Uranus got double-tapped?

By Phil Plait | October 7, 2011 9:37 am

One of the enduring mysteries of our solar system is why Uranus is tilted over on its side. If you measure the angle of a planet’s rotation axis (the location of its north pole) compared to the plane of its orbit, you find that all the planets in the solar system are tipped. Jupiter is only 3°, but Earth is at a healthy 23° angle; Mars is too. Venus is tipped so far over it’s essentially upside-down (we know this because it spins the wrong way).

Uranus, weirdly, is at 98°, like it’s rolling around the outer solar system on its side. The best guess is that it got hit hard by something planet-sized long ago, knocking it over (though there are other, more speculative, ideas). The problem with that is that its moons and rings all orbit around its equator, meaning their orbital planes are tipped as well. It’s hard to see how that might have happened, even if you assume the moons formed in that collision (as, apparently, our Moon formed in an ancient grazing impact with Earth by a Mars-sized body).

Well, a team of astronomers have come up with a new idea: maybe Uranus wasn’t hit by one big object. Maybe it was hit by two smaller ones.

It would’ve happened when the planet was still forming, and surrounded by a disk of leftover material that was in the process of forming its moons. A proto-planet could’ve hit it, knocking it over somewhat, and sending up a vast cloud of debris that puffed the disk up into a torus (that’s what us scientist-types call a donut). A second collision some time later would’ve completed the task. After more time elapsed things settled down and Uranus would’ve been rotating sideways, and the torus would’ve flattened back into a disk aligned with Uranus’ equator due to tidal forces.

It’s an interesting, if surprising idea. If there were only one collision at that time, the astronomers found the dynamics would’ve made the moons orbit the planet the wrong way (retrograde, against the spin of the planet). It would’ve taken a second hit to add enough momentum to the debris disk to get the moons orbiting prograde.

I wonder if this would also somehow explain the weird magnetic field of Uranus. It’s not aligned at all with the rotation axis, and is even off-center from the core of the planet! It’s unclear why this might be, though it may have to do with Uranus being an ice giant (PDF), with a different composition and structure than Jupiter and Saturn, the two gas giants. I’ll note Earth’s magnetic field isn’t well aligned with our spin axis either, but at least it has the courtesy to be centered on the center of our planet! One idea I’ve seen is that the magnetic field of Uranus isn’t generated in its core, like ours is (or, to be more accurate, in the outer layer of our core — this stuff gets complicated pretty quickly), but might be created higher up in the interior. Clearly, there’s a lot left to figure out here.

All of these things are clues to Uranus’ origin and evolution, its history. The characteristics we see today had some cause, and by piecing all this together we can, perhaps, understand the story of this giant planet. And we need to sometimes entertain unusual ideas — as long as the science supports them — because if there’s one thing that’s usual about the bodies inhabiting the solar system, it’s that they’re unusual.

Image credit: Erich Karkoschka (University of Arizona) and NASA


Related posts:

- Did Herschel see the rings of Uranus?
- Ooo-RAN-us
- Yes, yes, rings around Uranus, haha
- A new ring around Uranus (and this followup)

CATEGORIZED UNDER: Astronomy

Comments (34)

  1. Jess Tauber

    This is impossible. Only God makes planets, and He’s perfect. So the orientations of the planets have to be perfect too, in perfect circular orbits around the Earth, plus some epicycles we don’t talk about in polite company…..

  2. Tony

    really odd thought, but could it be possible that Uranus was one of these ‘rogue’ planets they were talking about recently? Something that drifted in and decided to stay? Probably not, or someone smarter than me would have thought about it, but an interesting thought.

  3. Landry Dugan

    Every time I read these articles it makes me want to play Osmos. It’s all about orbits and gravity. Great game.

    http://store.steampowered.com/app/29180/

  4. Gary Ansorge

    I wonder how those simulations would deal with Uranus as a captured planet? We already have some good indications that planets get ejected from their solar systems. It makes more sense to me to assume Uranus was one such that we picked up several billion years ago. Of course it would most likely have been formed from the same nebula that gave rise to our system, so its metalicity should be like our own. On the other hand, if we got lucky, it would be from a different nebular cloud with enough elemental differences to be detectable.

    I wonder how long it would take a captured planet to settle into resonance with our indigenous Sol system planets?

    We REALLY need to send an atmosphere probe to Uranus with return to earth capability.

    Gary 7

  5. Brian

    No — if Uranus were a captured planet, the odds that it would be orbiting on the ecliptic (and in the same direction) would be pretty small. In all the important ways, Uranus looks like a native planet.

  6. Maybe someone thought Uranus was a Zombie? You always have to double tap zombies!

  7. mike burkhart

    This is interesting.I can rember when the rings were discoverd at the time only Saturn was known to have rings. Uranus was the subject of the sci-fi movie ”Journey to the seventh planet”:Astronuts land on Urauns and find themselvs under the controll of a big alien brain the only inhabint (the movie was made in the 50s I know Uranus is a gas giant) Also I’m sick of the planet being the but of jokes (your anus) I would like to say STOP PICKING ON THE POOR PLANET!!!#1God dose create Planets but God likes endless vriety just like no two pepole are the same no two planets are the same it would be a dull universe if everything was the same.

