the Verb ??? Does Not Mean ‘to Create’ in Genesis 1.1-2.4a. Journal for the Study of the Old Testament 34 (1):3-23.
      The correct translation is closer to the English word “separate”.
      You appear to be relying on the King James translation.
      Please remember that (King James Who followed Queen Elizabeth the First) believed in witches,
      and there were witch hunts and other activities that were definitely against the Bible.
      (Remember, when Jesus was asked if the tares should be removed from the field, he responded No,
      they should be left because wheat could also be killed in removing the tares.)

      I would suggest that you read several translations of the Bible before you try and use it as a reference.
      Using the new Testament is much more reliable because the only major change was when somebody later added the passage about the woman caught in adultery.
      This passage was not in the original Gospel of Saint John.

  4. Vic says

    I still don’t understand how gas planets hold together in one piece, especially the ones with no core. I know how gravity works but still it’s hard to fathom. Concerning how and why things are the way they are, I believe the full answers lie in the “Origin” of life and matter.

  5. mci1983 says

    Why is Uranus on its Side? Try sleeping on your back.

  6. mikeg says

    I tried. Really.

    But once I saw:
    “Uranus is an incredibly worthwhile target to visit.”
    “We’ve seen Pluto up close, but there are no plans to visit Uranus? ”

    I just couldn’t stop laughing. It brought back memories of MST3K (and I’d love to see you do this on their new show!)

    “What could have done that to Uranus, how could it have happened?”

    Indeed…

  7. Smokey says

    “If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later.” …Which leads to an effect that is truly unique in our solar system: the poles receive more insolation over the course of a year than does the equator, although the equator is still warmer.

    So the real mystery is: why is middle of Uranus warmer than the rest of it . . . ?

  8. DrakTheDrake says

    Here, it’s easy, I can come up with two:

    OO-rah-nus
    YOO-rah-nus

  9. Forty says

    Hmmm. Never thought about the way English-speakers would pronounce Uranus. At least not the ones older than 10 years. Grow up, folks. And learn that you can’t just pronounce Greek or Latin words like they spoke English back then. Doesn’t work that way.

    I’d really love to see a Uranus mission in my lifetime. Fear it wont happen.

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