  8. Am I the only one to chuckle at the title?

    Phil, I’m surprised. I’d never have expected such a blue Uranus joke. I like it.

  9. Thomas Siefert

    Am I the only one to chuckle at the title?

    No you are not. Right now hundreds of people are suppressing the urge to comment. They will not lower themselves to cheap jokes about Uranus.

  10. I saw this headline in my updates list and immediately understood what it implied. Then I started reading the site, beginning with the previous entry on the WSJ false-equivalency editorial, scrolled down to the headline, and began to laugh. I guess the preceding article served as a set-up.

  11. viggen

    I’ll note Earth’s magnetic field isn’t well aligned with our spin axis either, but at least it has the courtesy to be centered on the center of our planet!

    Actually, that’s not quite true; the Earth’s dipole moment is not concentric with the planet. Read about the South Atlantic Anomaly:

    http://en.wikipedia.org/wiki/South_Atlantic_Anomaly

  12. vall

    If Venus spins opposite to us, would a day there be longer than one rotation period? Think about Uranus too, day and night would lose meaning.

  13. Pat

    It was the same planet every time. Little-known fact: Ego the Living Planet was originally named Dagwood.

  14. Togan

    I can’t wait for astronomers to finally rename the planet to Urrectum to end those jokes once and for all!

  15. It’s interesting to figure out what the day/night cycle on Uranus actually is. It varies according to where you stand (if you could stand, of course) on the surface. At either of the poles, you would see a sunrise with the sun moving right around the horizon once per Uranian day, gradually climbing up the sky in a spiral pattern until after 21 (earth) years of continuous sunlight it would reach almost the zenith, then it would take another 21 years to spiral back down to the horizon again. Then would follow an unbroken night lasting 42 (earth) years. The situation is totally different at the equator, with a standard day/night cycle but with the sun moving gradually from North to South in the sky. And if you are anywhere in between, you get a combination of these patterns. Try it in Celestia or Stellarium – it’s fascinating to see it!

  16. Why do we keep having to figure out why Uranus is the way it is, like it was modified later by its environment, or it was a choice? Why can’t we just accept that it was Born This Way and stop worrying about its orientation? Come on, people!

    (tongue planted firmly in cheek for the 1% of you who couldn’t tell)

  17. Wayne Robinson

    Has the theory how the Moon was formed changed? I was under the impression that it wasn’t a glancing collision. Theia was in the same orbit as the proto-earth, but following it. Gravity eventually caused Theia to catch up and combine in a low velocity collision, adding to the core and ejecting crust material to form the Moon, so the Earth has an unusually large core and a thinner crust, allowing tectonic plate movements.

  18. Jess Tauber

    You should leave Uranus alone, and stop picking at it. And stop sending probes there- just one step towards colonization. Every time people have pushed into new places, they’ve wrecked ‘em.

    Re 19: Thin crust eh? So the Moon is made of green cheese, but the Earth is the pizza?

  19. Infinite123Lifer

    Having a perfectly concentric dipole moment with the Earth seems a bit to rounded out and smooth. The courtesy could be in the result of the calculation and their only certainty or courtesy they follow is that dipoles appear exactly where they are supposed to.

  20. @ Jess Tauber

    “This is impossible. Only God makes planets, and He’s perfect. So the orientations of the planets have to be perfect too, in perfect circular orbits around the Earth, plus some epicycles we don’t talk about in polite company…..”

    I sincerely hope you are joking. Not only are you refuting intriguing scientific experiments and observations, but you are also saying to stop these in the name of “god”?
    One more proof that religious people are scared to death of science, they will end-up proving all your little 2000 years ago scriptures wrong.

    We’re not there yet, but definitely closer then you.

  21. Gary Ansorge

    22. Anonymous

    It’s tough being intelligent, yet having no sense of irony,,,

    Gary 7

  22. Thomas Siefert

    22. Anonymous

    My screen actually got moist from the irony oozing from that comment…

  23. Jess Tauber

    I’m sure I have a Bugs Bunny rejoinder somewhere, such as ‘he don’t know me very well, do he?’. If anything I make some fairly well known atheists seem like deists, on my good days. On my bad days, well, you’ve seen ‘Dante’s Peak’?

  24. Gary Ansorge

    25. Jess Tauber

    I just finished re-watching Pitch Black. I loved Vin Diesels comment, “I absolutely believe in God,,,and I absolutely hate the Fraker”.

    Gary 7

  25. Messier Tidy Upper

    @19. Wayne Robinson :

    Has the theory how the Moon was formed changed? I was under the impression that it wasn’t a glancing collision. Theia was in the same orbit as the proto-earth, but following it. Gravity eventually caused Theia to catch up and combine in a low velocity collision, adding to the core and ejecting crust material to form the Moon, so the Earth has an unusually large core and a thinner crust, allowing tectonic plate movements.

    It sure has! Over time .. ;-)

    Once we had a number of different ideas ranging from our Moon being part of our planet that was thrown off through to it being an independent planet itself that was captured into orbit.

    Current thinking is it was created in the “Big Splash” impact between the proto-Earth and a Mars sized impacter.As I see (#19.) Wayne Robinson has already mentioned in more detail. :-)

    Although the newest suggestion here :

    http://www.scientificamerican.com/article.cfm?id=two-moons-smaller

    is that we once (relatively briefly) had two moons before our current Moon collected it’s secondary Earthly partner forming its highland areas.

    BTW. Thinking latest news and items outer planets~wise this :

    http://www.space.com/13229-neptune-day-length-calculated.html

    from Neptune may be of interesty to some folks here perhaps? I hope. :-)

    @ 16. Togan :

    “All the planets except one are named after Roman gods, and Ouranos is the one exception. Unfortunately it has become the brunt of joke after joke due to a Latinization of the original Greek name, Ouranos. We do not call Poseidon Posidon, so why make Ouranos the exception? Ouranos is a magnificent planet that has been subject to a bromidic and stale joke for far too long.”

    Source : The ‘Ouranos not Uranus’ group on facebook.

    http://www.facebook.com/group.php?gid=189939051777

    I strongly agree with (& am a proud member of) that group.

    Actually I think the name 34 Tauri has priority – that being the very first name that planet was charted as – although that could potentially cause some confusion! ;-)

    (The 34 Tauri story is on P. 160, ‘Patrick Moore’s New Guid eto the Planets’,Sidgwick & Jackson, 1993.)

  26. All these theories eh. Mine is that Uranus is the correct way up and it was the rest of the Solar System which hot tipped up, probably by a giant space goat making its clumsy way to the next dimension through a portal which only special people know the whereabouts of (but its near the orbit of planet X).

  27. Actually I think the name 34 Tauri has priority – that being the very first name that planet was charted as – although that could potentially cause some confusion! (The 34 Tauri story is on page 160, ‘Patrick Moore’s New Guide to the Planets’,Sidgwick & Jackson, 1993.)

    Of course, restoring Flamsteed’s 34 Tauri moniker would be rather unfair to William Herschel given Flamsteed’s failure to understand the non-stellar nature of the green “star” he’d charted. Mind you, it took Herschel a while to understand what he’d found too – he first thought Ouranos was a comet and wrote it up as such. Curiously, Ouranos has thus been considered to be a star, a comet then a planet!

    Hmm .. better not tell the IAU this though – if Ouranos can be confused with a star and a comet clearly its planetary status must be questionable and needing a downgrade to mere “dwarf planetary” status! :-o ;-)

    Moving on, Herschel as Ouranos’es discoverer also suggested a name for his find which gives a third alternative for renaming the planet between Saturn and Neptune – ‘Georguium Sidus’ or “George’s Star” in honour of the monarch of the time. Slight problem there in that (I think) that was the “mad King” George III who lost the then colony of America to the Americans, which may not be a reign worth commorating really.

    So 34 Tauri, Georgium Sidus or Ouranos? Take your pick folks! :-)

    PS. Good video on the triple named planet linked to my name here now.

  28. @17. Elwood Herring : “It’s interesting to figure out what the day/night cycle on Uranus actually is.”

    Astronomer & author Neil F. Comins has written a series of hypotheticals about how our Earth would be different if various factors changed. (Click on my name for his works page) One of these included a scenario where Earth orbited “sideways” the way Ouranos does. That was titled “A New Slant on Earth” and published as an article in the July 1992 issue of ‘Astronomy’ magazine and makes for interesting reading if folks can get hold of a copy. It may be included in his text ‘What if the Moon Didn’t Exist’ (published by HarperCollins, 1993) although I’m not sure if it is or not, as I lack a copy myself.

  29. Frost Bite

    Uranus and Neptune collided during Gravitational scattering(Planetary migration). :)

  30. Jay

    Nice write-up, Phil. I’d also be curious to hear your take on the Conservation of Angular Momentum. I have a YEC at work goes around telling folks that the Big Bang model would never work and that Uranus is a prime example of that. He states that because of the CoAM and space being a vacuum, all the planets would orbit the same way. Now I know this is simply not true, but I’d like to hear your take on this sometime with the science to back it up. I figured that collisions could definitely make objects go into different orbits, but it would be nice to have some Plaitish science to throw back at this guy some day.

  31. immature

    Uranus is obvious a rouge planet with ulterior motives. As Americans I believe it is our duty to protect our planet from harm, therefore I too believe that it is necessary that we send spaceprobes to Uranus. Big probes, little probes, probes of varying circumference, we must not relent in the probing of Uranus. Like Navy Seals in the night we shall sneak up and thrust our probes into Uranus repeatedly until we get the answers we desire. Uranus shall learn to fear and submit to our glorious, throbbing, American probes!

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